From the values of H and S, predict which of the following reactions would be sp

Question

From the values of H and S, predict which of the following reactions would be spontaneous at 21°C:

reaction A: H = 10.5 kJ/mol, S = 30.0 J/K·mol;

spontaneous

nonspontaneous

impossible to tell

reaction B: H = 1.8 kJ/mol, S = 113 J/K · mol,

spontaneous

nonspontaneous

impossible to tell

If either of the reactions is nonspontaneous, can it(they) become spontaneous?

yes, reaction A can become spontaneous

yes, reaction B can become spontaneous

yes, both reactions can become spontaneous

no, neither reaction can become spontaneous

If either of the reactions is nonspontaneous but can become spontaneous, at what temperature might it become spontaneous?

??????°C

Explanation / Answer

1)

reaction A:

delta H = 10.5KJ/mol

delta S = 30.0J/mol.k = 0.0300 KJ/mol.K

T = 21.0 oC =(21.0+ 273) K = 294.0 K

use:

delta G = delta H - T*delta S

delta G = 10.5 - 294.0 * 0.0300

delta G = 1.7 KJ/mol

Since delta G is positive, the reaction A is non spontaneous

2)

reaction B:

delta H = 1.8KJ/mol

delta S = -113.0J/mol.k = -0.1130 KJ/mol.K

T = 21.0 oC =(21.0+ 273) K = 294.0 K

use:

delta G = delta H - T*delta S

delta G = 1.8 - 294.0 * (-0.1130)

delta G = 35.0 KJ/mol

Since delta G is positive, the reaction A is non spontaneous

3)

reaction A can be made spontaneous by increasing Temperature

Answer: yes, reaction A can become spontaneous

4)

use:

delta G = delta H - T*delta S

0 = 10.5 - T * 0.0300

T = 350 K

= (350 - 273) oC

= 77 oC

Answer: 77 oC

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