Chemical Reaction: The process in which new substances with new properties are formed from one
or more substances is called Chemical Reaction.
Signs of Chemical Reaction
Change of colour of substance
Evolution of heat
Absorption of heat
Evolution of gas etc
.
Reactant and Product:
Reactant: Substances which take part in a chemical reaction are called reactants.
Product: New substance formed after a chemical reaction is called product.
Chemical Equation: Representation of chemical reaction using symbols of substances is called chemical
equation.
The necessary conditions such as temperature, pressure or any catalyst should be written on arrow
between reactant and products.
Balancing Chemical Equation
Law of conservation of Mass. Matter can neither be created nor destroyed in a chemical reaction.
So number of elements involved in chemical reaction should remain same at reactant and product side.
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STEPWISE BALANCING (Hit and Trial)
Step 1. Write a chemical equation and draw boxes around each formula.
Fe + H2O → Fe3O4 + H2
* Do not change anything inside the box.
Step 2. Count the number of atoms of each element on both the sides of chemical equation.
Step 3. Equalise the number of atoms of element which has maximum number by putting in front of it.
Fe + 4H2O → Fe3O4 + H2
Step 4. Try to equalize all the atoms of elements on reactant and product side by adding coefficient in front
of it.
3Fe + 4H2O → Fe3O4 + 4H2
* Now all the atoms of elements are equal on both sides.
Step 5. Write the physical states of reactants and products.
3Fe (s) + 4H2O (g) → Fe3O4 ( s) + 4H2 (g)
Solid state = (s) Liquid state = (l) Gaseous state = (g) Aqueous state = (aq)
Step 6. Write necessary conditions of temperature, pressure or catalyst on arrow above or below.
Balance the given equations.
1. H2 + O2 ⇨ H2O
2. Fe + H2O ⇨ Fe2O3 + H2
3. CO2 + H2O ⇨ C6H12O6 + O2
4. Fe2O3 + C ⇨ Fe + CO2
Types of Chemical Reaction
Chemical reactions can be classified in following types:
1. Combination Reaction
2. Decomposition Reaction
3. Displacement Reaction
4. Double Displacement Reaction
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5. Exothermic and Endothermic Reaction
6. Slow and fast Reaction
7. Oxidation and Reduction Reaction
1. Combination Reaction: Reactions in which two or more reactants combine to form one product
are called COMBINATION REACTION.
A general combination reaction can be represented by the chemical equation given here.
A + B ⇨ AB
Exothermic Reactions : Reaction in which heat is released along with formation of products.
2. Decomposition Reaction: Reactions in which one compound decomposes in two or more
compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite
of combination reaction.
A general decomposition reaction can be represented as follows:
A ⇨ B + C
Thermal decomposition : When decomposition is carried out by heating.
Electrolytic Decomposition : When decomposition is
carried out by passing electricity.
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Photolysis or Photo Decomposition Reaction: Reactions in which a compound decomposes
because of sunlight are known as PHOTOLYSIS or PHOTO DECOMPOSITION REACTION.
Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.
2AgCl ⇨ 2Ag + Cl2
Similarly, when silver bromide is put under sunlight, it decomposes into silver metal and bromine gas.
2AgBr ⇨ 2Ag + Br2
Photographic paper has coat of silver chloride, which turns into grey when exposed to sunlight. It happens
because silver chloride is colourless while silver is a grey metal.
3. Displacement Reaction The chemical reaction in which more reactive element displaces less
reactive element from its salt solution.
A general displacement reaction can be represented using chemical equation as follows:
A + BC ⇨ AC + B
The iron nail becomes brownish in colour by deposition of Cu and blue colour of CuSO4 changes dirty green
colour due to formation of FeSO4.
Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’,
then ‘A’ will not displace ‘B’ from ‘BC’ and reaction will not be taken place.
Example: When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.
Zn + 2HCl ⇨ ZnCl2 + H2
When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.
