Question

3. At equilibrium, the free energy change, ΔGΔGDelta G, of any system is zero:

ΔG=0=ΔH−TΔSΔG=0=ΔH−TΔS

So, under equilibrium conditions, this equation can be rearranged as

T=ΔHΔST=ΔHΔS

This form of the equation can be used to calculate the temperature at which the equilibrium occurs. Since a phase change represents an equilibrium between two phases, melting and boiling points can be calculated by this method.

Part A

Glacial acetic acid is the concentrated form of acetic acid, the acid in vinegar. The term "glacial" refers to the appearance of the solid form, which resembles ice-like crystals. What is the melting point of this compound, in degrees Celsius, based on its thermodynamic data shown here?

ΔHfusΔHfusDelta H_fus (kJ/molkJ/mol)	ΔSfusΔSfusDelta S_fus [J/(K⋅mol)J/(K⋅mol)]
24.32	83.93

Express the melting point numerically in degrees Celsius.

Part B

Based on the following thermodynamic data, calculate the boiling point of ethanol in degrees Celsius.

Substance	ΔH∘fΔHf∘Delta H^{\circ}_f (kJ/molkJ/mol)	S∘fSf∘S^{\circ}_f [J/(K⋅mol)J/(K⋅mol)]
C2H5OH(l)C2H5OH(l)	−−277.7	160.6
C2H5OH(g)C2H5OH(g)	−−235.1	282.6

Express the boiling point numerically in degrees Celsius.

Answer 1