Question

6. From the values of ΔH and ΔS, predict which of the following reactions would be spontaneous at 25ºC: Reaction A: ΔH = 10.5 kJ/mol, ΔS = 30 J/K ∙ mol Reaction B: ΔH = 1.8 kJ/mol, ΔS = –113 J/K ∙ mol If any of the above reactions is nonspontaneous at 25ºC, at what temperature might it become spontaneous? (16 points)

Answer 1

Both reaction A and reaction B are non spontaneous.

Step-by-step explanation:

For a spontaneous reaction, change in gibbs free energy (
`\Delta G`) should be negative.

We know,
`\Delta G=\Delta H-T\Delta S`, where T is temperature in Kelvin scale.

Reaction A:
`\Delta G=(10.5* 10^(3))-(298* 30)J/mol=1560J/mol`

As
`\Delta G` is positive therefore the reaction is non-spontaneous.

If at a temperature T K , the reaction is spontaneous then-

`\Delta H-T\Delta S< 0`

or,
`T> (\Delta H)/(\Delta S)`

or,
`T> (10.5* 10^(3))/(30)`

or,
`T> 350`

So at a temperature greater than 350 K, the reaction is spontaneous.

Reaction B:
`\Delta G=(1.8* 10^(3))-(-113* 298)J/mol=35474J/mol`

As
`\Delta G` is positive therefore the reaction is non-spontaneous.

If at a temperature T K , the reaction is spontaneous then-

`\Delta H-T\Delta S< 0`

or,
`T> (\Delta H)/(\Delta S)`

or,
`T> (1.8* 10^(3))/(-113)`

or,
`T> -16`

So at a temperature greater than -16 K, the reaction is spontaneous.