Chemical Changes and Reactions Claass 9 ICSe Notes
•Condition for a Chemical Reaction and Change
The conditions for a chemical reaction or chemical change are :
1. Close contact. A chemical reaction occurs only when the substances are brought in close contact with each other. The close contact can be brought about by grinding the reactants together or dissolving the reactants in a suitable solvent like water, etc.
Example : Sodium at once reacts with water when it is brought in contact with water.
2. The state of the reactants. The kinetic energy of molecules is least in solids and maximum in gases. So, frequency of collision will also be maximum for gases and hence reaction will be facilitated most when the reactants are in the gaseous state.
1. Close contact. A chemical reaction occurs only when the substances are brought in close contact with each other. The close contact can be brought about by grinding the reactants together or dissolving the reactants in a suitable solvent like water, etc.
Example : Sodium at once reacts with water when it is brought in contact with water.
2. The state of the reactants. The kinetic energy of molecules is least in solids and maximum in gases. So, frequency of collision will also be maximum for gases and hence reaction will be facilitated most when the reactants are in the gaseous state.
3. Heat. Some reactions occur only by heating, e.g. potassium chlorate decomposes only on heating.
Similarly, iron and sulphur do not react on mixing but on heating combine to form iron sulphide.
Heat increases the temperature and it in turn increases kinetic energy. So with increase in temperature, number of collisions increase resulting in increase in rate of chemical reaction.
Generally, the rate of reaction doubles for a 10°C rise in temperature.
Similarly, iron and sulphur do not react on mixing but on heating combine to form iron sulphide.
Heat increases the temperature and it in turn increases kinetic energy. So with increase in temperature, number of collisions increase resulting in increase in rate of chemical reaction.
Generally, the rate of reaction doubles for a 10°C rise in temperature.
4. Light. Some reactions take place only when the reactants are exposed to light. Such reactions are called photochemical reactions.
Examples :
Examples :
5. Pressure. Some reactions take place only when the reactants are subjected to high pressure.
Examples :
Examples :
6. Electricity. Some reactions occur when an electric discharge is passed.
e.g. Acidulated water decomposes on passing electric current through it giving hydrogen and oxygen gases.
3. Displacement reaction : It is a chemical reaction in which an element replaces another element in a compound and takes its place.
Examples :
Displacement reactions are of the following types :
(i) Displacement of a less reactive metal by a more reactive metal (in the electrochemical series) :
Examples :
(ii) Displacement of a less reactive non-metal by a more reactive non-metal :
Examples :
2. Endothermic reaction : A chemical reaction which proceeds with the absorption of the heat energy is called an endothermic reaction. Heat absorbed is used for breaking of old bonds.
Examples :
e.g. Acidulated water decomposes on passing electric current through it giving hydrogen and oxygen gases.
7. Catalyst. A catalyst can increase or decrease the rate of reaction, without itself undergoing any change.
A catalyst is a substance which increases or decreases the rate of reaction but remains unchanged at the end of reaction. Example : Potassium chlorate decomposes at 230°C instead of 400°C if heated in the presence of manganese dioxide (Catalyst).
Promoter : A substance which increases the efficiency of a catalyst is known as promoter.
Example : Molybdenum acts as a promoter during manufacture of ammonia.
A catalyst is a substance which increases or decreases the rate of reaction but remains unchanged at the end of reaction. Example : Potassium chlorate decomposes at 230°C instead of 400°C if heated in the presence of manganese dioxide (Catalyst).
Promoter : A substance which increases the efficiency of a catalyst is known as promoter.
Example : Molybdenum acts as a promoter during manufacture of ammonia.
8. Concentration of the reactants. With the increase in the concentration of the reactants, the rate of a reaction increases. One probable reason is increase in the number of collisions. More is the cone, of reactants more is the number of collisions and faster is the rate of reaction.
•Types of Chemical Change
Chemical changes or chemical reactions are mainly of four types. These are :
1. Combination reaction : It is a chemical reaction in which two simpler reactants combine to form a single product. Such reactions can be brought by the action of heat, light, pressure, electricity, etc.
A general combination reaction can be represented as
1. Combination reaction : It is a chemical reaction in which two simpler reactants combine to form a single product. Such reactions can be brought by the action of heat, light, pressure, electricity, etc.
A general combination reaction can be represented as
The combination reaction in which a compound is formed from its constituent elements is called synthesis reaction.
(ii) Element and a compound
Examples :
Examples :
2. Decomposition reaction : It is a chemical reaction in which a chemical compound decomposes to give two or more new substances.
Examples :
Examples :
Decomposition reactions may be of the following types :
(i) Thermal decomposition. It is a decomposition reaction brought about by heat.
