CBSE Class 10 Science
Notes Chapter 1 Chemical Reactions and Equations
What is a chemical reaction
Class 10?
Chemical
Reaction: The transformation of chemical substance into another
chemical substance is known as Chemical Reaction. For example: Rusting of iron,
the setting of milk into curd, digestion of food, respiration, etc.
In a chemical reaction, a new substance is formed which is
completely different in properties from the original substance, so in a
chemical reaction, a chemical change takes place.
Only a rearrangement of atoms takes place in a chemical reaction.
·
The substances which take part in a chemical reaction are called
reactants.
·
The new substances produced as a result of a chemical reaction
are called products.
Example: The
burning of magnesium in the air to form magnesium oxide is an example of a
chemical reaction.
2Mg(s) + O2(g)
Before burning in air, the magnesium ribbon is cleaned by rubbing with
sandpaper.
This is done to remove the protective layer of basic magnesium carbonate from
the surface of the magnesium ribbon.
Reactant: Substances
which take part in a chemical reaction are called reactants.
Example: Mg and O2.
Product: New substance formed
after a chemical reaction is called a product.
Example: MgO.
Characteristics
of Chemical Reactions :
(i)
Evolution of gas: The chemical reaction between zinc and
dilute sulphuric acid is characterised by the evolution of hydrogen gas.
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.
The chemical reaction between sulphur dioxide gas and acidified potassium
dichromate solution is characterized by a change in colour from orange to
green.
(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). There are some chemical reactions which can show more than
one characteristics.
(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).
The chemical reaction between zinc granules and dilute sulphuric acid is also
characterised by a change in temperature (which is a rise in temperature).
(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.
BaCl2(aq) + H2SO4(aq) →
BaSO4(s)
(ppt) + 2HCl(aq)
What is a chemical
Equation Class 10?
Chemical
Equation: Representation of chemical reaction using symbols and
formulae of the substances is called Chemical Equation.
Example: A + B → C + D
In this equation, A and B are called reactants and C and D are called the
products. The arrow shows the direction of the chemical reaction. Condition, if
any, is written generally above the arrow.
When hydrogen reacts with oxygen, it gives water. This reaction
can be represented by the following chemical equation:
Hydrogen + Oxygen → Water
H2 +
O2 →
H2O
In the first equation, words are used and in second, symbols of substances are
used to write the chemical equation. For convenience, the symbol of substance
is used to represent chemical equations.
A chemical equation is a way to represent the chemical reaction in a concise
and informative way.
A chemical equation can be divided into two types: Balanced Chemical Equation
and Unbalanced Chemical Equation.
(a) Balanced Chemical Equation: A
balanced chemical equation has the number of atoms of each element equal on
both sides.
Example: Zn + H2SO4 →
ZnSO4 +
H2
In this equation, numbers of zinc, hydrogen and sulphate are equal on both
sides, so it is a Balanced Chemical Equation.
According to the Law of Conservation of Mass, mass can neither be created nor
destroyed in a chemical reaction. To obey this law, the total mass of elements
present in reactants must be equal to the total mass of elements present in
products.
(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.
Example: Fe + H2O → Fe3O4 +
H2
In this example, a number of atoms of elements are not equal on two sides of
the reaction. For example; on the left-hand side only one iron atom is present,
while three iron atoms are present on the right-hand side. Therefore, it is an
unbalanced chemical equation.
|
Name of atom |
No. of atoms in the
reactant |
No. of atoms in the
product |
|
Iron |
1 |
3 |
|
Hydrogen |
2 |
2 |
|
Oxygen |
1 |
4 |
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.
Balance the atom which is maximum in number on either side of a
chemical equation.
In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen, one needs to multiply the oxygen on the LHS by 4, so
that, the number of oxygen atoms becomes equal on both sides.
Fe + 4 × H2O → Fe3O4 +
H2
Now, the number of hydrogen atoms becomes 8 on the LHS, which is more than that
on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.
Fe + 4 × H2O → Fe3O4 +
4 × H2
After that, the number of oxygen and hydrogen atoms becomes equal on both
sides. The number of iron is one on the LHS, while it is three on the RHS. To
balance it, multiply the iron on the LHS by 3.
