Question

Question #bea74

Answer 1

You need to consider the magnitude of the number, and its sign (+ or -) separately when considering enthalpy.

Explanation:

The sign indicates whether the heat energy is being evolved (i.e. "given out") or involved ("taken in").

If the sign is negative, then it indicates that the reaction evolves a certain amount of energy (exothermic).

For example the combustion of ethanol to give carbon dioxide and water has an ethalpy change `DeltaH = -"1418 kJ/mol"` - this means that `"1418 kJ/mol"` of energy are evolved (given off) in the reaction.

Compare this with the enthalphy change taking place during the combustion of methanol, however - this is `-"725 kJ/mol"`, so it's still negative, meaning that `"725 kJ/mol"` of energy is evolved (given off) during the reaction.

So the amount of energy is smaller, but the direction (energy is evolved) is the same.

However, if the sign is positive, then the energy is taken in rather then being evolved (endothermic) - for example in dissolution of ionic salts or the cracking of alkanes.

In such cases the number magnitude indicates how much energy is involved, whilst the sign (+) indicates that the energy is being taken in rather than given out.