Energy changes in Chemical reactions IGCSE
All chemical reactions are accompanied by a change in energy.
Energy is given out or taken in by the chemical during a chemical reaction. Energy is
given out or taken in will be in to form of heat.
Exothermic reactions
If energy is given out during a reaction, the reaction is called exothermic reaction.
During exothermic reaction, the chemicals lose heat (energy) and the surrounding
gains the heat (energy).
So during an exothermic reaction, the temperature of the surrounding will be
increased.
Combustion of fuels, neutralisation reactions and respiration are examples of
exothermic reactions.
The energy given out is used for various purposes such as keeping room warm,
cooking food, keeping our body warm, working of motor engines etc.
When sodium hydroxide solution is neutralised by hydrochloric acid, there is an
increase in the temperature. This is because during the chemical reaction heat is
given out into the solution mixture.
Endothermic reaction
During endothermic reaction energy (heat) is taken in to the system. So the energy
of the chemicals increases. This energy is taken from the surrounding and hence
there is a cooling of the surrounding.
Melting a solid, decomposing limestone, melting of ice, splitting a gas molecule and
working of a refrigerator are examples of endothermic processes.
During endothermic process, heat is taken up by the chemical from the surrounding.
So there will be a decrease in the temperature in the surrounding (cooling).
Activation energy
The minimum amount of energy required for the reaction to start is called the
activation energy. Activation energy is mainly used to break the bonds of the
reactants. Breaking bonds is endothermic process. Once the bonds are broken,
new bonds are formed. Formation of bonds is exothermic process.
If Activation energy is larger than the energy released, the process is
endothermic.
If the energy released is larger than activation energy, the process is
exothermic.
Activation energy > total energy released (Endothermic reaction)
Activation energy < total energy released (Exothermic reaction)
The following energy diagrams show change in energy including activation energy.
Exothermic reaction
Endothermic reaction
Hydrogen is used as a fuel in hydrogen fuel cells
Water is produced in hydrogen fuel cells together with energy. Hydrogen is high in
energy, yet an engine that burns pure hydrogen produces almost no pollution. A fuel
cell combines hydrogen and oxygen to produce electricity, heat, and water. Fuel
cells are often compared to batteries. Both convert the energy produced by a
chemical reaction into usable electric power.
Radioactive isotopes such as Uranium 235 ( 235U) is used to give out energy during
its radioactive decay. (atomic reactors). This process is not a physical or chemical
change. It is a nuclear reaction.
Reversible Reactions
If in a reaction, all reactants are changed into products, the reaction is called
irreversible reaction. In irreversible reactions, products do not change back to
reactants.
2H2(g) + O2(g)
 2H2O(l)
If in a reaction the products change back to the reactants, the reaction is reversible.
In a reversible reaction, reactants change into the products (forward reaction) and at
the same time products change back to the reactants (reverse reaction). In a
reversible reaction,
reactions.
sign is used which shows both forward reaction and reverse
When the reaction starts, forward reaction rate is high because of more reactant
molecules and more collision. When the products start forming, reverse reaction
starts. With time, rate of forward reaction decreases and rate of reverse reaction
increases. When both reaction rates become equal, the system comes to
equilibrium. The stage at which both forward reaction and reverse reaction take
place at the same rate is called a CHEMICAL EQUILIBRIUM. Chemical equilibrium
is a dynamic equilibrium because both reactions take place at this stage. At
chemical equilibrium, concentration of reactants and products remain same.
A + B
C + D
If you change conditions, equilibrium position can be changed to right (more
products) or to left (more reactants).
For any exothermic reaction, low temperature favours to make more products
(equilibrium shifts to right)
For any endothermic reaction, high temperature is favourable for making more
products (equilibrium shifts to right)
Haber Process of ammonia
Ammonia is an important starting compound for the manufacture of various fertilizers
containing Nitrogen.
Nitrogen is the most important element for plant growth because nitrogen is a must
in the making of proteins by plants. Plants cannot absorb nitrogen from the air. So
nitrogen containing compounds (fertilizer) is added to the soil for proper plant growth.
