Introduction:
Chemical reactions are an important part of our daily life. In this chapter, we will learn about chemical reactions, how to write and balance chemical equations, and the different types of reactions.
What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different properties.
How do We Know a Reaction has Occurred??
A chemical reaction has occurred if one or more of the following changes are observed:
• Change in state (solid, liquid, gas)
• Change in colour
• Evolution of gas (bubbles form)
• Change in temperature (heat or light produced/absorbed)
• Formation of a precipitate
Chemical Equations – Representing Reactions
A chemical equation is a symbolic representation of a chemical reaction using symbols and formulas of substances.
Types of Chemical Equations
Word Equation :- Written in words
Example: Magnesium + Oxygen → Magnesium oxide
Skeletal Equation – Unbalanced equation.
Example: Mg + O₂ → MgO
Balanced Equation :- atoms are equal on both sides.
Example: 2Mg + O₂ → 2MgO
Parts of a Chemical Equation :-
Reactants :- The substance which take part in chemical reaction are called Reactants.
Reactant :- on the left-hand side.
Products :- The substance which are formed in a chemical reaction are Called Products.
Product :- On the right-hand side
Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide(MgO)
(Reactant) → (Product)
Steps to Balance a Chemical Equation :-
(i) Write the unbalanced equation
Example: H₂ + O₂ → H₂O
(ii) Count the atoms of each element on both sides.
| Element | Left | Right |
|---|---|---|
| H | 2 | 2 |
| O | 2 | 1 |
(iii) Balance one element at a time –
Start with metals → non-metals → hydrogen/oxygen (usually last).
Add coefficients (numbers in front), not subscripts.
Example:
H₂ + O₂ → 2H₂O
(iv) Recount atoms -
Check again after each change.
| Element | Left | Right |
|---|---|---|
| H | 2 | 4 |
| O | 2 | 2 |
Hydrogen is now unbalanced.
(v) Adjust coefficients again Fix hydrogen:
2H₂ + O₂ → 2H₂O
(vi) Final check
| Element | Left | Right |
|---|---|---|
| H | 4 | 4 |
| O | 2 | 2 |
Now the equation is Balanced.
TYPES OF CHEMICAL REACTIONS.
(i) COMBINATION REACTION :- The reaction in which two or more reactant combine to form a single product.
OR
It is a reaction in which a single product is formed from two or more reactants is known as a Combination reaction.
Example
(a) Burning of Coal
C (s) + O2 (g) → CO2 (g) + Heat
(b) Formation of water
2H2 (g) + O2 (g) → 2H2O (l) + Heat
(C) Formation of slaked lime
Calcium oxide (CaO) reacts vigorously with water to produce slaked lime (calcium hydroxide- Ca(OH)₂) releasing a large amount of heat.
CaO (s) + H2O (l) → Ca(OH)2(aq) + Heat
• Slaked lime is used for white washing walls.
• Calcium hydroxide reacts slowly with the Carbon dioxide in air to form a thin layer of Calcium carbonate on the walls.
Ca(OH)2(aq) + CO2 (g) → CaCO3(S) + H2O
Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls.
Chemical formula for :-
• Quick lime :- Calcium oxide (CaO)
• Slaked lime :- Calcium hydroxide- Ca(OH)₂
• Marble :- Calcium Carbonate - CaCO₃
Exothermic Reactions:-
Reaction in which heat is released along with formation of products.
e.g.:-
(i) Burning of natural gas:-
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g) + Heat
ii) Respiration is also an exothermic reaction:-
C6H12O6 (aq) + 6O2 (g) → 6CO2 + 6H2O(l) + Energy
(2) DECOMPOSITION REACTION :- The reaction in which a single compound splits into two or more simple substances is called decomposition reaction.
A → B + C
Thermal decomposition:- When decomposition is carried out by heating.
Eg.:-
(i) 2FeSO4 (s) Heat → Fe2O3 (s) + SO2 (g) + SO3 (g)
FeSO₄ (s): Ferrous sulphate (Green colour).
Fe₂O₃ (s): Ferric oxide (Red-brown colour).
SO₂ (s): Sulphur dioxide.
SO₃ (s): Sulphur trioxide.
