Source C- Page 1
Website:
CU3.2: Energy from fuels
http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_21c/chemical_synthesis/whychemicalsrev8.shtml
Energy changes in reactions
Date accessed: 28 May 2012
The animation shows an exothermic reaction between sodium
hydroxide and hydrochloric acid, and an endothermic reaction
When a chemical reaction occurs, energy is transferred to, or from, the
between sodium carbonate and ethanoic acid.
surroundings - and there is often a temperature change. For example,
when a bonfire burns it transfers heat energy to the surroundings.
In industry it is important for scientists to be able to predict if a reaction
Objects near a bonfire become warmer. The temperature rise can be
is going to be endothermic or exothermic. The main reasons for this are:
measured with a thermometer.

Exothermic reactions
These are reactions that transfer energy to the surroundings. The
to be used elsewhere.

Endothermic reactions require energy, therefore fuel is needed which
costs money.
energy is usually transferred as heat energy, causing the reaction mixture
and its surroundings to become hotter. The temperature increase can be
As energy is given out in exothermic reactions it could be harnessed

An increase in temperature makes chemical reactions go faster. So, if
detected using a thermometer. Some examples of exothermic reactions
the reaction is exothermic (giving out heat energy) the rate of
are:
reaction may increase to an unsafe point causing an explosion.

Burning

Neutralisation reactions between acids and alkalis

The reaction between water and calcium oxide
Endothermic reactions
These are reactions that take in energy from the surroundings. The
energy is usually transferred as heat energy, causing the reaction mixture
and its surroundings to get colder. The temperature decrease can also be
detected using a thermometer. Some examples of endothermic reactions
are:

Electrolysis

The reaction between ethanoic acid and sodium carbonate

The thermal decomposition of calcium carbonate in a blast
furnace
Source C- Page 2
Website:
CU3.2: Energy from fuels
http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_21c/chemical_synthesis/whychemicalsrev8.shtml
Date accessed: 28 May 2012
Energy-level diagrams

During a chemical reaction energy is either taken in or given out.

During exothermic reactions energy is given out. This means that
the energy of the products will be lower than the energy of the
reactants. This can be represented using an energy-level diagram.

This energy-level diagram above shows energy has been
absorbed in the reaction - is it endothermic

This energy-level diagram above shows energy has left the
system - it is exothermic.

During endothermic reactions energy is taken in. This means the
energy of the products will be higher than the energy of the
reactants.