Chemical Reaction And Equation

A chemical reaction takes place when different substances react and a chemical change occurs.

Examples:

• Food gets digested in our body.
• Iron articles undergo rusting.
• Milk changes into curd.

In these examples, substances undergo permanent change and new substances are formed.

Chemical Reaction

A chemical reaction is a process that transforms one or more substances into new substances.

General form:

A + B → C

Here:

• A and B are reacting species.
• C is the new substance formed.

Reactant

A reactant is a substance that takes part in a chemical reaction and undergoes change.

Example:

2Mg + O₂ → 2MgO

Here, Mg and O₂ are reactants.

Product

A product is a new chemical substance formed as a result of reaction between reactants.

In the reaction:

2Mg + O₂ → 2MgO

MgO is the product.

Characteristics Of Chemical Reactions

Chemical reactions are often accompanied by observable changes.

The main characteristics are:

• Change in state
• Change in colour
• Evolution of gas
• Change in temperature
• Formation of precipitate

Change In State

When candle wax burns, it changes from solid wax to melted liquid wax and then gaseous products.

This shows that a chemical reaction may involve change in state.

Change In Colour

The reaction between citric acid and purple-coloured potassium permanganate solution shows a colour change from purple to colourless.

Evolution Of Gas

When zinc granules react with dilute sulphuric acid, hydrogen gas is evolved.

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑

Change In Temperature

Some chemical reactions are accompanied by a change in temperature.

Example:

Quicklime reacts with water to form slaked lime and releases heat.

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

When barium hydroxide is added to ammonium chloride, the temperature of the reaction mixture decreases and the mixture becomes cold.

Formation Of Precipitate

When aqueous lead nitrate reacts with potassium iodide, a yellow precipitate of lead iodide is formed.

Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

Here, PbI₂ is the yellow precipitate.

Chemical Equation

A chemical equation is a way of representing a chemical reaction in words or symbols.

Example:

Word equation:
Magnesium + Oxygen → Magnesium oxide

Symbol equation:
2Mg + O₂ → 2MgO

All necessary conditions such as temperature, pressure or catalyst are written on the arrow between reactants and products.

Steps For Writing A Chemical Equation

• Reactants are written on the left-hand side.
• Products are written on the right-hand side.
• A plus sign is written between two or more reactants or products.
• An arrow is placed between reactants and products.

Example:

2Mg + O₂ → 2MgO

Balanced Chemical Equation

A chemical equation is balanced when the number of atoms of each element is equal on both sides.

Balancing is necessary because of the Law of Conservation of Mass.

Example:

Unbalanced equation:
Mg₃N₂ + H₂O → Mg(OH)₂ + NH₃

Balanced equation:
Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃

Types Of Chemical Reactions

Chemical reactions are classified into different types based on rearrangement of atoms.

Important types are:

• Combination reaction
• Decomposition reaction
• Displacement reaction
• Double displacement reaction
• Precipitation reaction
• Oxidation reaction
• Reduction reaction
• Redox reaction

Combination Reaction

A combination reaction is a reaction in which two or more reactants combine to form a single product.

Examples:

2Mg(s) + O₂(g) → 2MgO(s)

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Here, calcium oxide is quicklime and calcium hydroxide is slaked lime.

Formation of water:

2H₂(g) + O₂(g) → 2H₂O(g)

Burning of coal:

C(s) + O₂(g) → CO₂(g)

Combination reactions usually release heat because bond formation takes place. Therefore, many combination reactions are exothermic reactions.

Types Of Combination Reactions

Combination reactions may occur in three ways:

• Combination of two elements to form a compound.
• Combination of two compounds to form a compound.
• Combination of an element and a compound to form a new compound.

Decomposition Reaction

A decomposition reaction is a reaction in which a compound splits into two or more substances.

Digestion of food in our body is also an example of decomposition.

Since decomposition reactions require energy to break down reactants, they are generally endothermic reactions.

Types Of Decomposition Reactions

Thermal Decomposition

Decomposition by heating is called thermal decomposition.

Example:

2Pb(NO₃)₂(s) → 2PbO + 4NO₂ + O₂(g)

Lead nitrate decomposes to form lead oxide, nitrogen dioxide and oxygen.

Electrolytic Decomposition

Decomposition by passing electricity is called electrolytic decomposition.

Example:

2H₂O → 2H₂ + O₂

Electrolysis of water gives hydrogen and oxygen gases.

Photolytic Decomposition

Decomposition in the presence of sunlight is called photolytic decomposition or photochemical decomposition.

