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Heat of Reaction

When chemical reagants react to form one or more products heat is usually either released or absorbed. For example:

H2(g) + 1/2 02(g)---> H2O(g) ΔH=-241.8 kJ/mol

The negative value given to ΔH indicates that heat is released (not absorbed) when we burn hydrogen to create water vapour.

If we were to reverse this reaction by running an electric current through water in order to produce hydrogen gas and oxygen gas; the same amount of energy released in the combustion would become absorbed in the reconstituted reactants.

H2O(g) ---> H2(g) + 1/2 02(g) ΔH=+241.8 kJ/mol

The fact that heat/energy is absorbed is indicated by fact that a positive value is assigned to ΔH.

Chemical reactions which give off heat are called exothermic reactions;

Chemical reactions which absorb heat/energy are called endothermic reactions;



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