Zn + CuSO4 ⇨ ZnSO4 + Cu
When silver metal is dipped in copper nitrate, no reaction takes place because silver metal is less reactive
than copper. Ag + Cu (NO3)2 ⇨ No reaction takes place
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4. Double Displacement Reaction: Reactions in which ions are exchanged between two reactants
forming new compounds are called double displacement reactions.
AB + CD ⇨ AC + BD
Example: When solution of barium chloride reacts with the solution of sodium sulphate, white precipitate
of barium sulphate is formed along with sodium chloride.
BaCl2 + Na2SO4 ⇨ BaSO4 + 2NaCl
When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.
NaOH + HCl ⇨ NaCl + H2O
Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction.
Neutralisation reactions are also examples of double displacement reaction.
5. Exothermic and Endothermic Reaction
Reactions which produce energy are called exothermic reaction. On the other hand, reactions which
absorb energy are called endothermic reaction. Most of the combination reactions are exothermic. Most
of the decomposition reactions are endothermic. Respiration is a decomposition reaction in which energy
is released. When quick lime (calcium carbonate) is added to water, it decomposes and releases energy.
Cooking involves chemical reactions which are endothermic as cooking is possible because of heating.
6. Slow and fast Reaction:-
i). Slow Reaction:- Those reaction in which takes a lot of time to complete are called slow reaction.
Ex – Rusting of iron
4Fe + 3O2 + 6H2O ⇨ 2Fe2O3.3H2O
ii). Fast Reaction:- Those reaction which are completed in small period of time are called fast reaction
Ex – Neutralization reaction
HCl + NaOH ⇨ NaCl + H2O
7. Oxidation and Reduction Reaction:
Oxidizing agent: Compounds which can add oxygen or a non-metallic compound or remove hydrogen
or metallic element are known as oxidizing agents.
Reducing agent: Compounds or elements which can cause reduction are called reducing agents.
OXIDATION REDUCTION
1. Addition of oxygen
C+O2 ⇨ CO2
Removal of oxygen
2MgO ⇨ Mg + O2
2. Removal of hydrogen
H2S ⇨ H2 + S
CH3CH2OH ⇨ CH3CHO + H2
Addition of hydrogen
H2+Cl2 ⇨ 2HCl
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3. Loss of electron
Mg ⇨ Mg2+ + 2e-
2Cl- ⇨ Cl2 + 2e-
Addition of electron
S + 2e- ⇨ S2-
Fe3+ + 3e- ⇨ Fe
4. Increase of positive charge Decrease of positive charge
5. decrease in negative charge Increase in negative charge
Redox Reaction:- If in a chemical reaction oxidation of one substance and reduction of another
substance takes place simultaneously then such reactions is called Redox reaction.
oxidation
CuSO4 + Fe −−−−−→ FeSO4 + Cu
Reduction
In the word REDOX, ‘Red’ stands for reduction and ‘Ox’ stands for oxidation.
In this reaction CuO is reduced to Cu and H2 is oxidized to H2O. So, oxidation and reduction taking place
together is redox reaction.
Effects of Oxidation in Daily Life
1) Corrosion
When a metal is exposed to substances such as moisture, acid etc. for some time, a layer of
hydrated oxide is formed which weakens the metal and hence metal is said to be corrode
Rusting of iron, black coating on silver and green coating on copper is example of corrosion.
Corrosion can be prevented by galvanization, electroplating or painting.
2) Rancidity :
The oxidation of fats and oils when exposed to air is known as rancidity. It leads to bad smell and bad taste
of food.
Methods to Prevent Rancidity
i. By adding antioxidants
ii. Keeping food in air tight containers
iii. Replacing air by nitrogen
iv. Refrigeration
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QUESTIONS
VERY SHORT QUESTIONS (1 Mark)
1. What changes do you observe in the iron nails and colour of copper sulphate solution, if iron nails
are dipped in CuSO4 solution for 15 minutes ?