3 × Fe + 4 × H2O → Fe3O4 +
4 × H2
Now the number of atoms of each element becomes equal on both sides. Thus, this
equation becomes a balanced equation.
|
Name of atom |
No. of atoms in the
reactant |
No. of atoms in the
product |
|
Iron |
3 |
3 |
|
Hydrogen |
8 |
8 |
|
Oxygen |
4 |
4 |
After balancing, the above equation can be written as follows:
3Fe + 4H2O → Fe3O4 +
4H2.
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.
·
Gaseous state is represented by symbol (g).
·
Liquid state is represented by symbol (l).
·
Solid state is written by symbol (s).
·
Aqueous solution is written by symbol (aq).
·
Writing the condition in which reaction takes place: The
condition is generally written above and/or below the arrow of a chemical
equation.
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.
What are the types of a
chemical reaction Class 10?
Types
of Chemical Reactions: Combination Reaction, Decomposition
Reaction, Displacement Reaction, Double Displacement Reaction, Neutralization
Reactions, Exothermic – Endothermic Reactions and Oxidation-Reduction
Reactions.
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.
A general combination reaction can be represented by the chemical equation
given here:
A + B → AB
Examples:
When magnesium is burnt in the air (oxygen), magnesium oxide is formed. In this
reaction, magnesium is combined with oxygen.
Mg(s) + O2(g) →
2MgO(s)
Magnesium + Oxygen → Magnesium Oxide
When carbon is burnt in oxygen (air), carbon dioxide is formed. In
this reaction, carbon is combined with oxygen.
C (s) + O2(g) → CO2(g)
Carbon + Oxygen → Carbon dioxide
(ii) Decomposition Reaction: Reactions
in which one compound decomposes in two or more compounds or elements are known
as Decomposition Reaction. A decomposition reaction is just the opposite of
combination reaction.
A general decomposition reaction can be represented as follows :
AB → A + B
Examples:
When calcium carbonate is heated, it decomposes into calcium oxide and carbon
dioxide.
CaCO3(s)
Calcium carbonate → Calcium oxide + Carbon dioxide
When
ferric hydroxide is heated, it decomposes into ferric oxide and water
2Fe(OH)3(s)
Thermal Decomposition: The decomposition
of a substance on heating is known as Thermal Decomposition.
Example: 2Pb(NO3)2(s) heat−→− 2PbO(s)
+ 4NO2(g) + O2(g)
Electrolytic Decomposition: Reactions in which
compounds decompose into simpler compounds because of passing of electricity,
are known as Electrolytic Decomposition. This is also known as Electrolysis.
Example: When electricity is passed in water, it decomposes into hydrogen and
oxygen.
2H2O(l) Undefined
control sequence \xrightarrow 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.
Example: When silver chloride is put in sunlight, it decomposes into silver
metal and chlorine gas.
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. It happens because
silver chloride is colourless while silver is a grey metal.
(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. Displacement
reactions are also known as Substitution Reaction or Single Displacement/
replacement reactions.
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. If ‘B’
is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and
reaction will not be taking place.
Examples:
When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc
chloride.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
When zinc reacts with copper sulphate, it
forms zinc sulphate and copper metal.
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.
AB + CD → AC + BD
Examples:
When the solution of barium chloride reacts with the solution of sodium
sulphate, white precipitate of barium sulphate is formed along with sodium
chloride.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) (Precipitate) +
2NaCl(aq)
When sodium hydroxide (a base) reacts with
hydrochloric acid, sodium chloride and water are formed.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Note: Double Displacement Reaction, in
which precipitate is formed, is also known as precipitation reaction.
Neutralisation reactions are also examples of double displacement reaction.
Precipitation Reaction: The reaction in
which precipitate is formed by the mixing of the aqueous solution of two salts
is called Precipitation Reaction.
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.
Example:
(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.
Elements or compounds in which oxygen or non-metallic element is added or
hydrogen or metallic element is removed are called to be Oxidized.
Reduction: Addition of hydrogen or metallic
element or removal of oxygen or non-metallic element from a compound is called
Reduction.
The compound or element which goes under reduction in called to be Reduced.
Oxidation and Reduction take place together.
Oxidizing agent:
·
The
substance which gives oxygen for oxidation is called an Oxidizing agent.
·
The
substance which removes hydrogen is also called an Oxidizing agent.
Reducing agent:
·
The
substance which gives hydrogen for reduction is called a Reducing agent.
·
The
substance which removes oxygen is also called a Reducing agent.
