IX Chemistry - Chapter 8 - Electro-Chemistry

Electro-Chemistry

The branch of chemistry which deals with the study of chemical energy to electrical energy or electrical energy to chemical energy is called electro-chemistry.

Conductors

Those substances through which electric current can pass are called conductors. For example all metals are conductors.

Non-Conductors

Those substances through which electric current cannot pass are called non-conductors. For example plastic, wood are non-conductors.

Electrolysis

The process in which electricity passes through the aqueous or infused state of some substance. The substances itself decompose into its component. This process is called electrolysis.

Electrolyte

The compound in molten state or in aqueous solution through which electricity can pass are called electrolyte.

Non-Electrolyte

Those compounds through which electricity cannot pass are called non-electrolyte.

Strong Electrolyte

The substances which are highly soluble and completely ionized are called strong electrolyte. For example acids, bases and salts are strong electrolytes.

Weak Electrolyte

The substances which are not highly soluble and remain in un-ionized form are called weak electrolyte.

Electroplating

A process in which metal is deposited on the surface of another metal by electrolysis is called electroplating.

Objectives of Electroplating

---

Decoration

It is done for decoration. Noble and precious metals like gold or silver are deposited on the inferior metals to enhance their beauty and look beautiful.

Protection

Electroplating is done to protect the metals from rusting as well as from attack of other substance like organic acids and acidic gases.

Repair

It can be used to repair the broken machinery by electroplating with other metals. Usually the metals like copper, silver, chromium, nickel and gold are used for electroplating.

Procedure of Electroplating

---

The metal which is to be electroplated is first cleaned with sand and then washed with caustic soda solution and finally with a lot of water.

This metal is made cathode and the metal which is going to be deposited is made anode. The electrolyte is a salt of metal being deposited and electroplating is carried out in a tank made of cement, glass or wood. It is called an electrolytic tank.

The electrolyte should have following properties:

1. It must be very soluble in water.

2. It must be good conductor.

3. Cheap

4. May not easily oxidized or reduced or hydrolyzed.

(Diagram)

IX Chemistry - Chapter 7 - Solution and Suspension

Solution

A homogeneous mixture of different chemical substances which has uniform chemical composition through out and shows uniform physical properties is called solution. For example dissolve a small amount of copper sulphate in water the water will become blue. If this blue liquid is filtered, it will pass through the filter paper without leaving any solid. The mixture thus prepared is called a solution.

Binary Solution

A solution which is formed by mixing two substances is called binary solution. For example solution of glucose and water.

Solute

The component of a binary solution which is in lesser amount is called solute. For example in copper sulphate solution, copper sulphate is solute.

Solvent

The component of a binary solution which is in greater amount is called solvent. For example in copper sulphate solution, water is solvent.

Saturated solution

A solution in which maximum amount of a solute has been dissolved at a particular temperature and in which the dissolved form of solute is at equilibrium with its undissolved form is called saturated solution.

Unsaturated Solution

Solution which can dissolve further amount of a solute at a [particular temperature is called an unsaturated solution.

Supersaturated Solution

The solution which contains even more amount of solute required to prepare saturated solution is called super saturated solution. The hot saturated solution of compound like sodium thiosulphate does not crystallize its solute if cooled slowly without disturbance. Such a solution is called supersaturated solution.

Dilute Solution

A solution which contains small amount of a solute as compared to the solvent is called dilute solution.

Concentrated Solution

A solution which contains excess amount of a solute as compared to that of a solvent is called a concentrated solution.

Concentrated Solution

The amount of solute present in given quantity of solvent is called concentration of solution. The concentration of a solution can be expressed in many ways depending upon the amount o solute and solvent present in it.

Concentration of Solution

The amount of solute present in given quantity of solvent is called concentration of solution. The concentration of a solution can be expressed in many ways depending upon the amount of solute and solvent present in it.

Percentage by Mass

The percentage of solute by mass is the mass of solute present in hundred part of the solution. For example 5% hydrogen peroxide solution by mass means that 5g hydrogen peroxide are dissolved in 95g of water to give 100g of solution.

Percentage of Mass = (Mass of Solute/Mass of Solution) x 100

Percentage by Volume

The concentration unit expresses the volume of solute present in 100cm3 of solution. For example 15% solution of alcohol by volume will mean that 15cm3 alcohols are present in 100cm3 of solution. (Here 3 represents cube)

Percentage by Volume = (Volume of Solute/Volume of Solution) x 100

Molar Solution

The solution that contains one mole of solute in 1dm3 of solution is called a molar solution. The concentration of this solution is expressed as M.

Molarity

Molarity of a solution is the number of moles of solute present in 1dm3 of the solution. It is expressed as M.

M = Number of Moles of Solute/Volume of Solution in dm3

or

M = (Mass of solute/Molecular Mass) x (1/ Volume of Solution in dm3)

Crystallization

The process in which crystal separates from saturated solution on cooling is called crystallization. It is a useful process because it can be used to purify the impure solid compounds. It can also be used to separate a mixture of solids.

Hydration

The ions surrounded by solvent molecules in solution are called solvated ions. If water is a solvent these ions are called hydrated ions.

Suspension

A suspension in such a mixture in which solute particles do not dissolved in solvent and if filtrated its particles do not pass through the pores of filter paper.

Colloidal Solution

In a colloidal solution the solute particles are slightly bigger than those present in a true solution but not big enough to seen with naked eye.

Standard Solution

A solution whose molarity (strength) is known is called Standard Solution.

