Approximately 467 K. ∆G=∆H-T∆S and a rxn proceeds spontaneously when ∆G < 0
and is non-spontaneous when ∆G > 0. So if we set ∆G=0 and solve the equation for T, we will see that the crossover from spontaneous to non-spontaneous occurs when T=467K.

If Q is greater than K, the reaction has exceeded the equilibrium state. It will proceed nonspontaneously (since equilibrium has already been reached), and this must mean that the ΔG (gibbs free energy) must be positive, or greater than zero. 

A reaction is spontaneous if the Gibb's Free Energy of the reaction is negative.

If , the enthalpy, and , the entropy, are both negative, then the reaction will be spontaneous if and only if the magnitude of the enthalpy is greater than the magnitude of the entropy times the temperature.

The Gibb's free energy equation is used to determine the spontaneity of a reaction and is written as follows: .

is Gibb's free energy,  is enthalpy, and  is entropy. In order for a reaction to be spontaneous, Gibb's free energy must have a negative value. Based on the equation, we can see that a positive enthalpy in combination with a negative entropy will always result in a positive value for Gibb's free energy.

This means these are the conditions that will always result in a nonspontaneous reaction. 

A galvanic cell results in a positive cell potential from a spontaneous reaction. Spontaneous reactions always have a negative Gibb's free energy.

An equilibrium constant greater than one would indicate that the equilibrium concentration of products is greater than the equilibrium concentration of reactants, consistent with a spontaneous reaction.

You can find the Gibb's free energy of a galvanic cell by using the following equation:

 is the number of moles of electrons that are transferred in the reaction,  is Faraday's constant, and  is the potential of the cell.

We are given the constant value and the cell potential. The moles of electrons transferred is equal to the change in charge on the atoms in the reaction. In this reaction, copper is reduced from a charge of to zero, requiring that is gained two moles of electrons.

Using these values, we can find Gibb's free energy for the cell.

Notice that the value is negative. Galvanic cells are spontaneous, and will have negative Gibb's free energies at standard conditions.

If Q is less than K, then the reaction has not yet reached the equilibrium state. It will proceed spontaneously in the forward direction. Since it is proceeding spontaneously in the forward direction, this must mean that the ΔG (Gibbs free energy) must be negative, or less than zero.

Enthalpy and temperature are necessary when mathematically evaluating Gibbs free energy, but the reaction quotient and equilibrium constant provide enough information to qualitatively determine that the reaction is proceeding spontaneously.

Based on Gibbs Free Energy, ∆G = ∆H – T∆S, we find that ∆H– will contribute to spontaneity (∆G<0). Since we are looking at temperatures, in order for ∆S to make an nonspontaneous reaction spontaneous at only high temperatures, ∆H will be positive, and ∆S will be positive.

Given the equation for free energy, (delta)G = (delta)H-T(delta)S, we can determine that the reaction is nonspontaneous at all temperatures if H is positive and S is negative. This combination would always lead to a positive G value, meaning that free energy is required for the reaction to take place and it is therefore nonspontaneous. 

Change in free energy must always be negative for a spontaneous process. Additionally, Q must be less than K so that the reaction will proceed in the forward reaction, toward equilibrium.