Question

(assume all thermodynamic quantities refer to the system, except where indicated)

Carbon dioxide sublimes at -78.5 degrees C at 1 bar pressure. When 1 mole of solid carbon dioxide is converted gas at 0 degrees C and 1 bar, which of the following quantities will be POSITIVE? CHOOSE ALL THAT APPLY: w, q, delta S, delta S_surr , delta S_total , delta G, delta H.

Answer 1

Sublimation is a process where a solid is directly converted into it's vapour phase. We know that solids have molecules in a packed form and in gases the molecules move on randomly. Randomness is measured by a factor termed as entropy of the system. Higher the disorderliness, higher is the entropy. S_gas>>S_solid

So clearly S_sys= S_g - S_s >0 so, positive.

Now, we know that phase transitions are reversible processes. For reversible processes S_total = 0.

S_total = S_surr + S_sys = 0

So, S_surr = -S_sys

therefore, S_surr < 0. It is negative.

Now talking about the workdone, when work is done by the system, workdone is negative. w = -P_ex* dV

volume of gases >> volumes of solid

So, w is negative.

Talking about enthalpy, phase transition from solid to vapour needs heat. So the process is endothermic in nature. For endothermic processes H>0.

H and S are related by H_transition/T_transition = S_transition

q is the heat that is transferred from the surroundings to the system. In an endothermic process heat is given to the system. So, q is positive.

Now Change in Gibbs free energy.

For spontaneous reactions or processes, G <0

The process of sublimation is spontaneous too. So, G is negative.

Have a good day!