True Solution

A True Solution is such a mixture in which solute particles are completely homogenized in the solvent for example solution of sodium chloride or copper sulphate in water.

Solubility

Solubility o a solute in a particular solvent is defined as the amount of solute in grams, which can dissolve in 100g of the solvent at a particular temperature to give a saturated solution.

or

The amount of a solute in gram moles, which can dissolve in one kilogram of the solvent at a particular temperature, to give a saturated solution.

Factors Affecting the Solubility

---

Effect of Solvent

Similar solvents dissolve similar solutes, i.e. if the chemical structure and the electrical properties such as dipole moment of solute and solvent are similar, the solubility will increase. If there is dissimilarity in properties, then either the solute will not dissolve or there will be very little solubility.

Effect of Solute

Different solutes have different solubility's in a particular solvent e.g. if the saturated solutions of table sugar and sodium chloride are prepared, it is found that the concentration of sodium chloride solution is 5.3 molar while that of sugar solution is 3.8 molar. In other words, the solubility of sodium chloride in water is far greater than that of sugar. This is due to the fact that the attraction of sodium (Na+ and chloride (Cl-) ions with water is greater than that of sugar molecules with water.

Effect of Temperature

Change in temperature has different effects on the solubility of different compounds. Usually the solubility increase with the increase in temperature but it cannot be taken as a general rule. The solubility of compounds like lithium carbonate, calcium chromate decreases with the increase in temperature. The solubility of gases in water also decreases with the increase in temperature. On the other hand, there are a large number of compounds whose solubility in water increase with the increase in temperature e.g. sodium nitrate, silver nitrate, Potassium chloride etc. the solubility of sodium chloride in water does not increase appreciably with the increase in temperature.

IX Chemistry - Chapter 6 - States of Matter

States of Matter

Matter has three states:

1. Gas

2. Liquid

3. solid

These are physical states of matter. The three states of one matter may have different physical properties while their chemical properties are same. Water exists in three physical states solid (ice), liquid and gas(steam) has same chemical properties.

Kinetic Theory of Matter

The Kinetic theory was presented to explain the properties of gases and is called kinetic theory of gases. But this theory was also able to explain the composition of liquid and solid state of matter. So its is called Kinetic Theory of Matter.

According to Kinetic Theory of matter:

1. All matter is composed of atoms, molecules or ions.

2. These particles have kinetic energy due to which they are in the state of motion.

3. In gaseous state, these particles move in a straight line. They collide with one another and with the walls of container. In liquids the rate of their movement is very small but in solids, there is to and fro motion only.

4. Generally material particles can have three types of movements, i.e. translational, rotational and vibrational.

Solids

The state of matter which has definite shape and volume is called solid.

Properties of Solids

1. Definite Volume and Shape

The cohesive forces in solid substances are so strong that they keep their particles arranged in fixed positions. So due to restrict movements of particles, the solids have definite volume and shape.

2.Motion of Particles

The solid particles have vibrational motion only because these particles are held in fixed position by strong cohesive forces.

3. Effect of Heat

The physical state of solid substance can be changed by heating. On heating solid is converted to liquid and gaseous state. Heat increases the kinetic energy of the particles and they start vibrating at higher frequency. At a particular temperature the vibrational motions become fast that they overcome the cohesive forces and solid melts to liquid.

4. Melting Point

The temperature at which the solid is converted to liquid on heating is called melting point. At melting point, the particles of solid loose their means position and their arrangement. The solid collapses and turns to liquid.

5. Sublimation

The conversion of some solids directly into gaseous state on heating is called sublimation. Iodine, ammonium chloride and naphthalene change directly into vapour state upon heating.

Liquid

The state of matter having definite volume but indefinite shape is called liquid.

Properties of Liquid

1. Volume

Liquids have definite volume. In liquid particles are very close to one another and have cohesive forces among the particles. Due to the presence of cohesive forces, liquids have definite volume and keep their level as well.

2. Shape

Liquids do not have any specific shape. They adopt the shape of the container. The molecules of liquid are able to move. Due to this random motion the molecules of liquid do not have fixed position and as a result, a liquid does not have any specific shape.

3. Evaporation

Conversion of liquid into its vapours at any temperature is called evaporation. The molecules of liquid come to the surface of liquid and escape by overcoming cohesive forces. So liquid is converted to vapours at all temperature.

4. Boiling Point

The temperature of a liquid at which its vapour pressure becomes equal to the atmospheric pressure is called boiling point.

Gas

The state of matter which does not have definite shape and volume is called gaseous state.

Properties of Gaseous State

1. Indefinite Volume and Shape

In gaseous state, the molecules have insignificant cohesive forces among themselves. They move very fast in all possible directions. As a result, a gas neither has fixed shape nor a fixed volume.

2. Kinetic Energy of the Particle of a Gas

Gas particles have very high kinetic energy as compared to liquid and solid state.

3. Pressure

The molecules of a gas are in the state of random motion. The molecules of gas not only collide with one another but also with the walls of the container in which they are enclosed. Due to their collision, the velocity of the molecules changes every moment. The pressure exerted by gas is also due to the collision of its molecules with the walls of the container.

4. Elastic Collision

The collision of gas molecules is elastic in nature which means that the total energy of the colliding molecules remains the same before and after the collision.

5. Kinetic Energy

The kinetic energy of molecules of gas is very high as compared with solid and liquid.

Diffusion

The movement of molecules from a higher concentration to a lower concentration is known as Diffusion.

