Question

Predict whether the values of q; w; DU, and DH are positive, zero, or negative for each of the following processes: (a) melting of ice at 1 atm and 273 K, (b) melting of solid cyclohexane at 1 atm and the normal melting point, (c) reversible isothermal expansion of an ideal gas, and (d) reversible adiabatic expansion of an ideal gas

Answer 1

Answer: for (a) and (b); q= positive, w=positive, ∆U= negative, ∆H= positive.

(c) reversible isothermal expansion; q= negative ; w= negative, ∆U= zero, ∆H=zero

(d) adiabatic expansion: q= zero, ∆U= negative, w= negative, ∆H=negative

Explanation: for melting work is done on the system.

For isothermal changes, it occurs at constant temperature. q=∆E-w . At constant temperature, ∆E=0. Therefore q=w.

For adiabatic changes, it occurs such that there is no change in heat between the system and the surroundings. Hence, q=0. Which implies that ∆U= -w

The equation ∆U= -w simply means that in an adiabatic change any work done is at the expence of internal energy. An increase in workdone leads to a decrease in internal energy of a system.