Example:

2AgBr(s) → 2Ag(s) + Br₂(g)

Silver bromide decomposes into silver and bromine in sunlight.

Another example:

2HgO → 2Hg + O₂

Uses Of Decomposition Reactions

Decomposition reactions are used in metallurgical processes.

Examples:

• Extraction of zinc from zinc carbonate:
  ZnCO₃(s) → ZnO(s) + CO₂(g)
• Decomposition of molten aluminium oxide by electric current:
  2Al₂O₃ → 4Al + 3O₂
• Decomposition of molten sodium chloride by electricity:
  2NaCl → 2Na + Cl₂

Displacement Reaction

A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound.

Example:

Fe + CuSO₄ → Cu + FeSO₄

Here, iron displaces copper from copper sulphate solution.

Another example:

Zn + 2HCl → ZnCl₂ + H₂

Zinc displaces hydrogen from hydrochloric acid.

Double Displacement Reaction

A double displacement reaction occurs when two compounds exchange ions to form two new compounds.

Example:

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

Here, barium sulphate is formed as an insoluble white precipitate.

Another example:

Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃

Here, yellow precipitate of lead iodide is formed.

Precipitation Reaction

A reaction in which an insoluble solid is formed is called a precipitation reaction.

Example:

BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl

The insoluble solid BaSO₄ is the precipitate.

Neutralisation Reaction

A neutralisation reaction occurs when an acid reacts with a base to form salt and water.

Example:

HNO₃ + KOH → KNO₃ + H₂O

This reaction is both neutralisation and double displacement.

Exothermic Reaction

An exothermic reaction is a reaction in which heat is released.

Examples:

• Burning of coal
• Reaction of quicklime with water
• Respiration

Reaction of quicklime with water:

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat

Endothermic Reaction

An endothermic reaction is a reaction in which heat is absorbed.

Examples:

• Decomposition reactions
• Sublimation of dry ice
• Evaporation of water

Oxidation

Oxidation may involve:

• Addition of oxygen
• Removal of hydrogen
• Increase in oxidation number
• Addition of a more electronegative element
• Loss of electrons

Examples:

2Mg + O₂ → 2MgO

Here, magnesium gains oxygen.

2H₂S + O₂ → 2H₂O + 2S

Here, hydrogen is removed from hydrogen sulphide.

Fe²⁺ → Fe³⁺ + e⁻

Here, iron loses an electron and oxidation number increases.

Reduction

Reduction may involve:

• Addition of hydrogen
• Removal of oxygen
• Decrease in oxidation number
• Addition of a more electropositive element
• Gain of electrons

Examples:

H₂ + S → H₂S

Here, hydrogen is added.

ZnO + C → Zn + CO

Here, oxygen is removed from zinc oxide.

Fe³⁺ + e⁻ → Fe²⁺

Here, iron gains an electron.

Redox Reaction

When oxidation and reduction occur simultaneously, the reaction is called a redox reaction.

Example:

ZnO + C → Zn + CO

Here:

• ZnO is reduced to Zn.
• Carbon is oxidised to CO.

Another example:

SO₂ + 2H₂S → 2H₂O + 3S

Here:

• SO₂ is reduced.
• H₂S is oxidised.

Corrosion

Corrosion is the slow damage of metals due to reaction with substances present in the environment such as air, moisture and chemicals.

Rusting of iron is a common example of corrosion.

During rusting, iron reacts with oxygen and moisture to form hydrated iron oxide.

Corrosion damages metal articles and weakens them.

Rancidity

Rancidity occurs when fats and oils are oxidised.

It produces an unpleasant smell and taste in food.

Rancidity can be reduced by:

• Keeping food in airtight containers.
• Using antioxidants.
• Flushing packets with nitrogen gas.
• Keeping food away from air, light and heat.

Nitrogen or helium can be used for storing fresh oil samples for a long time because they do not support oxidation easily.

Important Chemical Reactions From The Chapter

Reaction	Type
2Mg + O₂ → 2MgO	Combination
CaO + H₂O → Ca(OH)₂	Combination, exothermic
C + O₂ → CO₂	Combination, combustion
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂	Thermal decomposition
2H₂O → 2H₂ + O₂	Electrolytic decomposition
2AgBr → 2Ag + Br₂	Photolytic decomposition
Fe + CuSO₄ → Cu + FeSO₄	Displacement
Zn + 2HCl → ZnCl₂ + H₂	Displacement
BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl	Double displacement, precipitation
Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃	Double displacement, precipitation
HNO₃ + KOH → KNO₃ + H₂O	Neutralisation, double displacement

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