If the concentration of molecules at a particular place is higher, they start moving towards a place where their concentration is lower. When the concentration of molecules at both the places becomes equal the process of diffusion stops.

Diffusion in Gases

The molecules of one gas can diffuse easily into the molecules of other gas. For example if an open bottle of a perfume is kept in a room, its smell will spread uniformly throughout the room. The liquid perfume present in the bottle volatilized slowly and its vapours diffuse through out the room.

Graham's Law of Diffusion

Scottish Chemist, Thomas Graham (1833) discovered that lighter gs can diffuse through porous pot faster than the heavier one. This is called Graham's Law of Diffusion.

Hydrogen being lighter gas will diffuse faster than oxygen or carbon dioxide.

Diffusion in Liquids

Liquid molecules can also diffuse because they have free movement. Since the molecules of liquid move comparatively slowly than gas molecule, their rate of diffusion are also lesser than gases.

Brownian Movement

Robert Brown (1927) discovered this phenomenon:

The free movement of the molecules of gases and liquid is called Brownian Movement."

When a pollen grain is put in water. The movement of pollen grain in water is observed by microscope. It is observed that pollen grain is continuously moving in all directions. This free movement of pollen grain was due to the free movement of water molecules. The colliding water molecules will also force pollen grain to move as well. The students can observe Brownian movement with the help of simple experiment.

Experiment

Put a drop of milk on a microscope slide and cover it with cover slip. Put it under microscope and observe it. You will see small particle of fat moving randomly in milk. The movement of fat particles is actually due to the movement of water molecules in milk.

IX Chemistry - Chapter 4 - Periodicity of Elements and Periodic Table

Definitions

Periodic Table

A table of elements obtained by arranging them in order of their increasing atomic number in which elements having similar properties are placed in the same group is called Periodic Table.

Group

The vertical column of elements in the periodic table are called Groups.

Period

The horizontal rows of elements in the periodic table are called Periods.

Periodicity

The repetition of physical and chemical properties of elements periodically is called Periodicity of Properties.

Periodic Law

Physical and chemical properties of elements are periodic function of their atomic masses.

Metal

Elements which are good conductors of heat and electricity are malleable and ductile and have a metallic luster are called Metals like Sodium, Potassium, Gold, Copper etc.

Non-Metals

Elements which are non or bad conductor of heat and electricity are neither malleable or ductile and have no metallic luster are called Non-Metals like Carbon, Nitrogen, Chlorine etc.

Metalloids

Metalloids are semi metals have the properties which are intermediate between a metal and non-metal like Boron, Silicon, Germanium, Arsenic, Antimony etc.

Law of Triads

A German Chemist, Dobereiner (1829), arranged chemically similar elements in groups of three on the basis of their atomic masses called Triads and it was found that atomic mass of the middle element was approximately equal to the average of atomic masses of other two elements. This is known as Law of Triads.

Drawback or Defect

As very few elements could be arranged in such groups, this classification did not get wide acceptance.

Law of Octaves

An English Chemist Newland (1864) stated that if the elements were arranged in the ascending order of their atomic masses, every eight element will have similar properties to the first. This is knows as Law of Octaves.

Drawback or Defects

1. Noble gases were not discovered at that time and no place was reserved for the undiscovered noble gases.

2. In the same way no blank spaces for the undiscovered elements were present in his table.

Mendeleyv's Period Table and Periodic Law

Russian Chemist, Mendeleyv's (186) who wa working separately from Lother Mayer published a table of elements.

According to Mendeleyv's when the element were arranged in order of their increasing atomic mases, the elements with similar properties were repeated after regular interval and were placed one above the other.A table obtained in this manner is called Periodic Table. Mendeleyv's stated this periodicity in the form of Periodic Law.

Important Features of Mendeleyv's Periodic Table

---

The important features of Mendeleyv's Periodic table are:

Periods and Groups

The horizontal rows which run from left to right in Periodic Table are called Periods and they are twelve in number.

The vertical rows which run from top to bottom in periodic table are called groups and they are eight in number.

Vacant Spaces

Mendeleyv's left many vacant spaces for the still unknown elements. For example, next to Calcium (40) should be Titanium (48) but it resembled silicon (28) instead of Aluminium (27). He left vacant space for element with atomic mass 44.

Discovery of New Element

Mendeleyv's discovered new elements and also guessed their atomic mass and properties.

Atomic Mass Correction

Mendeleyv's corrected the atomic masses of certain elements on basis of their properties and provided proper place to them in the periodic table.

Defects in Mendeleyv's Periodic Table

---

The Mendeleyv's Period Table has following defects:

Irregular Position of Some Elements

According to Mendeleyv's Periodic Law Potassium (39) should be placed before Argon (40) but he placed Argon (40) before Potassium (39) which goes against his law.

Position of Isotopes

Mendeleyv's periodic table gives no indication about the position of isotopes.

Structure of Atom

Mendeleyv's Periodic table gives no idea about structure of atoms.

Position of Lanthanides and Actinides

Lanthanides and Actinides have not been given proper place in Periodic Table.

Coinage and Alkali Metals

Alkali metals and coinage metals with different properties are placed in the same group. This defect has been replaced by placing them into two sub groups.

Modern Periodic Law and Modern Periodic Table

Modern Periodic Law

Physical and chemical properties of the elements are periodic function of their atomic number. Mosely (1913) says that atomic mas is not fundamental property. Due to some defects present in Mendeleyv's periodic law, Mosely introduced the concept of anomic number for the elements.

