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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
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e
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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
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It helps
identify the reactants and products in a chemical reaction
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"

Symbols of elements and their valencies
A symbol is the chemical code for an element
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Valency is the combining capacity of an element
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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
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H2SO4(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 "(↑)"
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In other words, the total mass of the products
formed in a chemical reaction is equal to the total mass of the reactants participated in a chemical
reaction
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Steps for balancing chemical equations
Hit and trial method: While balancing the equation, change the coefficients (the numbers in front of
the compound or molecule) so that the number of atoms of each element is same on each side of
the chemical equation
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Ca: a=3c
C: a=d
O: 3a+4b=8c+3d
H: 3b=2d
P: b=2c
Let's set c=1
Then a=3 and
d=a=3
b=2c=2
So a=3; b=2; c=1; d=3
The balanced equation is
3CaCO3 + 2H3PO4 → Ca3(PO4)2 + 3H2CO3

Chemical Reactions and Equations II
Types of chemical reactions
Taking into consideration different factors, chemical reactions are grouped into multiple categories
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H2 + Cl2 → 2HCl
element + element → compound

2CO + O2 → 2CO2
compound + element → compound
NH3 + HCl → NH4Cl
compound + compound → compound

Decomposition reaction
A single reactant decomposes on the application of heat or light or electricity to give two or more
products
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Decomposition reactions which require heat - thermolytic decomposition or thermolysis
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Decomposition reactions which require light - photolytic decomposition or photolysis
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Decomposition reactions which require electricity - electrolytic decomposition or electrolysis
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i)
ii)

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

Double displacement reaction
An exchange of ions between the reactants takes place to give new products
...

For example, P b(NO3)2(aq) + 2KI(aq) → 2KNO3(aq) + P bI2(↓)(s)(yellow)

Redox reaction
Oxidation and reduction take place simultaneously
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Reduction: Substance gains electrons or loses oxygen or gains hydrogen
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Reducing agent - a substance that reduces another substance and self-gets oxidised
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Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
(Blue)
(Green)
Fe → Fe+2 + 2e − (oxidation) ; Fe - reducing agent
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2
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Most of the combination reactions are
exothermic
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6CO2 + 6H2O + Sunlight → C6H12O6 + 6O2
Glucose
Most of the decomposition reactions are endothermic
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Rusting:
4Fe(s) + 3O2(from air) + xH2O(moisture) → 2Fe2O3
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Cu(OH)2(green)
Corrosion of silver:
Ag(s) + H2S(from air) → Ag2S(black) + H2(g)

Rancidity
It refers to oxidation of fats and oils in food that is kept for a long time
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Rancid food causes stomach infection on consumption
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BY- AARUSHI SHARMA



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