Examples :
(i) Thermal decomposition. It is a decomposition reaction brought about by heat.
Examples :
(ii) Photolysis. It is a decomposition reaction brought about by light.
Examples :
(iii) Electrolysis. It is a decomposition reaction brought about by electricity.
Examples :
(iii) Electrolysis. It is a decomposition reaction brought about by electricity.
3. Displacement reaction : It is a chemical reaction in which an element replaces another element in a compound and takes its place.
Examples :
Displacement reactions are of the following types :
(i) Displacement of a less reactive metal by a more reactive metal (in the electrochemical series) :
Examples :
(ii) Displacement of a less reactive non-metal by a more reactive non-metal :
Examples :
4. Double decomposition or double displacement reaction : It is a chemical reaction in which two compounds in their aqueous solutions react to form two new compounds by the mutual exchange of their radicals.
In general double decomposition reaction can be represented as
AB + CD -----> AD + BC
Double displacement reactions may involve the
(i) formation of a precipitate
(ii) evolution of a gas (Hi) change in colour
Examples :
In general double decomposition reaction can be represented as
AB + CD -----> AD + BC
Double displacement reactions may involve the
(i) formation of a precipitate
(ii) evolution of a gas (Hi) change in colour
Examples :
Isomerism : The process in which simple rearrangement of the atoms takes place leading to new products in known as isomerism.
Simultaneous occurrence of physical and chemical changes :
It may be mentioned here that in many cases a visible change may be the result of simultaneous occurrence of physical as well as chemical changes.
Simultaneous occurrence of physical and chemical changes :
It may be mentioned here that in many cases a visible change may be the result of simultaneous occurrence of physical as well as chemical changes.
Examples :
1. Action of heat on nitre (KNO3)
When potassium nitrate is heated, it fuses (melts) and then decomposes into potassium nitrite and oxygen. The melting process is a physical change while decomposition is a chemical change.
1. Action of heat on nitre (KNO3)
When potassium nitrate is heated, it fuses (melts) and then decomposes into potassium nitrite and oxygen. The melting process is a physical change while decomposition is a chemical change.
2. Action of heat on ammonium chloride
When solid ammonium chloride is heated, it undergoes partial dissociation resulting in the formation of ammonia and hydrogen chloride gas. This is a chemical change. At the same time, a part of ammonium chloride undergoes sublimation forming the vapours of ammonium chloride. This is a physical change.
When solid ammonium chloride is heated, it undergoes partial dissociation resulting in the formation of ammonia and hydrogen chloride gas. This is a chemical change. At the same time, a part of ammonium chloride undergoes sublimation forming the vapours of ammonium chloride. This is a physical change.
•Energy Changes During Chemicals Reactions
The chemical changes or reactions are accompanied by energy changes which may be in the form of heat, light, electricity, sound, work, etc.
On the basis of energy change the chemical reactions are of the following types :
1. Exothermic reaction : A chemical reaction which proceeds with evolution of heat energy is called an exothermic reaction. Heat is evolved when new bonds are formed.
Examples :
On the basis of energy change the chemical reactions are of the following types :
1. Exothermic reaction : A chemical reaction which proceeds with evolution of heat energy is called an exothermic reaction. Heat is evolved when new bonds are formed.
Examples :
2. Endothermic reaction : A chemical reaction which proceeds with the absorption of the heat energy is called an endothermic reaction. Heat absorbed is used for breaking of old bonds.
Examples :
3. Photochemical reaction : A reaction which proceeds with the absorption of light energy is called a photochemical reaction.
Examples :
Examples :
4. Electrochemical reaction : The chemical reaction which proceed with the absorption of electrical energy is called electrochemical reaction.
Examples :
Examples :
•Burning or Combustion
It is a chemical change in which a combustible substance reacts with oxygen to produce new compounds called oxides alongwith the liberation of large amount of energy in the form of heat or light or both. The oxygen gas is mainly responsible for burning.
For example :
1. Burning of magnesium ribbon in oxygen
2. Burning of methane gas in air or oxygen
For example :
1. Burning of magnesium ribbon in oxygen
2. Burning of methane gas in air or oxygen
Conditions for burning :
1. The substance must be in the environment which supports combustion (such as oxygen or air).
2. The temperature of the substance must be above the ignition temperature.
Ignition temperature : It is the temperature to which a substance must be raised before burning.
1. The substance must be in the environment which supports combustion (such as oxygen or air).
2. The temperature of the substance must be above the ignition temperature.
Ignition temperature : It is the temperature to which a substance must be raised before burning.
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