Ap Chem Problem

Question

For the boiling of methanol, CH3OH, ∆H = 37.6 kJ/mol and ∆S = 111 J/mol·K.

CH3OH (l) → CH3OH (g)

(a) (i) Why is the ∆H value positive for this process?

(ii) Why is the ∆S value positive for this process?

(b) At what temperature, in °C, does this process become spontaneous (known as the boiling point)?

Answer 1

(a)

i. ∆H is positive because heat is being added to the system in order to boil the methanol. Adding heat is represented by a positive value for ∆H (endothermic)

ii. ∆S is positive because you are going from a liquid to a gas during this phase change and a gas has a higher entropy than does a liquid. This makes ∆S positive.

(b)

For the reaction to be spontaneous, ∆G has to be negative, and ∆G = ∆H - T∆S

If we set ∆G = 0, we have 0 = 37.6 kJ/mol -T(0.111 kJ/mol) ... note the change in units for ∆S to agree with ∆H

0 = 37.6 - 0.111T

T = 339K - 273 = 65.7ºC