How do I approach these chemistry questions about reactions?

Question

- In the following reaction:

Ni2 (aq) 6NH3(aq) -> [Ni(NH3)_6]^2 ∆Gº = – 49.9 kJ/mol (298K)

I am asked:

If a mixture of the species, each with a concentration of 2M is allowed to equilibrate, will more or less NH3 be produced? Will the non-standard ΔG be positive or negative under these initial concentration conditions (2 M each)?

- How do I interpret this question? It is also given that a mixture of this reaction at 298K is found to have a non-standard ΔG value equal to 50 J, though I do not know whether it is relative.

And another one: You are given a potential energy diagram with reaction progress vs PE (G) for two reactions/routes of reaction. One is faster and one is slower. It is asked: at equilibrium, which product will be higher in concentration? Is the product with the higher concentration the one for the reaction which has a faster rate?

Thanks.

Answer 1

∆G° = -RTlnKeq; Substituting,

-49,900 J/mol = - (8.314 J/K*mol) (298 K) lnKeq; Solving,

Keq = 5.6x10^8.

Keq is a huge number, so the reaction will proceed to the right, consuming the NH3. Since equimolar amounts of reagents were mixed, Ni++ is in excess.

For the second question, remember ∆G = ∆G° + RTlnQ, where Q is the reaction quotent. Given ΔG = 50 J/mol, we know:

1) Since ΔG doesn't equal 0, the reaction mixture is not in equilibrium

2) Since ΔG > 0, the reaction is not spontaneous in the direction of the equation. In other words, the reaction would proceed in the opposing direction.

3) We have enough data to calculate Q if you wish, just substitute.

Dynamic reaction equilibria means that the rates of the opposing reactions are equal. Since rate = k [reactants], we can write:

kf [reactants] = kr [products],

where kf and kr are the rate constants for the forward and reverse reactions. If kf > kr, then [products] > [reactants] to keep the rates equal, and vice versa. There will be more reagents on the side of the slower reaction, which would have a larger activation energy Ea.