Chemical reactions and equations
A reaction in which 2 or more substances react to form new
products with new properties is called a chemical reaction
Burning of a magnesium ribbon in air
When magnesium ribbon is burnt in air it burns with a
dazzling white flame and this flame changes into a white
powder called magnesium oxide
The magnesium ribbon has to be cleaned before performing
a chemical rxn because it is coated with a layer of magnesium
oxide to prevent it from reacting with atmospheric oxygen
and to get the desired chemical reaction it has to be cleaned.
Reaction between lead nitrate solution and potassium
iodide
When lead nitrate is added to potassium iodide 2 new
compounds are formed.
1. Yellow solid –lead iodide
2. White solid –potassium nitrate.
Reaction between zinc and sulphuric acid
When sulphuric acid is added to zinc granules it results in the
formation of zinc sulphate in aqueous form and evolution of
hydrogen gas.
Observations in a chemical reaction
Change in state
Change in temperature
Change in colour
Evolution of gas
Chemical formulae
The chemical formula of a substance is the symbolic
representation of the actual number of atoms present in one
molecule of a substance.
Formula of one molecule of water is H2O. It shows that
one molecule of water is made up of 2 atoms of
hydrogen and one atom of oxygen.
Chemical equation
The symbolic representation of a chemical rxn is called a
chemical equation
The description of a chemical reaction is quite long.
Therefore it can be written in the form of a chemical
equation.
Reactants: The substances that undergo a chemical
reaction are called reactants.
Products: The substances that are formed as a result of
a chemical equation are called products.
The word equation shows the change of reactants into
products through an arrow placed between them.
The reactants are in the LHS with a + b/w them
The products are in the RHS with a + b/w them.
The arrow head points towards the products showing
the direction of the equation.
Unbalanced chemical equation
If the number of atoms on the reactant side and the number
of atoms on the products side are not equal then the
reaction is said to be an unbalanced equation.
Balanced chemical equation
According to the law of conservation of mass on a chemical
reaction the number of atoms in the reactant side and the
product side are same.
Therefore a chemical reaction has to be balanced
If the number of atoms on the reactant side and the product
side are equal then the reaction is said to be a balanced
equation.
Example :
Zn + H2SO4
3Fe + 4H2O
ZnSO4 + H2
Fe3O2 + 4H2
Information conveyed by a chemical equation
Names and formula of various reactants and products.
Relative number of moles of reactants and products.
Relative masses of reactants and products.
Relative volumes of gaseous reactants and products.
Limitations of a chemical equation
It does not mention the state of the substances
The reaction may or may not be complete
It does not reveal the speed of the reaction.
It does not give the concentration of the substance.
It does not give conditions such as temperature,
pressure etc.,
Making a chemical reaction more informative
To make a chemical reaction more informative the physical
states are often mentioned along with the formulae.
Notations
Word
l
Liquid
g
Gas
s
solid
aq
aqueous
Sometimes reaction conditions such as temperature,
pressure, catalyst etc., are also mentioned above or below
the arrow.Example : CO(g) + 2H2O(l) 340 atm CH3OH(l)
6CO2 +12H2O CHLOROPHYLL C6H12O6 + 6O2 + 6H2O
Exothermic reaction: Reactions in which heat is produced
along with the formation of products is called an exothermic
reaction.
i.
ii.
Burning of natural gas: CH4 + 2O2
Respiration: C6H12O6 + 6O2
CO + 2H2O
6CO2 + 6H2O
Types of chemical reactions
Combination reaction
A reaction in which 2 or more reactants combine to form a
single product is called a chemical reaction.
Calcium oxide reacts vigorously with water to produce
calcium hydroxide and a large amount of heat
CaO + H2O
Ca(OH)2
In this calcium oxide reacts with water to form calcium
hydroxide
Such a reaction in which a single reactant is formed is
called combination reaction.
i. Burning of coal: C + O2
CO2
ii. Formation of water: 2H2 + O2
2H2O
iii. Formation of ammonium chloride
NH3 + HCl
NH4Cl
Decomposition reaction
A reaction in which a single reactant breaks down into two or
more products is called decomposition reaction.
Ferrous sulphate crystals on heating loses water and the
colour of the crystals change from light green to white.
On further heating it decomposes into ferric oxide and
sulphur dioxide and sulphur trioxide.
Ferrous sulphate crystals –(FeSO4, 7H2O)
2FeSO4
HEAT
Fe2O3 + SO2 + SO3
Decomposition of calcium carbonate to calcium oxide
and carbon dioxide on heating is an important
decomposition reaction used in various industries.
Calcium oxide is called quick lime
It is used in the manufacture of cement.
CaCO3
HEAT
CaO + CO2
Lead nitrate powder on heating decomposed into lead
oxide, nitrogen dioxide and oxygen.
The emission of brown fumes is nitrogen dioxide.
