Question

Define enthalpy using clear mathematical expressions and explain why it is used when performing chemical reactions at constant pressure.

Answer 1

Enthalpy(H) is a thermodynamic quantity that is equal to the sum of internal energy(U) and the product of pressure and volume(PV).

H = U + PV

Enthalpy is an extensive property i.e. it is dependent on the mass of the system.

Now a change in the internal energy of the system is brought about by keeping the pressure and volume of the system constant. and PdV is the work done on the system when there is a change in the volume of the system. Here, if we include a change in the pressure of the system, we can no longer be able to calculate the internal energy or the raise in temperture, in case of heating, of the system.

So, enthalpy is a quantity that is defined to measure the internal energy and the PdV work done on or by the system at constant pressure.