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CLASS 10 CHEMISTRY NOTES
T
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For example: Rusting of iron, the setting of milk into curd,
digestion of food, respiration, etc
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Only a rearrangement of atoms takes place in a chemical reaction
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The new substances produced as a result of a chemical reaction are called
products
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2Mg(s) + O2(g) △→ 2MgO(s)
Before burning in air, the magnesium ribbon is cleaned by rubbing with sandpaper
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Chemical Reactions and Equations: Balanced and unbalanced chemical equations and
balancing of chemical equations
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Example: Mg and O2
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Example: MgO
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Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) ↑
(ii) Change in Colour: The chemical reaction between citric acid and purple coloured
potassium permanganate solution is characterised by a change in colour from purple
to colourless
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(iii) Change in state of substance: The combustion reaction of candle wax is
characterised by a change in state from solid to liquid and gas (because the wax is a
solid, water formed by the combustion of wax is a liquid at room temperature whereas,
carbon dioxide produced by the combustion of wax is a gas)
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(iv)Change in temperature: The chemical reaction between quick lime water to form
slaked lime is characterized by a change in temperature (which is a rise in
temperature)
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(v) Formation of precipitate: The chemical reaction between sulphuric acid and barium
chloride solution is characterised by the formation of a white precipitate of barium
sulphate
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Example: A + B → C + D
In this equation, A and B are called reactants and C and D are called the products
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Condition, if any, is written generally above the
arrow
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This reaction can be represented by
the following chemical equation:
Hydrogen + Oxygen → Water
H 2 + O2 → H 2 O
In the first equation, words are used and in second, symbols of substances are used to
write the chemical equation
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A chemical equation is a way to represent the chemical reaction in a concise and
informative way
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Balanced Chemical Equation: A balanced chemical equation has the number
of atoms of each element equal on both sides
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According to the Law of Conservation of Mass, mass can neither be created nor
destroyed in a chemical reaction
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Balanced Chemical Equation depends on conservation of mass which
is very important
Every equation should be made on the bases of law of conservation of
mass
b) Unbalanced Chemical Equation: If the number of atoms of each element
in reactants is not equal to the number of atoms of each element present in the product, then
the chemical equation is called Unbalanced Chemical Equation
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example: on the left-hand side only one iron atom is present, while three iron atoms are
present on the right-hand side
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Balancing a Chemical Equation: To balance the given or any chemical
equation, follow these steps:
Fe + H2O → Fe3O4 + H2
Write the number of atoms of elements present in reactants and in products in a table as
shown here
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of atoms in reactant
No
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In this equation, the number of oxygen atom is the maximum on the RHS
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Fe + 4 × H2O → Fe3O4 + H2
Now, the number of hydrogen atoms becomes 8 on the LHS, which is more than that
on the RHS
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Fe + 4 × H2O → Fe3O4 + 4 × H2
After that, the number of oxygen and hydrogen atoms becomes equal on both sides
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To balance it,
multiply the iron on the LHS by 3
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Thus, this
equation becomes a balanced equation
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of atoms in the reactant
No
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To Make Equations More Informative:
Writing the symbols of physical states of substances in a chemical equation:
By writing the physical states of substances, a chemical equation becomes more
informative
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Liquid state is represented by symbol (l)
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Aqueous solution is written by symbol (aq)
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Thus, by writing the symbols of the physical state of substances and condition under
which reaction takes place, a chemical equation can be made more informative
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Types of Chemical Reactions:
Chemical reactions can be classified in following types:
(i) Combination Reaction: Reactions in which two or more reactants combine to
form one product are called Combination Reactions
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In this
reaction, magnesium is combined with oxygen
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In this reaction, carbon
is combined with oxygen
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A
decomposition reaction is just the opposite of combination reaction
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CaCO3(s) heat CaO(s) + CO2(g)
Calcium carbonate → Calcium oxide + Carbon dioxide
When ferric hydroxide is heated, it decomposes into ferric oxide and water
2Fe(OH)3(s) △→ Fe2O3(s) + 3H2O(l)
Thermal Decomposition: The decomposition of a substance on heating is known
as Thermal Decomposition
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This is also known as Electrolysis
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2H2O(l) 2H2(g) + O2(g)
Photolysis or Photo Decomposition Reaction: Reactions in which a
