Heat of combustion?

[fridge]

For an exothermic reaction, is the heat of combustion positive or negative? The textbook definition says that heat of combustion is the energy released (and so it should be positive, right?), and the table of values for certain fuels also has positive values. But in the questions, all the answers seem to be negative (like a deltaH value). Heinemman Chem 2 btw.

[thushan]

For an exothermic reaction, is the heat of combustion positive or negative? The textbook definition says that heat of combustion is the energy released (and so it should be positive, right?), and the table of values for certain fuels also has positive values. But in the questions, all the answers seem to be negative (like a deltaH value). Heinemman Chem 2 btw.

This can be quite confusing; the terms 'heat' and 'enthalpy' are sometimes used interchangeably, when they really shouldn't.

Heat is a form by which energy is transferred; energy is released as heat when combustion occurs. Hence, the heat of combustion is positive.

However, enthalpy (which is delta H) is a measure of the chemical energy present in a chemical. The energy that is released as heat was originally chemical energy (H). Hence, in combustion, chemical energy is lost, and H decreases.

So, the heat of combustion is positive (we use the symbol q to denote heat), the enthalpy (H) of combustion is negative.

[fridge]

Thanks thushan, cleared up a lot for me.