Enthalpy change calculations

When you are calculating Enthalpy change by q/n how do you know what to use for moles?

For example, when adding zinc powder to copper sulphate, would the Enthalpy change depend on moles of Zn, CuSO4, ZnSO4, Cu, or some combination within those?

When it is a neutralisation reaction you use moles of water formed, and when it's combustion you use moles of stuff burnt... but what if it's less specific?

Many enthalpy changes are well defined as you point out, another example is the enthalpy change of formation which has a very specific definition.

In less specific cases I would simply write out the equation of the reaction you're interested in and define the enthalpy change of reaction as the enthalpy change for the specific reaction you've chosen to write.

So, for a reaction, lets choose the test for sulphate ions using BaCl2

you could define an enthalpy of reaction for the process

BaCl2 + Na2SO4 --> BaSO4 + 2NaCl
(ie forming one mole of BaSO4)

or you might choose

1/2BaCl2 + 1/2Na2SO4--> 1/2BaSO4 + NaCl
(ie forming one mole of NaCl instead)

The enthalpy change of the second reaction would simply be half the enthalpy change of the first. As long as you make it clear which equation you're working out the enthalpy change for its not a problem.

In terms of the Zn and CuSO4 reaction you are considering, as long as you know the balanced equation and can work out which reagent is in excess, then it's probably sensible to do the enthalpy change for a 1:1 reaction, seeing as that's the stoichiometry of the reaction.

so you would calculate delta H for the process

Zn(s) + CuSO4(aq) ---> ZnSO4(aq) + Cu(s)