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Chemical Reactions & Equations
CBSE Board
Science , Class -10
Chemical Reactions
A chemical reaction occurs when one or more reactants (also known as reactants) are changed into one or mor
e products (also known as products)
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Physical and Chemical changes
Chemical change – one or more new substances with new physical and chemical properties are formed
...
e
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Physical change – change in colour or state occurs but no new substance is formed
...
This involves only a change in state (liquid to vapour)
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A chemical reaction is a process that causes one set of chemical components to change into another
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At a given
temperature and chemical concentration, chemical reactions occur at a predictable rate
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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
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The
names of the reactants are shown on the left side of a word equation
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Products are shown on the right side of a word
equation
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For example,
Sodium + Chlorine → Sodium chloride
The above equation means: “Sodium reacts with chlorine to form sodium chloride
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Each element has one or two-letter atomic symbol, which is the
abbreviated form of its name
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It can be considered as the number of electrons lost, gain or
shared by an atom when it combines with another atom to form a molecule
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𝑍𝑛(𝑠) + 𝑑𝑖𝑙
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• For liquids, it is “(l)”
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• For aqueous solutions, it is “(aq)”
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• For precipitate formed 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
The changes that occur during a chemical reaction are represented by a chemical equation
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The equilibrium of all chemical equations must be maintained
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Chemical equations are balanced using coefficients
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It indicates the number of atoms or molecules of the material involved in the
process
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For example,
Zn + HCl → ZnCl2 + H2
Balanced Equation is
Zn + 2HCl → ZnCl2 + H2
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 equations
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 → 𝑁𝐻4 𝐶𝑙
compound + compound → compound
When quicklime or calcium oxide (CaO) reacts with water, slaked lime [Ca(OH)2] is formed
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So, this reaction is an exothermic Reaction
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Types of decomposition reactions:
a
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Thermal decomposition of HgO
b
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Photolytic decomposition of H2O2
c
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Electrolytic decomposition of H2O

Displacement reaction
More reactive element displaces a less reactive element from its compound or solution
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Example - 𝐴𝑙2 (𝑆𝑂2 )3 (𝑎𝑞) + 3𝐶𝑎(𝑂𝐻)2 (𝑎𝑞) → 2𝐴𝑙(𝑂𝐻)3 (𝑎𝑞) + 3𝐶𝑎𝑆𝑂4 (𝑠)
An insoluble compound called precipitate forms when two solutions containing soluble salts are combined
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This is the insoluble salt formed as a product of
the precipitation reaction
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AgNO3(aqueous) + KCl(aqueous) —–AgCl(precipitate) + KNO3(aqueous)

Redox reaction
A redox reaction occurs when the oxidation states of the substrate change
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The gain of electrons or a decrease
in the oxidation state of a chemical or its atoms is referred to as reduction
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Oxidation: Substance loses electrons or gains oxygen or loses hydrogen
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Oxidising agent – a substance that oxidises another substance and self-gets reduced
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Examples :-

1
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𝐶𝑢+2 + 2𝑒− → 𝐶𝑢(𝑠)(𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛); 𝐶𝑢 − 𝑜𝑥𝑖𝑑𝑖𝑠𝑖𝑛𝑔 𝑎𝑔𝑒𝑛𝑡
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 the 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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Carbon reacts with Oxygen (O2 ) to give carbon dioxide
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2
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It is a decomposition reaction that occurs in the presence of sunlight and hence it is a photochemical
decomposition reaction
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In electrolysis of water (acidified), the name the gasses that are evolved at anode and
cathode respectively?
In electrolysis of water (acidified), the gases that are evolved at anode and cathode respectively are
oxygen and hydrogen
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