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Exothermic and Endothermic reactions
Exothermic reactions GIVE OUT / transfer energy to the surroundings usually in the form of
heat usually shown by a rise in temperature as they cause the reaction mixture and the
surroundings to become hotter.
Endothermic reactions TAKE IN heat energy from the surroundings usually shown by a fall in
Burning Fuels [or combustion] is an exothermic reaction as it gives out lots of heat energy
to the surroundings. For example burning fuels.
Neutralisation reactions where an acid is reacted with a base to form a neutral solution is
also an exothermic reaction.
Many Oxidation reactions are exothermic also. This is when a substance joins with oxygen
e.g. adding water to anhydrous copper sulphate is exothermic.
Thermal Decomposition reactions are endothermic.
For example the thermal decomposition of calcium carbonate to make quicklime is
endothermic as it needs heat to decompose the compound about 800 degrees in a kiln
The thermal Decomposition of Hydrated copper sulphate
If you heat up blue,hydrated copper sulphate crystals it drives the water away as this is a
thermal decomposition reaction and heat is taken in as the energy is needed to break down
the crystal structure and drive off the water. This is an endothermic reaction.
If you then add water to the anhydrous copper sulphate hydrated copper sulphate will reform.
This is an oxidation reaction and is exothermic. (Anhydrous doesn't contain water of
This is a reversible reaction the forward reaction is endothermic the reverse reaction is
exothermic. (The forward and the reverse reaction release and take in the same energy.)
blue hydrated copper(II) sulphate + heat white anhydrous copper(II) sulphate +
CuSO4.5H2O(s) CuSO4(s) + 5H2O(g
Notice on the right is plus 5h20 whereas on the left it is part of the compound. It is part of the
compound on the left as this is hydrated copper sulphate and contains water (5 lots in every
molecule).Whereas on the right this water is driven away due to the thermal decomposition
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Exothermic and Endothermic
In exothermic and endothermic reactions the starting materials have to be decomposed by
breaking chemical bonds (overcoming forces of attraction)to release the elements so that
they can recombine in a different way to make the product.
During the chemical reaction old bonds are broken and new bonds are formed.
Breaking bonds requires energy usually in the form of heat as heat energy is taken in to
break bonds,bond breaking is an endothermic process as heat is taken in from the
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In order for a chemical reaction to occur the reacting particles need to collide with enough
energy to start the reaction. The minimum amount of energy needed to start a reaction is
called the activation energy. Catalysts work by lowering the activation energy.…read more