Example

When isotopes were discovered, it was thought advisable to arrange the elements on basis of their atomic number instead o increasing atomic mases. Isotopes were needed different position in the Mendeleyv's periodic table. Hence Mendeleyv's periodic law was modified.

Modern Periodic Table

When Mendeleyv's periodic law was modified and new elements were discovered. This forcd the scientists to change Mendeleyv's periodic law.

The electronic configuration of atoms also played an important role in he arrangement of the modern periodic law. This form of periodic table is called "Long form of Periodic Table" because it contains eighteen groups instead of eight but seven periods instead of twelve.

Group I - The Alkali Metals

The elements of group I are called "Alkali Metals". The word alkali is derived from an Arabic word meaning Ashes.

Elements of Group I

• Lithium

• Sodium

• Potassium

• Rubidium

• Cesium

• Francium

Properties of Group I

1. They are mono atomic.

2. They exist in solid metallic state.

3. Outer most shell of these elements is incomplete having one electron.

4. Elements of this group are highly reactive.

5. Elements of this group have large tendency to form compounds.

6. Elements of this group are strongly electro-positive.

Group II - The Alkaline Earth Metals

The elements of group II are called Alkaline Earth Metals. These elements occur in nature as silicate mineral and their oxides and hydroxides are strongly basic. Therefore these elements are called Alkaline Earth Metals.

Elements of Group II

• Beryllium

• Magnesium

• Calcium

• Strontium

• Barium

• Radium

Properties of Group II

1. They are mono atomic.

2. They exist in solid state.

3. Outer most shell of these elements is incomplete having two electrons.

4. Elements of this group are moderately reactive.

5. Elements of this group have moderate tendency to form compounds.

Group III - The Boron or Aluminium Family

The elements of group III exist in solid state.

Elements of Group III

• Boron Metalloid

• Aluminium Metal

• Gallium Metal

• Indium Metal

• Thallium Metal

Properties of Group III

1. They are mono atomic.

2. They exist in solid state.

3. Outer most shell of these elements is incomplete having three electrons.

4. Elements of this group are quite reactive.

5. Elements of this group have moderate tendency to form compounds.

Group IV - The Carbon and Silicon Family

Elements of Group IV

• Carbon

• Silicon

• Germanium

• Tin

• Lead

Properties of Group IV

1. They are mono atomic.

2. They exist in solid state.

3. Outermost shell of these elements is incomplete.

4. Elements of this group are quite reactive.

5. Elements of this group have moderate tendency to form compounds.

Group V - The Nitrogen Family

Elements of Group V

• Nitrogen

• Phosphorus

• Arsenic

• Antimony

• Bismuth

Properties of Group V

1. Some are mono atomic and some are di-atomic.

2. Some of them exist in gaseous and some are in solid state.

3. Outermost shell of these elements is incomplete having five electrons.

4. elements of this group are quite reactive.

5. Elements of this group have quite tendency to form compound.

Group VI - The Oxygen Family

Elements of Group VI

• Oxygen

• Sulphur

• Selenium

• Tellurium

• Polonium

Properties of Group VI

1. Some are mono atomic and some are di-atomic.

2. Some of them exist in gaseous and some are in solid state.

3. Elements of this group have quite tendency to form compounds.

4. The tendency of forming covalent bond decreases from oxygen to polonium.

5. There is a gradual decrease in the ionization potential down the group.

Group VII - The Halogen Family

Elements of Group VII

• Fluorine Gas

• Chlorine Gas

• Bromine Liquid

• Iodine Solid

• Astatine Radioactive

Properties of Group VII

1. They are diatomic except At.

2. Halogens are very active non-metals.

3. Outer most shell of these elements is incomplete having seven electrons.

4. Elements of this group are highly reactive.

5. There is a gradual decrease in the ionization potential down the group.

Transition Elements

Definition

Elements in Group IB, IIB, through VIIB are known as Transition Elements because they show their properties which are transitional between higly reactive and strong electro-positive elements of S-block which form ionic compounds and p-block elements which form largely covalent compounds.

Properties of Transition Elements

1. Transition Elements have incomplete inner electron shells.

2. They show variable valency.

3. They show similar behaviour.

4. They all are metals.

5. They have strong inner atomic bonds.

Group 0, The Noble Gases

The elements of Group VIII A are called "Noble Gases" or "Inert Gases" or "Zero Group Elements".

Elements of Group 0

• Helium

• Neon

• Argon

• Krypton

• Xenon

• Radon

Properties of Group 0

1. They are mono atomic.

2. They exist in gaseous state.

3. Outer most shell of these elements is either complete or contains eight electrons.

4. These elements are mostly chemically non-reactive.

5. These elements have no tendency to form compounds (only a few of these compounds are known).

Atomic Radius

Definition

One half of the distance between the nucleus of two identical atoms when these are in close contact with each other is called Atomic Radius.

Unit

It is measured in angstrom unit A.

Trend in Period

The atomic radii decreases from left to right within a period in the periodic table. This is because nuclear charge increases with the increase of atomic number. But the number of shells remains same within a period.

Trend in Group

Atomic radius increases from top to bottom in a group. This is because, although nuclear charge increases from top to bottom but at the same time on new shell is also added for each successive element down the group.

Ionization Energy (I.E) or Ionization Potential (I.P)

Definition

The minimum energy needed to remove an electron from an isolated, gaseous atom in its ground state is called Ionization Energy.

Unit

It is expressed in electron volts or kilo-joules permole.

