Question

Define the term ‘Enthalpy’. What will happen to the internal energy if work is done by the system.

Answer 1

Hint: Enthalpy is the measure of the total energy of a system. Also, for work done by the system on the surroundings, the value of w is negative. (-w)

Complete step by step answer:
1: Enthalpy is a thermodynamic state function. In simple language, you can also take it to be a measure of heat in the system.
2: It is the sum of system’s internal energy, U and the product of its pressure, P and volume, V. The product of volume and pressure is work, w.
3: Enthalpy is denoted by H. The formula is $H = U + PV$
Where, U is the internal energy of the system.
     P is the pressure of the system.
     V is the volume of the system.
4: For the second part, if the system performs work on the surroundings, the internal energy of the system decreases. This implies that the system will use its own energy to do work and as a result, there is a decrease in internal energy. Internal energy is given as: $\Delta U = q + w$,
where, q is heat, w is work and ∆U= change in internal energy of the system.
5: Since in the problem it is given as work is done by the system, w is negative. (-w) We put it in the formula and it becomes: $\Delta U = q - w$
6: Thus, negative value of work (-w) signifies that energy has left the system as work. So, the internal energy of the system decreases.

Note:
The above solution can be easily explained by understanding the 1^st law of Thermodynamics. The energy is converted in the form of heat and work. It can neither be created nor destroyed. It can only be converted from one form to another.