Ammonia is also used in the manufacture of nitric acid.
Manufacture of ammonia is called Haber Process.
In Haber process, Nitrogen gas and Hydrogen gas are mixed in the ratio 1:3 ratio by
volume and the reaction takes place in the presence of Iron catalyst.
Raw materials
Air is the raw material for Nitrogen gas. Air is liquefied at high pressure and low
temperature. Fractional Distillation of liquid air will yield Nitrogen gas
Natural gas and water are the raw materials for Hydrogen gas. Natural gas
(Methane) is mixed with heated steam to produce carbon monoxide and Hydrogen
gas.
CH4 + H2O  CO + 3H2
Metal iron is used as the catalyst for Haber Process. Forward reaction goes on at a
higher rate in the beginning and the reaction comes to equilibrium. At this stage
Nitrogen, Hydrogen and Ammonia are present as a mixture. Following changes are
made to obtain more ammonia.
1. Temperature: Ammonia manufacture is an exothermic reaction. So a low
temperature is favourable to produce more ammonia. But the reaction will be
very slow. Therefore an optimum temperature of 450oC is used in Haber
Process. At this temperature, reasonably good amount of ammonia will be
produced at a good rate.
2. Pressure: In the Haber Process reactants and products are all gases. For
the forward reaction, 1 mole of Nitrogen reacts with 3 moles of Hydrogen to
form 2 moles of Ammonia according to the balanced equation. When the
moles decrease from 4 to 2, volume will also decrease (mole is proportional to
volume).
High pressure favours decrease in volume. Because very high pressure is
too expensive, an optimum pressure of 200-300 atmosphere is used in Haber
process.
3. Catalyst: Metal iron is used as a catalyst. Without the catalyst, the reaction is
very slow. Catalyst will not help to make more ammonia but it helps the
reaction go faster.
4. Concentration: Ammonia produced during Haber process is changed into
liquid and removed from the mixture. When the product concentration is
decreased, more nitrogen and hydrogen react to replace the removed
ammonia so that the equilibrium ratio will be maintained. Normally the
reactants are recycled to produce more ammonia.
Contact Process of Sulphuric acid
Large quantities of sulphuric acid are used every day. It is used a dehydrating
agent. Sulphuric acid is used in fertislizer industry, and in the manufacture of
paints, ceramics, plastics etc. Sulphuric acid is the electrolyte used in car
batteries.
Sulphuric acid is manufactured by Contact Process.
Step 1: Making sulphur dioxide
Sulphur is burnt in air to produce sulphur dioxide gas.
S + O2  SO2
SO2 is also formed when iron sulphide (another ore of iron) is heated in air.
Sulphur dioxide gas is also produced as by-product during the petroleum
industry.
Step 2: conversion of sulphur dioxide to sulphur trioxide
This is an exothermic reaction and is reversible. So we need to consider the
conditions to get more sulphur dioxide.
2SO2(g) + O2(g)
2SO3(g)
Temperature: Low temperature is favourable for this reaction because it is an
exothermic reaction. But for the catalyst to work, 4500C is used in the contact
process.
Pressure: During the above reaction volume of gas reactants is decreased from
3 to 2. High pressure favours decrease in volume. But a pressure of around 1-2
atmosphere is used in contact process due to economic reasons.
Catalyst: Vanadium(V) oxide is used as a catalyst in the process. The catalyst
is used in the powdered form to increase surface area. It lowers the activation
energy and makes reaction faster.
Concentration: SO3 formed is removed from the mixture to convert more SO2 to
SO3
Step 3: Dissolving SO3 in conc sulphuric acid
Sulpur trioxide does not dissolve in water easily. Therefor it is first mixed with
concentrated sulphuric acid to get fuming oleum.
SO3 + H2SO4  H2S2O7
Step 4: Mixing Oleum with water
Oleum is mixed with water to get sulphuric acid molecules
H2S2O7 + H2O  2H2SO4
Sulphuric acid has the following uses:
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Manufacture of fertilizers e.g ammonium sulphate
Manufacture of paints, pigments and dyes
Making soaps and detergents
Used as electrolyte in batteries.