1 ev = 96.49kj

Factors Affecting Ionization Energy

The ionization energy of elements depends upon the following factors:

1. Effect of Nuclear Charge on I.E

The greater the nuclear charge the higher is the ionization energy.

2. Effect of Atomic Size

The larger the size of atom the lower is the ionization energy.

Trend of I.E in Period

Ionization energy increases from left to right in a period due to increase in nuclear change and decrease in atomic size.

Trend of I.E in Group

I.E decreases from top to bottom in a group due to increase in atomic size.

Electronegativity

Definition

The tendency of each atom in a covalent molecule to attract a shared pair of electrons towards itself is known as its electronegativity.

Factors Affecting Electronegativity

Electronegativity depends upon the following factors:

• Atomic size

• Atomic Number

• Electron Affinity

• Ionization Energy

Trend or Variation in the Period

Electronegativity increases from left to right within a period due to increase in nuclear charge and decrease in atomic size.

Trend or Variation in the Group

Electronegativity values decreases from top to bottom within a group due to increase in atomic size.

Electron Affinity

Definition

The energy change that occurs when an electron is gained by an atom in the gaseous state is known as Electron Affinity.

Electron Affinity for the addition of first electron is negative i.e. energy is released but for further addition of electrons it is positive because energy has to be added to over come repulsion between negative ion and electron.

Unit

It is measured in KJ/mol or in e.v per atom.

Factors Affecting Electron Affinity

• Atomic Size

• Nuclear Charge

Tend or Variation of Electron Affinity in Group

Down the group in the periodic table, electron affinity decreases because the addition of a new shell to each atom decreases its force of attraction.

Trend or Variation of Electron Affinity in Period

In a period, the electron affinity increases from left to right because the incoming successive atoms have higher nuclear charge and attract electron more towards itself.

IX Chemistry - Chapter 3 - Atomic Structure

Dalton's Atomic Theory

The important postulates of Dalton's atomic theory are:

1. All elements are composed of atoms. Atom is too small so that it could not be divided into further simpler components.

2. Atom cannot be destroyed or produced.

3. Atoms of an element are similar in all respects. They have same mass and properties.

4. Atoms of different elements combine in a definite simple ratio to produce compounds.

Discovery of Electron

A discharge tube is a glass tube. It has two electrode, a source of electric current and a vacuum pump.

(Diagram)

Sir William Crooks (1895 performed experiments by passing electric current through gas in the discharge tube at very low pressure. He observed that at 10-4 (-4 is power to 10) atmosphere pressure, shining rays are emitted from cathode. These rays were named cathode rays. Cathode rays are material particles as they have mass and momentum.

Properties of Cathode Rays

The properties of these particles are given below:

1. These particles are emitted from cathode surface and move in straight line.

2. The temperature of the object rises on which they fall.

3. They produce shadow of opaque object placed in their path.

4. These particles are deflected in electric and magnetic fields.

5. These particles are deflected towards positive plate of electric field.

Discovery of Proton

Gold Stein (1886) observed that in addition to the cathode rays, another type of rays were present in the discharge tube. These rays travel in a direction opposite to cathode rays. These rays were named positive rays. By using perforated cathode in the discharge tube the properties of these rays can be studied. Positive rays are also composed of metered particles. The positive rays are not emitted from anode. They are produced by the ionization of residual gas molecules in the discharge tube. When cathode rays strike with gas molecule, electrons are removed and positive particles are produced.

Properties of Positive Rays

1. They are deflected towards negative plate of electric field. Therefore these rays carry positive charge.

2. The mass of positive rays is equal to the mass of the gas enclosed in the discharge tube.

3. The minimum mass of positive particles is equal to the mass of hydrogen ion (H+). These positive ions are called Protons.

4. The charge on proton is equal to +1.602x10-19 Coulomb. (-19 is power of 10)

Natural Radioactivity

The phenomenon in which certain elements emit radiation which can cause fogging of photographic plate is called natural radioactivity. The elements which omit these rays are called radioactive elements like Uranium, Thorium, Radium etc. There are about 40 radioactive elements. Henri Bequrel (1896) discovered radioactivity.Madam Curei also has valuable contribution in this field.

In natural radioactivity nuclei of elements are broken and element converted to other elements. Natural radioactivity is nuclear property of the elements.

Alpha Rays

1. They are helium nuclei. They are doubly positively charged, He2+.

2. They move with speed equal to the 1/10th of the velocity of the light.

3. They cannot pass through thick-metal foil.

4. They are very good ionizer of a gas.

5. They affect the photographic plate.

Beta Rays

1. They are negatively charged.

2. They move with the speed equal to the velocity of light.

3. They can pass through a few millimeter thick metal sheets.

4. They are good ionizer of a gas.

5. They can affect the photographic plate.

Gamma Rays

1. They are electromagnetic radiations.

2. They travel with speed equal to velocity of light.

3. They carry no charge.

4. They have high penetration power than alpha and beta rays.

5. They are weak ionizer of gas.

Rutherford Experiment and Discovery of Nucleus

Lord Rutherford (1911) and his coworkers performed an experiment. They bombarded a very thin, gold fail with Alpha particles from a radioactive source. They observed that most of the particles passed straight through the foil undeflected. But a few particles were deflected at different angles. One out of 4000 Alpha particles was deflected at an angle greater than 150.

(Diagram)

Conclusion

Following conclusions were drawn from the Rutherford's Alpha Particles scattering experiment.

1. The fact that majority of the particles went through the foil undeflected shows that most of the space occupied by an atom is empty.