compound decomposes because of sunlight are known as Photolysis or Photo
Decomposition Reaction
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2AgCl(s) (white) Sunlight 2Ag(s) (grey) + Cl2(g)
Photographic paper has a coat of silver chloride, which turns into grey when exposed
to sunlight
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(iii) Displacement Reaction: The chemical reactions in which a more reactive
element displaces a less reactive element from a compound is known as Displacement
Reactions
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A general displacement reaction can be represented by using a chemical equation as
follows
A + BC → AC + B
Displacement reaction takes place only when ‘A’ is more reactive than B
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Examples:
When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride
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Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
(iv) Double Displacement Reaction: Reactions in which ions are exchanged
between two reactants forming new compounds are called Double Displacement
Reactions
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BaCl2(aq) + Na2SO4(aq) → BaSO4(s) (Precipitate) + 2NaCl(aq)
When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and
water are formed
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Neutralisation reactions are also examples of double
displacement reaction
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Example:
Neutralization Reaction: The reaction in which an acid reacts with a base to form
salt and water by an exchange of ions is called Neutralization Reaction
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one of the reason of causing
indigestion is when our stomach produces more amount of HCL acid when we use
antacid Neutralization Reaction
Antacid example: milk of magnesium , etc
(v) Oxidation and Reduction Reactions:
Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or
metallic element from a compound is known as Oxidation
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Reduction: Addition of hydrogen or metallic element or removal of oxygen or nonmetallic element from a compound is called Reduction
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Oxidation and Reduction take place together
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The substance which removes hydrogen is also called an Oxidizing agent
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The substance which removes oxygen is also called a Reducing agent
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When copper oxide is heated with hydrogen, then copper metal and hydrogen are
formed
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Oxygen is being removed from copper
oxide
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(ii) In this reaction, H2 is changing to H2O
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Addition
of oxygen to a substance is called Oxidation, so hydrogen is being oxidised to water
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The substance which gets reduced is the oxidizing agent
Exothermic and Endothermic Reactions:
Exothermic Reaction: Reaction which produces energy is called Exothermic Reaction
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Example:
Respiration is a decomposition reaction in which energy is released
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Endothermic Reaction: A chemical reaction in which heat energy is absorbed is called
Endothermic Reaction
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Effects of Oxidation Reactions in Everyday life: Corrosion and Rancidity
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present in the
atmosphere is called Corrosion
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Rusting: Iron when reacts with oxygen and moisture forms red substance which is
called Rust
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Corrosion (rusting) weakens the iron and steel objects and structures such as railings,
car bodies, bridges and ships etc
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Methods to Prevent Rusting
By painting
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By galvanisation
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Cu(OH)2 when exposed to air
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Rancidity: The taste and odour of food materials containing fat and oil changes
when they are left exposed to air for a long time
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It is caused
due to the oxidation of fat and oil present in food materials
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Vacuum packing
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Refrigeration of foodstuff
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Chemical Reaction: During chemical reactions, the chemical composition of
substances changes or new substances are formed
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Chemical Equation: Chemical reactions can be written in chemical equation form
which should always be balanced
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Types of Chemical Reactions:
Combination reaction: A single product is formed from two or more reactants
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Zn + CuSO4 → ZnSO4 + Cu
Double displacement reaction: Exchange of ions between reactants
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Thermal decomposition: 2Pb(NO2)2 → 2PbO + 4NO2 + O2
Electrolysis: 2H20 → 2H2 + O2
Photochemical reaction: 2AgBr → 2Ag + Br2
Displacement reaction: One element is displaced by another element
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AgNO3 + NaCl → AgCl + NaNO3
Redox reaction: Both oxidation and reduction take place simultaneously
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C + O2 → CO2 (g) + heat
Endothermic reaction: A chemical reaction in which heat energy is absorbed
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4
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5
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C is oxidized to CO—Oxidation
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Effects of Oxidation Reactions in Our Daily Life:
Corrosion: It is an undesirable change that occurs in metals when they are
attacked by moisture, air, acids and bases
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nH2O (Hydrated iron oxide)
Rancidity: Undesirable change that takes place in oil containing food
items due to the oxidation of fatty acids
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