Unit 6: Topic 6 - Introduction to Enthalpy of Reaction

Question 1

What is enthalpy?

Answer 1

Enthalpy is equivalent to the sum of internal energy U, and the product of pressure and volume PV. Enthalpy itself is very difficult to work with, but the change in enthalpy is very important. We have ∆H = ∆U + ∆(PV) = ∆U + P∆V if the pressure of the system holds constant. This is also equivalent to the heat flow (q) of the process, i.e. ∆H = q.

Question 2

How can the change in enthalpy of a reaction determine whether it is exothermic or endothermic?

Answer 2

At a constant pressure, the enthalpy change ∆H is equivalent to the heat flow q. If ∆H > 0, then q > 0, so that the reaction consumes heat from the surroundings, and is endothermic. If ∆H < 0, then q < 0, the reaction releases heat into the surroundings, and the reaction is exothermic.