2. The deflection of a few particles over a wide angle of 150 degrees shows that these particles strike with heavy body having positive charge.

3. The heavy positively charged central part of the atom is called nucleus.

4. Nearly all of the mass of atom is concentrated in the nucleus.

5. The size of the nucleus is very small as compared with the size of atom.

Defects of Rutherford Model

Rutherford model of an atom resembles our solar system. It has following defects:

1. According to classical electromagnetic theory, electron being charged body will emit energy continuously. Thus the orbit of the revolving electron becomes smaller and smaller until it would fall into the nucleus and atomic structure would collapse.

2. If revolving electron emits energy continuously then there should be a continuous spectrum but a line spectrum is obtained.

(Diagram)

Bohr's Atomic Model

Neil Bohr (1913) presented a model of atom which has removed the defects of Rutherford Model. This model was developed for hydrogen atom which has only proton in the nucleus and one electron is revolving around it.

Postulates of Bohr's Atomic Model

The main postulates of Bohr's Model are given below:

1. Electrons revolve around the nucleus in a fixed orbit.

2. As long as electron revolves in a fixed orbit it does not emit and absorb energy. Hence energy of electron remains constant.

3. The orbit nearest to the nucleus is the first orbit and has lowest energy. When an electron absorbs energy it jumps from lower energy orbit to higher energy orbit. Energy is emitted in the form of radiations, when an electron jumps from higher energy orbit to lower energy orbit. The unit of energy emitted in the form of radiations is called quantum. It explains the formation of atomic spectrum.

4. The change in energy is related with the quantum of radiation by the equation :

E2 - E1 = hv

where

E1 = Energy of first orbit

E2 = Energy of the second orbit

h = Planck's constant

v = Frequency of radiation

Atomic Number

The number of protons present in the nucleus of an atom is called atomic number or proton number. It is denoted by z. The proton in the nucleus of an atom is equal to number of electrons revolving around its nucleus.

Mass Number

The total number of the protons and neutrons present in the nucleus of an atom is called mass number. The protons and neutrons together are called nucleon. Hence it is also known as nucleon number. It is denoted by A. the number of neutrons present in the nucleus of an atom is rperesented by N.

Mass Number = No of Protons + No of neutrons

A = Z + N

Isotopes

The atoms of same elements which have same atomic number but different mas number are called Isotopes. The number of protons present in the nucleus of an atom remains the same but number of neutrons may differ.

Isotopes of Different Elements

Isotopes of Hydrogen

---

Hydrogen has three isotopes:

1. Ordinary Hydrogen or Protium, H.

2. Heavy Hydrogen or Deutrium, D.

3. Radioactive Hydrogen or Tritium, T.

Protium

Ordinary naturally occurring hydrogen contains the largest percentage of protium. It is denoted by symbol H. It has one proton in its nucleus and one electron revolve around the nucleus.

• Number of Protons = 1

• Number of Electrons = 1

• Number of Neutrons = 0

• Atomic Number = 1

• Mass Number = 1

Deutrium

Deutrium is called heavy hydrogen. The percentage of deutrium in naturally occuring hydrogen is about 0.0015%. It has one proton and one neutron in its nucleus. It has one electron revolving around its nucleus. It is denoted by symbol D.

• Number of Proton = 1

• Number of Electron = 1

• Number of Neutrons = 1

• Atomic Number = 1

• Mass Number = 2

Tritium

Radioactive hydrogen is called tritium. It is denoted by symbol T. The number of tritium isotope is one in ten millions. It has one proton and 2 neutrons in its nucleus. It has one electron revolving around its nucleus.

• Number of Proton = 1

• Number of Electron = 1

• Number of Neutron = 2

• Atomic Number = 1

• Mass Number = 3

IX Chemistry - Chapter 2 - Chemical Combinations and Chemical Equation

Laws of Chemical Combinations

There are four laws of chemical combinations these laws explained the general feature of chemical change. These laws are:

1. Law of Conservation of Mass

2. Law of Definite Proportions

3. Law of Multiple Proportions

4. Law Reciprocal Proportions

Antoine Lavoiser has rejected the worn out ideas about the changes that take place during a chemical reaction. He made careful quantitative measurements in chemical reactions and established that mass is neither created nor nor destroyed in a chemical change.

Law of Conservation of Mass

Statement

It is presented by Lavoiser. It is defined as:

"Mass is neither created nor destroyed during a chemical reaction but it only changes from one form to another form."

In a chemical reaction, reactants are converted to products. But the total mass of the reactants and products remains the same. The following experiment easily proves law of conservation of mass.

Practical Verification (Landolt Experiment)

German chemist H. Landolt, studied about fifteen different chemical reactions with a great skill, to test the validity of the law of conservation of mass. For this, he took H.shaped tube and filled the two limbs A and B, with silver nitrate (AgNO3) in limb A and Hydrochloric Acid (HCl) in limb B. The tube was sealed so that material could not escape outside. The tube was weighed initially in a vertical position so that the solution should not intermix with each other. The reactant were mixed by inverting and shaking the tube. The tube was weighed after mixing (on the formation of white precipitate of AgCl). He observed that weight remains same.

HCl + AgNO3 ----------> AgCl + NaNO3

Law of Definite Proportions

Statement

It is presented by Proust. It is defined as:

"When different elements combine to give a pure compound, the ratio between the masses of these elements will always remain the same."

Proust proved experimentally that compound obtained from difference source will always contain same elements combined together in fixed proportions.