(ii) 2Pb(NO3) 2(s) Heat → 2PbO (s) + 4NO2 (g) + O2 (g)
2Pb(NO₃)₂ (s) : Lead nitrate
2PbO(s) : Lead oxide.
4NO2(g) : Nitrogen dioxide.
Electrolytic decomposition:- When decomposition is carried out by passing electricity.
Eg.:-
2H2O (l) Electric current → 2H2 (g) + O2 (g)
Photolytic decomposition :- When decomposition is carried out in presence of sunlight.
Eg.:-
2AgCl (s) sunlight → 2Ag (s) + Cl2 (g)
2AgBr (s) sunlight → 2Ag (s) + Br2 (g)
Endothermic Reactions :- The reactions which require energy in the form of heat or light to break reactants are called endothermic reactions.
(3) DISPLACEMENT REACTION :- The chemical reaction in which more reactive element displaces less reactive element from its salt solution.
Fe (s) + CuSO4(aq) → FeSO4 (aq) + Cu(s)
• Iron is more reactive element than copper. They displace copper from its compounds.
Iron - Fe (s) : Greyish Black / Grey
Copper Sulphate:- CuSO4 (s) : Dark Blue.
Iron Sulphate :- FeSO4 : Pale green.
Copper:- Cu : Red-Brown.
Zn(s) + CuSO4(aq) → ZnSO4 (aq) + Cu(s)
Pb (s) + CuCl2(aq) → PbCl2 (aq) + Cu(s)
• Zinc and lead are more reactive elements than copper. They displace copper from its compounds.
(4) DOUBLE DISPLACEMENT REACTION :- A reaction in which new compounds are formed by mutual exchange of ions between two compounds.
Na2SO4(aq) + BaCl2(aq) → BaSO4 (s) + 2NaCl(aq)
Na2SO4 (aq) : Sodium Sulphate.
BaCl2 (aq) : Barium Chloride.
BaSO4 (s) : Barium Sulphate.(White ppt)
2NaCl (aq) Sodium Chloride.
During reaction a white substance (BaSO₄) Which is insoluble in water, is formed is called precipitate and reaction is called a precipitation reaction.
• More Example of Double Displacement Reaction :-
AgNO3(aq) + NaCl(aq) → AgCl (s) + NaNO3(aq)
AgNO3(aq) : Silver nitrate.
NaCl (aq) : Sodium Chloride.
AgCl (s) : Silver Chloride.(White ppt)
NaNO3(aq) Sodium nitrate.
CuSO4(aq) + H2S (g) → CuS (s) + H2SO4(aq)
CuSO4(aq) : Copper sulphate.
H2S (g) : Hydrogen sulphide.
CuS (s) : Copper sulphide.(Black ppt)
H2SO4(aq) Sulphuric acid.
2KI + (PbNO3)2 (aq) → PbI2 (s) + 2KNO3(aq)
2KI(aq) : Potassium Iodide.
(PbNO3)2 (aq) : Lead Nitrate.
PbI2 (s) : Lead Iodide.(Yellow ppt)
2KNO3(aq) Potassium nitrate.
• All Double Displacement reactions are not Precipitation reactions.
OXIDATION AND REDUCTION :-
OXIDATION :- (i) The addition of oxygen to substance.
OR
(ii) The removal of hydrogen from a substance.
2Cu + O 2 heat → 2CuO
• Cu is Oxidized to CuO
• If a substance gains oxygen during a reaction, it is said to be Oxidised.
• The Substance which either gives Oxygen or removes hydrogen in an oxidation reaction is known as Oxidising agent.
Reduction :- (i) The addition of hydrogen to substance.
OR
(ii) The removal of oxygen from a substance.
2Cu + H2 heat → Cu + H2O.
• Cu is reduced to Cu.
• If a substance loses oxygen during a reaction, it is said to be reduced.
• The Substance which either gives hydrogen or removes Oxygen in a reduction reaction is known as reducing agent.
Oxidation and Reduction Reaction
In this reaction, one substance gets oxidised while the other gets reduced.
Explanation:
CuO + H₂ → Cu + H₂O
• CuO loses oxygen → Reduction
• H₂ gains oxygen → Oxidation
• Hydrogen ( H₂) is a reducing agent as it undergo Oxidation.
