Class 10 Science Notes | Chemical Reactions and Equations Class 10 | CBSE
byJay's Mathematics Classes•
15
Chemical Reactions and Equations
Introduction
Chemical changes result from chemical reactions taking place between
substances.
Chemical reactions: The processes in which a substance or substances undergo change to
produce new substances with new properties are known as chemical
reactions.
For example, when calcium carbonate is heated, calcium oxide and carbon
dioxide are formed.
`CaCO_3 \overset{\triangle}{\rightarrow} CaO + CO_2`
The breaking up of calcium carbonate into calcium oxide and carbon
dioxide is, thus, a chemical reaction because calcium carbonate
changes into new substances, calcium oxide, and carbon
dioxide.
Burning of a magnesium ribbon in air and collection of magnesium oxide in a watch-glass
You must have observed that magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen present in the air.
The reaction between lead nitrate and potassium iodide 👉 yellow precipitate of lead iodide is formed.
`Pb(NO_3)_2 + 2KI rightarrow PbI_2 + 2KNO_3`
Reactant:
The substance which takes part in a chemical reaction is called a
reactant.
`Na + H_2O \rightarrow NaOH + H_2`
Here, `Na` and `H_2O` are reactants.
Product: A product is a new substance formed in a chemical reaction.
`Na + H_2O \rightarrow NaOH + H_2`
Here, `NaOH` and `H_2` are the products.
Characteristics of chemical reactions
There are some characteristics by which we can identify the occurrence of a chemical
reaction. These are described below.
1. Evolution of gas Some
of the chemical reactions occur with the evolution of gas or a mixture of
gases. Metals like zinc, magnesium etc react with dilute
hydrochloric acid with the evolution of hydrogen gas.
`Zn + 2HCl \rightarrow ZnCl_2 + H_2 \uparrow`
`Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2 \uparrow`
2. Formation of
precipitate
Sometimes, when two solutions are mixed together, a solid get separated
from the solution.
The solid thus separated is called the
precipitate.When an aqueous solution of sodium sulphate is mixed with a solution of
barium chloride, a white precipitate of barium sulphate is produced.
`Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl`
3.Change in temperature There
are several reactions that occur with a change in temperature. When quick
lime (calcium oxide) is treated with water in a beaker, a large quantity
of heat is produced.Such reactions in which heat is produced are called exothermic
reactions.
`CaO + H_2O \rightarrow Ca(OH)_2 + \triangle`
(Calcium oxide + Water `\rightarrow` Calcium hydroxide)
4. Change in colour In some
chemical reactions a change in color is observed.
The reaction between lead nitrate and potassium iodide 👉 yellow precipitate of lead iodide is formed.
`Pb(NO_3)_2 + 2KI rightarrow PbI_2 + 2KNO_3`
Chemical equation:
A chemical equation is a representation of the chemical reaction
using symbols and formulae.
`Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2`
Unbalanced chemical equation (Skeletal equation):
When the number of atoms of the elements on the two sides of
the equation are not the same.
`Mg + O_2 \rightarrow MgO`
Balanced chemical equation:
When the number of atoms of the elements on the two sides of
the equation is the same.
`2Mg + O_2 \rightarrow 2MgO`
Balancing chemical equation
Balance the equation `Mg + H_2O \rightarrow Mg(OH)_2 + H_2`
Count each type of atom
H and O are unbalanced.
To balance O, 2 is set just before `H_2O`
`Mg + 2H_2O \rightarrow Mg(OH)_2 + H_2`
Now check the atoms on both the side. The equation is balanced
now.
Ferrous
sulphate crystals (`FeSO_4 .7H_2O`) lose water when heated and the colour
of the crystals changes. It then decomposes to ferric oxide (`Fe_2O_3`),
sulphur dioxide (`SO_2`) and sulphur trioxide (`SO_3`).
👉Ferrous sulphate crystals (`FeSO_4 .7H_2O`) green colour.
👉 Ferric oxide (`Fe_2O_3`) brown colour.
b) Electrolytic decomposition
Some compounds are decomposed by passing
electricity.
c) Photochemical decomposition
There are some reactions which take place
in the presence of light. These are called
photochemical reaction.
3. Displacement reaction A reaction, in which an atom or a group of atoms present in a
molecule is displaced by another atom, is known as
displacement reaction.
a) Displacement of less reactive metal by a more reactive
metal
When a piece of iron is added to a solution of copper
sulphate, iron displaces copper from copper
sulphate.
`Fe + CuSO_4 \rightarrow FeSO_4 + Cu`
(Iron + Copper sulphate `\rightarrow` Iron sulphate +
Copper)
Displacement occurs because iron is more reactive than
copper.
Iron displaces copper from copper sulphate
The blue color of copper sulphate solution disappears due to the
formation of `FeSO_4`, which is light green. A reddish
coating of copper metal is deposited on the surface of iron
metal.