2Pb(NO3)2
HEAT
2PbO + 4NO2 + O2
The decomposition reaction which is carried out by heating
is called thermal decomposition.
Electrolysis of water
When water taken in an electrolytic cell, is acidified with a
small quantity of sulphuric acid and a direct current is
passed through it it undergoes decomposition to yield
hydrogen and oxygen.
Electrolysis of water is the decomposition of water
into hydrogen and oxygen gas due to the passage of
current. It is also called water splitting.
It ideally requires a potential difference of 1.23 volts
to split water .
The volume of gas in the hydrogen test tube is twice
the volume of gas in the oxygen test tube.
Black and white photography (photolysis)
When silver chloride is exposed to sunlight for some
time white silver chloride turns grey in sunlight
This is the decomposition of silver chloride by light into
silver and chlorine.
This is the reason why silver chloride should be stored in
air tight containers.
2AgCl SUNLIGHT 2Ag + Cl2
Silver bromide also behaves the same way
Endothermic reactions
Reactions in which energy is absorbed are called
endothermic reactions.
If blue crystals of copper nitrate absorb heat it decomposes
into black copper oxide, nitrogen dioxide and oxygen.
Displacement reaction
A reaction in which a high reactive element displaces the low
reactive element is called displacement reaction
When iron nail is immersed in copper sulphate solution the
iron nail becomes brownish in colour and the blue colour of
copper sulphate solution starts fading.
In this iron has displaced copper from copper sulphate soln
Fe + CuSO4
FeSO4 + Cu
Other examples
Zinc + copper sulphate gives zinc sulphate + copper
Lead + copper chloride gives lead chloride + copper
Double displacement reaction
Reactions in which exchange of ions take place between the
reactants are called double displacement reactions.
Na2SO4 + BaCl2
BaSO4 + 2NaCl
When sodium sulphate is added to barium chloride it results
in the formation of sodium chloride and white precipitate of
barium sulphate.
When lead nitrate is added to potassium iodide it results in
the formation of precipitate of yellow lead iodide and white
potassium nitrate.
Pb(NO3)2 + 2KI
PbI2 + 2KNO3
Precipitation reaction
Any reaction that produces a precipitate is called
precipitation reaction.
Neutralization reaction
Neutralization reaction is reaction in which the reactants are
an acid and a base and the products are salt and water. In
this reaction the positive charge of the hydrogen ions of the
acid and the negative charge of the hydroxide ions of the
base get neutralised.
Oxidation, reduction and redox reactions.
Oxidation reaction
A reaction in which oxygen is added or hydrogen is removed
is called an oxidation reaction.
Addition of O2: 2Mg + O2
Removal of H2: 2HI
2MgO
H2 + I2
Reduction reaction:
A reaction in which hydrogen is added or oxygen is removed
is called a reduction reaction.
Removal of O2: 2CuO
2Cu + O2
Addition of H2: H2 +2 Cl
2HCl
Redox reaction
A reaction in which oxidation and reduction takes place
simultaneously is called a redox reaction.
Mg + H2SO4
MgSO4 + H2
Oxidising agent –the substance responsible for oxidation.
Reducing agent the substance responsible for reduction.
Oxidised substance –substance that gains O2 or loses H2
Reduced substance –substance that gains H2 or loses O2
The effects of oxidation in our day to day life
1. Corrosion
Some metals when exposed to moisture, oxygen, acids
etc., are tarnished due to the formation the respective
metal oxide on their surface. This is called corrosion.
Iron articles after some time get coated with a
reddish brown powder called rust.
The corrosion of iron is called rusting.
The problem with iron is that the oxide formed
does not firmly stick to the metal surface.
This causes the metal surface to flake out
This eventually weakens the metals.
Copper gets coated with a black colour copper
oxide.
Silver reacts with sulphide and gets tarnished
forming black colour silver sulphide.
Effects of corrosion
It causes damage to car bodies, railways, bridges and
to all objects made of metals.
Prevention
Painting
Alloy making
Galvanisation
Anodising
Electroplating
2. Rancidity
When the substance containing oils and fats are
exposed to air they get oxidised and become rancid due
to which their smell and taste change. This process is
called rancidity.
Prevention
This can be prevented by adding antioxidants to the
substances containing fats and oil.
Keeping food in air tight containers also prevent
rancidity.
Chips manufacturers usually flush bags of chips with gas
such as nitrogen to prevent the chips from getting
oxidised.
Formation of calcium carbonate from calcium oxide.
Calcium oxide is mixed with water to form calcium
hydroxide and a large amount of heat.
This calcium hydroxide also known as slaked lime is
used for white washing.
Calcium hydroxide after been used for white
washing slowly reacts with atmospheric CO2
Calcium carbonate is formed as a result after 2 -3
days.
This calcium carbonate gives the walls a shiny finish.
Chemical formula of marble is CaCO3