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C3: Energy in Chemical Reactions
Knowing the amount of energy involved in chemical reactions is
useful so that resources are used efficiently and economically.
It is possible to measure the amount of energy experimentally or
to calculate it.
Controlling the amount of energy intake in our diet is important in
The relative amounts of energy released when substances burn
can be measured by simple calorimetry, e.g. by heating water in a
glass or metal container. This method can be used to compare
the amount of energy produced by fuels and foods.
Energy is normally measured in joules (J). Some dietary
information is given in calories, which are equal to 4.2 joules.
Different foods produce different amounts of energy. Foods with
high proportions of carbohydrates, fats and oils produce relatively
large amounts of energy.
Eating food that provides more energy than the body needs can
lead to obesity.
The amount of energy produced by a chemical reaction in solution
can be found by mixing the reagents in an insulated container and
measuring the temperature change of the solution. This method
can be used for reactions of solids with water or for neutralisation
During a chemical reaction:
energy must be supplied to break bonds
energy is released when bonds are formed.
These changes can be represented on an energy level diagram.
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In an exothermic reaction, the energy released from forming new
bonds is greater than the energy needed to break existing bonds.
In an endothermic reaction, the energy needed to break existing
bonds is greater than the energy released from forming new