The reaction in which oxidation and
reduction both take place simultaneously is called Redox reaction.
When copper oxide is heated with hydrogen, then copper metal and hydrogen are
formed.
CuO + H2 → Cu + H2O
(i) In this reaction, CuO is changing into Cu. Oxygen is being removed from
copper oxide. Removal of oxygen from a substance is called Reduction, so copper
oxide is being reduced to copper.
(ii) In this reaction, H2 is changing to H2O. Oxygen is being added to hydrogen.
Addition of oxygen to a substance is called Oxidation, so hydrogen is being
oxidised to water.
·
The
substance which gets oxidised is the reducing agent.
·
The
substance which gets reduced is the oxidizing agent.
(vi) Exothermic and
Endothermic Reactions:
Exothermic Reaction: Reaction which produces energy is
called Exothermic Reaction. Most of the decomposition reactions are exothermic.
Example:
Respiration is a decomposition reaction in which energy is released.
When quick lime (CaO) is added to water, it releases energy.
Endothermic Reaction: A chemical reaction
in which heat energy is absorbed is called Endothermic Reaction.
Example: Decomposition of calcium carbonate.
Effects of Oxidation
Reactions in Everyday life: Corrosion and Rancidity.
Corrosion: The process of slow conversion of
metals into their undesirable compounds due to their reaction with oxygen,
water, acids, gases etc. present in the atmosphere is called Corrosion.
Example: Rusting of iron.
Rusting: Iron when reacts
with oxygen and moisture forms red substance which is called Rust.
The rusting of iron is a redox reaction.
Corrosion (rusting) weakens the iron and steel objects and structures such as
railings, car bodies, bridges and ships etc. and cuts short their life.
Methods to Prevent Rusting
·
By
painting.
·
By
greasing and oiling.
·
By
galvanisation.
Corrosion of Copper: Copper objects lose
their lustre and shine after some time because the surface of these objects
acquires a green coating of basic copper carbonate, CuCO3.Cu(OH)2 when exposed to air.
Corrosion of Silver
Metal: The
surface of silver metal gets tarnished (becomes dull) on exposure to air, due
to the formation of a coating of black silver sulphide(Ag2S) on its surface by the action of H2S gas present in the air.
Rancidity: The taste and odour
of food materials containing fat and oil changes when they are left exposed to
air for a long time. This is called Rancidity. It is caused due to the
oxidation of fat and oil present in food materials.
Methods to prevent rancidity:
·
By
adding anti-oxidant.
·
Vacuum
packing.
·
Replacing
air by nitrogen.
·
Refrigeration
of foodstuff.
1. Chemical Reaction: During chemical
reactions, the chemical composition of substances changes or new substances are
formed.
2. Chemical Equation: Chemical reactions
can be written in chemical equation form which should always be balanced.
3. Types of Chemical
Reactions:
Combination reaction: A single product is
formed from two or more reactants.
2Mg + O2 → 2MgO
Decomposition reaction: A single reactant
breaks down to yield two or more products.
·
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.
Zn + CuSO4 → ZnSO4 + Cu
Double displacement
reaction: Exchange
of ions between reactants.
AgNO3 + NaCl → AgCl +
NaNO3
Redox reaction: Both oxidation and
reduction take place simultaneously.
CuO + H2 → Cu + H2O
Exothermic reaction: A chemical reaction
in which heat energy is evolved.
C + O2 → CO2 (g) + heat
Endothermic reaction: A chemical reaction
in which heat energy is absorbed.
ZnCO3 + Heat → ZnO + CO2
Redox reaction: Chemical reaction
in which both oxidation and reduction take place simultaneously.
4. Oxidation: Reaction that
involves the gain of oxygen or loss of hydrogen.
5. Reduction: Reaction that shows
the loss of oxygen or gain of hydrogen.
ZnO + C → Zn + CO
ZnO is reduced to Zn—reduction. C is oxidized to CO—Oxidation.
6. 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.
Example, Corrosion (rusting) of Iron: Fe2O3. nH2O (Hydrated iron oxide)
·
Rancidity: Undesirable change
that takes place in oil containing food items due to the oxidation of fatty
acids.
Preventive methods of rancidity: Adding antioxidants to the food materials,
storing food in the airtight container, flushing out air with nitrogen gas and
refrigeration.
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