Example

Water can be obtained from different sources such as river, ocean, well, canal, tube well, rain or by the chemical combination of hydrogen and oxygen. If different samples of water are analyzed, it will have two elements, hydrogen and oxygen and the ratio between their mass is 1:8.

Law of Multiple Proportions

Statement

This law is defined as:

"When two elements combine to give more than one compounds, the different masses of one element, which will combine with the fixed mass of other element, will be in simple whole number ratio."

Two different elements can combine to form more than one compound. They can do so by combining in different ratios to give different compounds.

Example

Hydrogen and oxygen combine with one another to form water (H2O) and hydrogen peroxide (H2O2). In water and hydrogen oxide 2 g of hydrogen combine with 16g and 32g of oxygen respectively. According to law of multiple proportions, the different masses of oxygen (16g and 32g) which have reacted with fixed mass (2g) of hydrogen will have a simple ratio between each other i.e. 16:32 or 1:2. It means that hydrogen peroxide contains double the number of oxygen atoms than water. This law proves this point of Dalton's Atomic Theory that atoms do not break in a chemical reaction.

Law of Reciprocal Proportions

Statement

This law is defined as:

"When two element A, B combine separately, with the mixed mass of the third element E, the ratio in which these elements combine with E is either the same or simple multiple of the ratio in which A and B combine with each other."

Example

Hydrogen and Nitrogen separately combine to form ammonia (NH3) and dinitrogen oxide (N2O), in these compounds, fixed mass of nitrogen is 14g and combines with 8 g of oxygen and 3 g of hydrogen. The ratio between the mass of oxygen and hydrogen is 8:3. Hydrogen and oxygen also combine with one another to form water (H2O). The ratio between hydrogen and oxygen in water is 16:2. These ratios are not same. Let us observe whether these ratios are simple multiple to each other or not following mathematical operation is carried out.

8:3 ::16:2

8/3 : 16/2

or

8/3 x 2/16

or

1/3 => 1:3

Definitions

Atomic Mass

The mass of an atom of the element relative to the mass of some reference or standard element is called atomic mass. Atoms are very small particles. They have very small mass. If the masses of atoms were to be expressed in gram. It is a very big unit for this very tiny object. Then it was decided by the chemists that masses of the atoms were to be found after comparing with mass to some standard form.

Hydrogen being the lightest element is taken as standard. The mass of the hydrogen atom taken as one.

The atomic mass could be defined as

"Atomic mass of an element is the mass of an atom of that element as compared to the mass of an atom of hydrogen taken as one."

Example

The atomic mass of sodium is 23. It means that an atom of sodium is 23 times heavier than hydrogen atom. Similarly atomic mass of oxygen is 16. It means that an atom of oxygen is 16 times heaviest than that of hydrogen.

Atom

The smallest particle of an element which cannot exist independently and take part in a chemical reaction is known as Atom.

Examples

Hexogen(H), Carbon (C), Sodium (Na), Gold (Au) etc.

Molecule

The particle of a substance (Element or Compound) which can exist independently and show all the properties of that substance is called molecule.

Atoms of the same or different elements react with each other and form molecule.

Atoms of some elements can exist independently, since they have property of molecule so they are called mono atomic molecule.

Examples

Examples of Molecules of the elements are Hydrogen (H2). Nitrogen (N2), Sulphur (S8) etc.

Molecules of different elements are called compounds. For example HCl, H2O, CH4 etc.

Valency

The combining capacity of all elements with other elements is called valency.

Example

H = 1

C = 4

Al = 3

Mg = 2

Na = 1

Chemical Formula

"A brief name used for full chemical name at a compound is called Chemical Formula."

A chemical formula is used to represent an element or a compound in terms of symbols. It also represents the number and type of atoms of elements present in the smallest unit of that substance.

Example

The chemical formula of hydrogen sulphide is H2S. It shows two types of elements (H and S) and number of atoms of element (2H and 1S). Similarly the formula of NaCl show number and type of different atoms present in its smallest unit.

Empirical Formula

"The formula which shows the minimum (simple) ratio between atoms present in a compound is known as Empirical Formula."

Example

For example the empirical formula of hydrogen peroxide is HO that of water is H2O and benzene is CH.

Molecular Formula

The formula of an element or a compound which represents the actual number of atoms present in the molecule of these substances is called molecular formula.

Example

Water, Hydrogen Peroxide, Ethylene Benzene and Sulphur have molecular formula H2O, H2O2, C2H4, C6H6 and S8 respectively.

Molecular Mass

Molecular mass of an element or a compound is defined as the mass of its molecule relative to 1/12th of the mass of C-12. It is the sum of the atomic masses of all the atoms presents in its molecular formula.

Example

Molecular mass of water (H2O) = 2 + 16 = 18 a.m.u

Mass of hydrogen sulphide (H2S) = 2 + 32 = 34 a.m.u

Formula Mass

Formula mass of a compound is the mass of its formula unit relative to 1/12th of the mass of C-12.

Example

Formula mass of Sodium Chloride NaCl = 23 + 35.5 = 58.5 a.m.u

Formula mass of Calcium Chloride CaCl2 = 40 + 35.5x2 = 111a.m.u

Molar Mass

The mass of one mole of a substance is called molar mass.

Example

1 mole of Hydrogen atom (H) = 1.008g

1 mole of Hydrogen molecule (H2) = 2.016g

Thus mass of substance is related to the particles by mole.

Chemical Reaction

A chemical change in which reactants are converted to products is called chemical reaction.