• CuO is an Oxidising agent as undergo reduction.
Explanation:
ZnO + C → Zn + CO (on heating)
• ZnO loses oxygen → Reduction
• Carbon gains oxygen → Oxidation
• Carbon (C) acts as a Reducing Agent
• ZnO acts as an Oxidising Agent
Explanation:
MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O
• MnO₂ loses oxygen → Reduction
• HCl (Cl⁻) loses electrons and forms Cl₂ → Oxidation
• MnO₂ acts as an Oxidising Agent
• HCl acts as a Reducing Agent
Explanation:
Fe₂O₃ + 3CO → 2Fe + 3CO₂
• Fe₂O₃ loses oxygen → Reduction
• CO gains oxygen and forms CO₂ → Oxidation
• Fe₂O₃ acts as an Oxidising Agent
• CO acts as a Reducing Agent
Effects of Oxidation Reaction in Daily Life :-
• Corrosion :- The Process of Slow and gradual Conversion of metals into their undesirable compounds due to their reaction with air , moisture(water) present in the atmosphere is called Corrosion.
Examples of corrosion :- Rusting of iron, black coating on silver and green coating on copper.
Rusting of Iron
• Rusting is the process in which iron reacts with oxygen and moisture (water) to form a reddish-brown substance called rust (iron oxide).
Chemical Reaction:
4Fe + 3O2 + xH2O → 2Fe2O3·xH2O
Conditions Required for Rusting:
• Presence of oxygen (air)
• Presence of water (moisture)
Prevention of Rusting:
• Painting
• Oiling or greasing
• Galvanization (coating with zinc)
• Alloy formation (e.g., stainless steel)
Corrosion of Copper:
• Copper objects lose their lustre and shine after some time because their surface acquires a green coating of basic copper carbonate, CuCO₃·Cu(OH)₂, when exposed to air.
Chemical Reaction:
2Cu(s) + CO₂(g) + O₂(g) + H₂O(l) → CuCO₃·Cu(OH)₂(s)
Copper + Moist Air → Basic Copper Carbonate (Green layer)
Explanation:
Copper reacts with moist air (O₂, CO₂, and H₂O) to form a green coating of basic copper carbonate
(CuCO₃·Cu(OH)₂) on its surface. This process is known as corrosion of copper.
Corrosion of Silver Metal:
• The surface of silver metal gets tarnished (becomes dull) on exposure to air due to the formation of a black coating of silver sulphide Ag₂S by the action of H₂S gas present in the air.
Chemical Reaction:
2Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g)
Silver + Hydrogen Sulphide → Silver Sulphide (Black)
Rancidity:
• The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is called rancidity. It is caused due to the oxidation of fat and oil present in food materials.
Methods to prevent rancidity:
(i) By adding antioxidants
(ii) Keeping food in air tight containers
(iii) Replacing air by nitrogen
(iv) Refrigeration
Conclusion:
In this chapter, we learned that chemical reactions are processes in which substances change into new substances with different properties. We studied how to represent these reactions using chemical equations and the importance of balancing them according to the law of conservation of mass. We also explored different types of chemical reactions such as combination, decomposition, displacement, and redox reactions. These concepts help us understand many natural and everyday processes in a scientific way and form the foundation for further study in chemistry.
Frequently Asked Questions (FAQ)
Q1. What is a chemical reaction?
Ans. A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different properties.
Q2. What is a balanced chemical equation?
Ans. A balanced chemical equation is one in which the number of atoms of each element is equal on both sides of the equation showing that mass is conserved.
Q3. What is a combination reaction?
Ans. The reaction in which two or more reactants combine to form a single product.
Q4. What is a decomposition reaction?
Ans. A decomposition reaction is a reaction in which a single compound breaks down into two or more simpler substances using heat, light, or electricity.
Q5. What is a displacement reaction?
Ans. A displacement reaction is a reaction in which a more reactive element replaces a less reactive element from its compound.
Q6. What is rancidity?
Ans. Rancidity is the process in which fats and oils in food get oxidised, resulting in a bad smell and unpleasant taste.
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