Question

Which one of the following (unbalanced) equations describes a spontaneous reaction?

a.) Au(s) + HCl(aq) → AuCl(s) + H2(g)

b.) Hg(ℓ) + Al3+(aq) → Hg2+(aq) + Al(s)

c.) Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)

d.) Ag(s) + H2O(ℓ) → Ag(OH)2(s) + H2(g)

e.) None of the above describes a spontaneous reaction.

Please show and explain work

Answer 1

Condition of spontaneous reaction :-

1) reaction should be exothermic.

2) entropy should be increase.

3) energy should be decrease.

Condition of non - spontaneous rection :-

1) reaction should be endothermic.

2) entropy should be decrease.

3) energy should be increase.

Entropy :- randomness of molecules present in a system of a given reaction.

Gas has highest entropy.

Liquid has less entropy than gas.

Solid has no entropy, or negligible entropy.

Only on the basis of entropy we can easily find, which reaction is spontaneous or which is non - spontaneous. Lets start -

In given reaction :-

a) solid and aqueous (liquid) goes to solid and gas.

Hence entropy increase so spontaneous reaction.

b) liquid and aqueous ( liquid ) goes to aqueous (liquid) and solid.

Hence entrooy decrease so non- spontaneius reaction.

c) liquid and solid goes to solid and liquid.

Hence entropy neither increase nor decrease. So equilibrium reaction.

d) solid and liquid goes to solid and gas.

Hence entropy increase, so spontaneous reaction.

Therfore from the above conclusion we can say that reaction (a) & (d) are spontaneous reaction.