Thermodynamics heat, enthalpy, Gibbs
When a reaction occurs at constant pressure and no work other than expansion work  (such as electrochemical) is performed, then it can be shown, from the definition of the enthalpy and the first law of thermodynamics applied to a closed system, that the change in enthalpy is equal to the heat:
$$\begin{align}dH&amp;=dU+d(pV)\\&amp;=dw_{pV}+dw_{other}+dq+pdV+Vdp\\&amp;=-p_{ext}dV+dw_{other}+dq+pdV+Vdp\\&amp;=dq+dw_{other} \end{align}$$
The last equality follows when the pressure is constant and equal to the external pressure. When only pV work is done then clearly heat and enthalpy change are equal. Readily measured heats of reaction provide a very convenient way to determine values of the enthalpy state function which can be tabulated and employed in further thermodynamic computations.