When a strip of copper is added to a solution of silver
nitrate, silver is displaced by copper.
In this reaction, copper is displaced from copper suphate
by zinc because zinc is more reactive than copper. A
reddish deposit of copper is observed on the zin strip. The
blue color of copper sulphate disappears due to the
formation of colorless zinc sulphate.
b) Displacement of less reactive nonmetal from its
compound.
When `Cl_2` gas is passed through a solution of
sodium bromide `(NaBr)`, sodium chloride and bromine
are formed. The solution becomes brown due to the
liberation of bromine in the solution.
This reaction proves that
chlorine is more reactive than bromine and
iodine.
c) Displacement of hydrogen from acids by active
metals.
When a piece of zinc is added to a dilute solution
of sulphuric acid `(H_2SO_4)`, hydrogen gas is
evolved and zinc sulphate is formed.
`Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2 \uparrow`
Zinc can displace hydrogen from other acids, such as
hydrochloric acid `(HCl)`, phosphoric acid `(H_3PO_4)`,
and acetic acid `(CH_3COOH)`. This can be demonstrated
by the following experiments.
It is hydrogen gas coming out from each test tubes. Thus
hydrogen gas has been displaced from acids by
zinc.
4. Double displacement reaction
The reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions.
XY + AB `\rightarrow` XB + AY
Here, X and A are the positive ions and Y and B are
the negative ions.
A double displacement reaction usually occurs in
solution and one of the products, being
insoluble precipitate out (separate as a
solid).
Examples
When a solution of silver nitrate is added to a
solution of sodium chloride, a white precipitate
of silver chloride is formed along with sodium
nitrate.
When a solution of barium chloride is added to a
solution of sodium sulphate, a white precipitate
of barium sulphate is obtained, sodium chloride is
formed is left behind the solution.
When ferrous sulphide reacts with sulphuric acid,
a double displacement reaction takes place with
the evolution of hydrogen sulphide gas, ferrous
sulphate remains in solution.
When potassium iodide solution is added to lead
nitrate solution, then a yellow precipitate of
lead iodide is produced along with potassium
nitrate solution.
When ammonium hydroxide solution is added to
aluminium chloride solution, the white precipitate
of aluminium hydroxide is formed along with
ammonium chloride solution.
Oxidation is defined as (i) the addition of
oxygen to a substance (ii) the removal of
hydrogen from a substance
Reduction is defined as (i) the addition of
hydrogen to a substance (ii) the removal of
oxygen from a substance.
When phosphorus burns in the air, oxygen is added to
it to form phosphorus pentoxide (`P_2O_5`), so
phosphorus gets oxidised.
The process of reduction is just the opposite of
oxidation.
Oxidising agent
The substance which gives oxygen for oxidation is
called oxidising agent.
The substance which removes hydrogen is also
called an oxidising agent
Reducing agent
The substance which gives hydrogen for reduction
is called a reducing agent.
The substance which removes oxygen is called a
reducing agent.
The oxidation and reduction reactions are also
called redox reactions.
In the name 'redox', the term 'red' stands for
'reduction' and 'ox' stands for 'oxidation'.
Example 1
When copper oxide is heated with hydrogen, the
copper metal and water are formed.
`CuO + H_2 \overset {\triangle}{\rightarrow} Cu +
H_2O`
In this reaction, `CuO` is changing into `Cu`.
That is, oxygen is being removed from copper
oxide. So, we can say that copper oxide is reduced to
copper.
`H_2` is changing into `H_2O`. That is oxygen is
being added to hydrogen, so
we can say that hydrogen is being oxidised to
water.
We find that hydrogen is being oxidised to
water and at the same time copper oxide is being
reduced to copper.
This shows that oxidation and reduction occur
together.
Copper oxide is giving the oxygen required for the
oxidation of hydrogen, therefore
copper oxide is the oxidising agent.
Hydrogen is responsible for removing oxygen from
copper oxide, therefore
hydrogen is the reducing agent.
Substance oxidised : `H_2`
Substance reduced : `CuO`
Oxidising agent : `CuO`
Reducing agent : `H_2`
Example 2
When hydrogen sulphide reacts with chlorine, then
sulphur and hydrogen chloride is formed.
In this reaction,
`H_2S` is changing to `S`.
Hydrogen is being removed from hydrogen
sulphide, so we can say that hydrogen
sulphide is being oxidised.
In this reaction, `Cl_2` is changing to `HCl`.
Hydrogen is being added to chlorine. So we
can say that chlorine is being reduced to
hydrogen chloride.
Redox reaction
Chlorine is oxidising agent and hydrogen
sulphide the reducing agent.
