Question

For each process, state whether ΔG_sys, ΔH_sys, and ΔS_sys are positive, negative or zero.

a. An ideal gas is compressed in one step from 1 atm to 5 atm a constant temperature.

b. Water freezes at its normal freezing point.

c. Benzene dissolves in toluene.

Answer 1

a. An ideal gas is compressed in one step from 1 atm to 5 atm a constant temperature.

Compression means that work is being done ON the system.

Thus, dG = +ve             ; dH = +ve                    ; dS = -ve

+dG means that the process (freezing) is non-spontaneous

The +ve sign, of +dH indicates that the system gains heat/energy.

The -ve sign of dS indicates that the gas has lower entropy i.e. freedom to move because compression causes the molecules to come closer and reduce the available volume for movement.

b. Water freezes at its normal freezing point.

dG = 0 ; dH = -ve         ; dS = -ve

dG = 0 means that the solid and liquid phase exist at equilibrium at normal freezing point

The -ve sign, of -dH indicates that the system loses heat/energy.

The -ve sign of dS indicates that the ice has lower entropy because H-bonding reduces relative movement of water molecules in solid phase.   

c. Benzene dissolves in toluene.

dG = -ve           ; dH = 0            ; dS = +ve

-dG means that the process (mixing of benzene and hexane) is spontaneous

dH = 0 means enthalpy change

The +ve sign of dS indicates that the mixed solution has greater randomness than the pure solvents alone. It is because the solvent molecules have greater volume for movement (1 vol benzene + 1 volume hexane = 2 volume solution).