Zn + 2HCl --------> ZnCl2 + H2

The fact that a chemical reaction is taking place can be inferred from the following observation.

1. Evolution of a gas

2. Change in colour

3. Change in temperature.

4. Emission of light.

Types of Chemical Reaction

The chemical reaction is classified into following types:

1. Displacement Reaction

The reaction in which an atom or group of atoms is displaced by another atom or group of atoms in a compound is called displacement reaction.

Fe + CuO ---------> Cu + FeO

2. Double Displacement Reactions

The reactions in which reacting substances exchange their radicals or ions are double displacement reaction. Insoluble salts are formed by mixing soluble salts.

3. Addition Reactions

When two different compounds or elements react together to give only one confound, the reaction will be called addition reaction.

2Mg + O2 --------> 2MgO

4. Decomposition Reaction

The reaction in which some compounds may decompose into elements or simpler compounds on heating is called decomposition reaction.

CaCO3 ---------> CaO + CO2 (Heat)

Chemical Equation

Symbolic representation of chemical change in terms of symbols and formulae is called Chemical Equation.

Method of Equation Writing

A chemical equation can be written as follows:

1. Write the formulae and symbols of the reactants on the left hand side.

2. Write the formulae and sympols of the products on the right hand side.

3. Separate the reactants and products by an arrow which is directed towards the products.

Characteristics of Chemical Equation

1. Chemical equation must be representative of a chemical reaction.

2. It should represent molar quantities.

3. It should be balanced in terms of atoms/molecules of reactants and products.

Reactants

Those substances, which react together in a chemical reaction, are called reactants.

Zn + 2HCl ------> ZnCl2 + H2

In the above reaction Zn and HCl are the reactants.

Products

Those substances, which are formed in a chemical reaction, are called products.

Zn + 2HCl ------> ZnCl2 + H2

In the above reaction, ZnCl2 and H2 are products.

Information obtained from a Chemical Equation

1. A balanced equation indicates that which reactant undergo chemical change. It indicates that which products are formed.

2. It indicates that how many moles of reactants under go chemical change. It indicates that how many moles of products are formed.

Why are Chemical Equations Balanced

A chemical equation must be balanced in order to satisfy the law of conservation of matter, which states that matter can neither be created nor be destroyed during a chemical reaction.

IX Chemistry - Chapter 1 - Introduction to Chemistry

Chemistry

The branch of science which deals with the composition and properties of matter, changes in matter and the laws or principles which govern these changes is called Chemistry.

Branches of Chemistry

Physical Chemistry

The branch of chemistry which deals with the physical properties and physical behavior of material things is called physical chemistry.

Inorganic Chemistry

The study of all elements and their compounds except carbon is called inorganic chemistry.

Organic Chemistry

The branch of chemistry in which we study the compounds of carbon is called organic chemistry.

Analytical Chemistry

The branch of chemistry which discusses the analytical methods for getting information about chemical compounds and chemical processes is called analytical chemistry.

Biochemistry

The study of chemical compounds present in living things is called biochemistry.

Industrial Chemistry

The application of chemical knowledge in technology and industry and the preparation of industrial products are called industrial chemistry.

Steps Involved in Getting Information in the Scientific Method

Science is not only an integrated knowledge of physical and biological phenomena but also the methodology through which this knowledge is gathered. The process of scientific discoveries is a cyclic process.

In science the facts are gathered through observations and experiments and then theories or law are deduced. The scientific method include following four steps:

1. Observation

2. Inference

3. Prediction

4. Experiment

Observation

The observations are made by the five senses of man. Men made equipments are also used for making observations. For example microscope is used for observing minute objects. Thermometer is used to measure temperature. Sensitive balance is used to determine the mass of a very light object. The capacity of man made instruments is also limited. But it can be improved by improving technology. Thus better and more reliable information are given to the scientists who produce better result. Information acquired through careful observations are called facts. These facts are foundation of scientific knowledge.

Inference

The facts gathered through observations are carefully arranged and properly classified. Correlating the knowledge thus acquired with previous knowledge, we try to think of a tentative solution to explain the observed phenomenon. The tentative solution is called hypothesis. The validity of this hypothesis is tested through the results obtained from experiments. The results are discussed by the scientists and the hypothesis is accepted or rejected. The accepted hypothesis then takes the form of theory. A theory when repeatedly gives the same results after experimentation and gives correct explanation of the scientific facts becomes a law or principle.

A theory remains valid until contrary informations are given on the basis of experimentation. Thus a hypothesis requires experimental support. But Avogadro's hypothesis has been accepted as law without any experimental support.

Prediction

Facts, theories and laws which are deduced from observation can help in deducing more facts and phenomenon. This process is called prediction.

Experiment

An experiment is an integrated activity, which is performed under suitable conditions with specially designed instruments to get the required information. Such information is used to test the validity of the hypothesis. If a hypothesis is proved correct. It increases the reliability of known facts. If it is proved wrong, it stil can give information which can be used to deduce other results.

Chemistry and Society

Chemistry has played important role for well being of mankind in the form of food, clothing, shelter, medical treatment and chemical fertilizers, crops protected by insecticides, refined food and production of artificial fiber. Production of cement, iron bricks, glass, paint etc are all due to chemistry.

The hazards of chemistry are so vast that no aspect of human life has remained unaffected. The smoke coming from chimneys of chemial industries and from vehicles pollute the air. It is very dangerous to breath in that air. Similarly waste water from industry, pollute canals, rivers and has bad effect on land. Excessive chemical spray on plants also has bad effect.