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Chemical Reactions and Equations
Introduction to Chemical Reactions and Equations

Physical and chemical changes
Chemical change - one or more new substances with new physical and chemical properties
are formed
...
e
...

4

4

Physical change - change in colour or state occurs but no new substance is formed
...
This involves only change in state (liquid
to vapour)
...


Types of chemical reactions

Taking into consideration different factors, chemical reactions are grouped into multiple
categories
...
 It helps identify the reactants and products in a chemical reaction
...
" 

Symbols of elements and their valencies
A symbol is the chemical code for an element
...

Valency is the combining capacity of an element
...


Writing chemical equations
Representation of a chemical reaction in terms of symbols and chemical formulae of the
reactants and products is known as a chemical equation
...
H2 SO4 (aq) → ZnSO4 (aq) + H2 (↑)

 (Reactants) 
 (Products)
• For solids, the symbol is "(s)"
...

• For gases, it is "(g)"
...

• For gas produced in the reaction, it is represented by "(↑)"
...


Balancing of a Chemical Reaction
Conservation of mass
According to the law of conservation of mass, no atoms can be created or destroyed in a
chemical reaction, so the number of atoms for each element in the reactants side has to
balance the number of atoms that are present in the products side
...


Balanced chemical equation
The chemical equation in which the number of atoms of each element in the reactants side
is equal to that of the products side is called a balanced chemical equation
...
 

Short-cut technique for balancing a chemical equation
Example:
aCaCO3 + bH3 P O4 → cCa3 (P O4 )2 + dH2 CO3

Set up a series of simultaneous equations, one for each element
...

Few examples are:
● Combination
● Decomposition
● Single Displacement
● Double displacement
● Redox
● Endothermic
● Exothermic
● Precipitation
● Neutralisation

Combination reaction
In a combination reaction, two elements or one element and one compound or two
compounds combine to give one single product
...

Types of decomposition reactions:
a
...


Thermal decomposition of HgO

b
...


Photolytic decomposition of H

2 O2

c
...


Electrolytic decomposition of H

2O

Displacement reaction
More reactive element displaces a less reactive element from its compound or solution
...

For example, Al (SO ) (aq) + 3Ca(OH ) (aq) → 2Al(OH ) (aq) + 3CaSO (s)
2

4

3

2

3

4

Precipitation reaction
An insoluble compound called precipitate forms when two solutions containing soluble salts
are combined
...

Oxidation: Substance loses electrons or gains oxygen or loses hydrogen
...

Oxidising agent - a substance that oxidises another substance and self-gets reduced
...

Examples:
1
...

+ 2e− → Cu(s) (reduction) ; Cu - oxidising agent
...
ZnO + C → Zn + CO
ZnO reduces to Zn → reduction
C oxidises to CO → oxidation
ZnO - Oxidising agent
C - Reducing agent

Endothermic and exothermic reaction
Exothermic reaction - heat is evolved during a reaction
...

Al + F e2 O3 → Al2 O3 + F e + heat
CH4 + 2O2 → CO2 + 2H2 O + heat

Endothermic - Heat is required to carry out the reaction
...


Corrosion
Gradual deterioration of a material, usually a metal, by the action of moisture, air or
chemicals in the surrounding environment
...
xH2 O(rust)

Corrosion of copper:
Cu(s) + H2 O(moisture) + CO2 (f rom air) → CuCO3
...
It gives foul smell
and bad taste to food
...

Prevention:
(i) Use of air-tight containers

(ii) Packaging with nitrogen
(iii) Refrigeration
(iv) Addition of antioxidants or preservatives



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