difference between 'q' and 'ΔH' in thermochemistry?

Abdul Quadeer

What is the basic difference between 'q' and 'ΔH' in thermochemistry? I get confused between them!
Is there any criteria for ΔH to become equal to q?

alxm

Q is an amount of heat being transferred and only heat. The change in enthalpy (ΔH), is the change in total energy of the system. That includes heat, but also pressure/volume work and entropy.

Gibbs' free energy, ΔG, is a measure of the change of the useful (i.e. work-producing) energy of the system, given no change in temperature or pressure. So it's simply the enthalpy minus the entropy.

So the heat transferred to a system in a reaction, Q, equals ΔH only if there is no change in pressure/volume or entropy. (You also neglect how the change in temperature caused by the heat from ΔH changes ΔH itself)