Substance oxidised: `H_2S`
Substance reduced: `Cl_2`
Oxidising agent: `Cl_2`
Reducing agent: `H_2S`
Example 3
When Zinc oxide is heated with carbon, then zinc
metal and carbon monoxide are formed.
`ZnO + C \overset{\triangle}{\rightarrow} Zn +
CO`
Substance oxidised: `C`
Substance reduced: `ZnO`
Oxidising agent: `ZnO`
Reducing agent: `C`
Example 4
When copper is heated in the air, it reacts with
oxygen to form a black compound copper
oxide.
`2Cu + O_2
\overset{\triangle}{\rightarrow} 2CuO`
Substance oxidised: `Cu`
Oxidising agent: `O_2`
Effects of oxidation
reactions in everyday
life.
Oxidation has a damaging
effect on metals as well as
on food. The damaging effect of
oxidation on metals is studied
as corrosion and that on food
is studied as rancidity. There are two common
effects of oxidation
reactions:
Corrosion
Rancidity
1. Corrosion:
Corrosion is the
process in which metals
are eaten up gradually
by the action of air,
moisture.
Corrosion is caused mainly
by the oxidation of metals.
Rusting of iron is the most
common form of
corrosion.
During the corrosion of
iron, iron metal is oxidised
by the oxygen of air in the
presence of water (moisture)
to form
hydrated iron (III)
oxide ( `Fe_2O_3.
xH_2O`) called rust.
`4Fe + 3O_2 + 2xH_2O
\rightarrow 2Fe_2O_3.
xH_2O`
The number of water
molecules (`x`) in the rust
varies, it is not fixed. The
rusting of iron is a redox
reaction. Corrosion weakens
the iron objects and
structure such as railing,
car bodies, bridges, and
ships, etc., and cuts short
of their life. A lot of
money has to be spent every
year to prevent the
corrosion of iron and to
replace the damaged
part.
2. Rancidity:When fats and oils are
oxidized, they become
rancid. When the food
materials prepared in fats
and oils are kept for a long
time, they start giving an
unpleasant smell and taste.
The condition produced by
aerial oxidation of fats
and oils in foods marked
by unpleasant smell and
taste is called
rancidity.
Prevention of
rancidity
Rancidity can be
prevented by adding
antioxidants to foods
containing fats and
oils.
An antioxidant is a
substance that prevents
oxidation. Antioxidants
are reducing agents. When
antioxidants are added to
food, then the fats and
oils present in them do
not get oxidised easily
and hence do not turn
rancid. BHA (Butylated
Hydroxy- Anisole ) and BHI
(Butylated Hydroxy -
Toluene) are the two
common antioxidants used
in food to prevent the
development of
rancidity.
Rancidity can be
prevented by packaging
fat and oil-containing
foods in nitrogen gas. When the packed food is
surrounded by unreactive
nitrogen. there is no
oxygen to cause its
oxidation and make it
rancid. The manufacturers
of potato chips fill the
plastic bags containing
chips with nitrogen gas to
prevent the chips from
being oxidised and turn
rancid.
Rancidity can be
retarded by keeping food
in a refrigerator.
When the food is kept in a
refrigerator, the
oxidation of fats and oils
in it is slowed down due
to low
temperature.
Rancidity can be
retarded by storing food
in air-tight containers. When food is stored in
air-tight containers, then
there is little exposure
to the oxygen of the air.
Due to reduced exposure to
oxygen, the oxidation of
fats and oils present in
food is slowed down and
hence the development of
rancidity is
retarded.
Rancidity can be
regarded by storing food
away from light.
In the absence of light,
the oxidation of fats and
oils present in food is
slowed down.
Few other reactions
Exothermic reaction:
Reactions in which heat is produced is called
an exothermic reaction.
`CaO + H_2O \rightarrow Ca(OH)_2 +
\triangle`
(Calcium oxide + Water `\rightarrow` Calcium
hydroxide)
Do you know that respiration is an exothermic
reaction?
For example, rice, potatoes, and bread contain
carbohydrates. These carbohydrates are broken down
to form glucose. This glucose combines with oxygen
in the cells of our body and provides energy. The
special name of this reaction is respiration.
`C_6H_12O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O` +
energy
Endothermic Reaction: Reactions in which heat is absorbed is known
as an endothermic reaction.
The decomposition reactions require energy
either in the form of heat, light or
electricity for breaking down the reactants
are the examples of endothermic
reactions.
Precipitation
You will observe that a white substance,
which is insoluble in water, is formed. This insoluble substance formed is known
as a precipitate. Any reaction that produces a precipitate
can be called a precipitation reaction.
White precipitate of
`BaSO_4` is formed by the reaction
of `SO_4^{2-}` and `Ba^{2+}`.
The other product formed is sodium
chloride which remains in the
solution.
Any reaction in which an insoluble
solid (called precipitate) is formed
that separates from the solution are
called a precipitation reaction.
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