2. Identify the chemical change : Melting of ice or conversion of milk into curd.
3. Why is respiration considered an exothermic reaction ?
4. Why do copper vessel lose shine when exposed to air ?
5. Potato chips manufacturers fill the packet of chips with nitrogen gas. Why ?
6. Why we store silver chloride in dark coloured bottles ?
7. Write a chemical equation of double displacement reaction.
8. N2 + 3H2 → 2NH3 name the type of reaction.
9. What happens when milk is left open at room temperature during summers ?
10. What happens when quick lime is added to water ?
SHORT TYPE QUESTIONS (2 Marks)
1. Define combination reaction. Give one example of combination reaction which is exothermic in
nature.
2. What is decomposition reaction ? Explain with the help of an example.
3. Name and state the law which is kept in mind when we balance a chemical equation.
4. Give one example of each :
a) Chemical reaction showing evolution of gas.
b) Change in substance’s colour during a chemical reaction.
5. What is rancidity ? Write two ways by which it can be prevented.
6. What are two conditions which promotes corrosion ?
7. A small amount of ferrous sulphate is heated in hard glass tube.
a) Write the chemical equation.
b) What type of reaction is taking place.
8. What happens when Zn strip is dipped in CuSO4 solution ?
SHORT TYPE QUESTIONS (3 Marks)
1. What is redox reaction ? Write down a chemical reaction representing it.
2. In electrolysis of water :
a) Name the gas collected at cathode and anode.
b) Why is volume of one gas collected at one electrode is double of another ?
c) Why are few drops of dil. H2SO4 added to water ?
3. In the reaction CuO (s) + H2 (g) → Cu(s) + H2O (g)
a) Name the oxidized substance.
b) Name the reduced substance.
c) Name the oxidizing agent.
4. Give reasons :
a) White silver chloride turns grey in sunlight.
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b) Brown coloured copper powder on heating in air turns into black coloured substance.
5. Compound ‘X’ decomposes to form compound ‘Y’ and CO2 gas. Compound Y is used in
manufacturing of cement.
a) Name the compounds ‘X’ and ‘Y’.
b) Write the chemical equation for this reaction.
6. A metal salt MX when exposed to light splits upto to form metal M and gas X2. Metal M is used to
make ornaments whereas gas X2 is used in making bleaching powder. The salt MX is used in black &
white photography.
a) Identify the metal M and gas X2.
b) Identify MX.
c) Write down the chemical reaction when salt MX is exposed to sunlight.
7. A metal strip X is dipped in blue coloured salt solution YSO4. After some time a layer of metal ‘Y’ is
formed on metal strip X. Metal X is used in galvanization whereas metal Y is used for making
electric wires.
a) What could be metal ‘X’ and ‘Y’ ?
b) Name the metal salt YSO4.
c) What type of chemical reaction takes place between X and YSO4 ? Write the balanced
chemical equation.
LONG TYPE QUESTIONS (5 Marks)
1. White wash was being done at Mukesh’s house. Mukesh saw that the painter added quick lime to
drum having water. Mukesh touched outer surface of drum, it is unbelievably hot.
a) Write the chemical equation for above reaction.
b) What type of reaction is it ?
c) What is utility of this reaction ?
2. What types of reactions are represented by following :
a) CaCO3 → CaO + CO2
b) 2Ca + O2 → 2CaO
c) Pb + CuCl2 → PbCl2 + Cu
d) 2FeSO4 → Fe2O3 + SO2 + SO3
e) Na2SO4 + BaCl2 → BaCl2 + 2NaCl
3. Balance the following equations :
a) H2 + O2 → H2O
b) MnO2 + HCl → MnCl2 + H2O + Cl2
c) Pb(NO3) → PbO + NO2 + O2
d) AgNO3 + NaCl → AgCl + NaNO3
e) Ca(OH)2 + HNO3 → Ca(NO3)2 + H2O
4. Write down the balanced chemical equation for the following :
a) Silver chloride is decomposed in presence of sunlight to give silver and chlorine gas.
b) Calcium oxide reacts with water to give lime water.
c) Sodium hydroxide reacts with hydrochloric acid to give sodium chloride and water.