# How can heat change be measured under constant pressure?

#### TheEugenius

2+ Year Member
So enthalpy is the heat content of a system at constant pressure. Enthalpy change is equal to the heat absorbed or evolved by the system at constant pressure. If my understanding is correct, a system whose temperature goes up will return back to that starting temperature if pressure is kept constant (i.e., its volume is allowed to expand). Therefore, is enthalpy, or heat change, measured in terms of the expansion?

Additionally, would enthalpy change and internal energy change (which is equal to the heat absorbed or evolved by the system at constant volume) not have the same values? After all, in the former, the amount of energy released (if the reaction inside releases energy) can be calculated by multiplying the change in volume with the outside pressure, and this value of energy, as far as I know, should be equal to the energy of the change in temperature inside the latter system.

#### Flufpot12

If there’s no change in either pressure or volume, then enthalpy change would equal to internal energy change

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