Question

15. The entropy change for a certain nonspontaneous reaction at 50 °C is 140 J/K.
(a) Is the reaction endothermic or exothermic?
(b) What is the minimum value of ΔH (in kJ) for the reaction?

16. What is the sign of ΔS in each of the following?
(a) Freezing of water at 2 °C
(b) Corrosion of iron metal
(c) Expansion of a gas to fill the available volume
(d) Separation of an unsaturated aqueous solution of potassium chloride into solid KCl and liquid water

Answer 1

15) the reaction is non-spontaneous at 50C

delta S= 140J/K

a) The reaction is endothermic as the delta S is positive , at low temperture of 50C , the reaction is non-spontaneous. That shows delta H is also positive so that at low temperatures delta G is positive.

b)we know delta S = delta H /T

Thus delta H = T. delta S

= 323K x 140J /K

=45.22kJ

The minimum delta H for this reaction is 45.22kJ

16)

a) freezing of water at 2 C

delta S is negative as liquid changes to solid

b) corrosion of iron metal

delta S is positive  

Iron reacts with oxygen to give ironoxide which is formed randomly on the surface.

c)delta S positive

expansion of gas to fill availble volume.

more space, more randomness, more entropy

d) Delta S is negative

As separation of Kclsolution int o solide KCl and liquid h2O decreases randomness, entropy decreases.