## K and Q and G

$\Delta G^{\circ}= \Delta H^{\circ} - T \Delta S^{\circ}$

$\Delta G^{\circ}= -RT\ln K$

$\Delta G^{\circ}= \sum \Delta G_{f}^{\circ}(products) - \sum \Delta G_{f}^{\circ}(reactants)$

MaryBanh_2K
Posts: 101
Joined: Wed Sep 18, 2019 12:21 am

### K and Q and G

I understand that reaction quotient (Q) measures the relative amounts of products and reactants at any time in the reaction at any particular point and equilibrium constant (K) describes the reaction at equilibrium. I also know that K and Q can be compared so that we can tell which way the reaction is shifting. However, in terms of the Gibbs Free Energy equation, why do we use K and Q? Can someone explain this three-way relationship for these three variables conceptually?

Amy Pham 1D
Posts: 103
Joined: Fri Aug 09, 2019 12:15 am

### Re: K and Q and G

Perhaps this question can be revisited after we have covered this portion of the Gibb's Free Energy topic, but Dr. Lavelle actually hasn't covered those versions of the Gibb's Free Energy equation yet. The only one he has gone over so far has been the deltaG= deltaH - T*deltaS.

ThomasNguyen_Dis1H
Posts: 102
Joined: Fri Aug 30, 2019 12:17 am

### Re: K and Q and G

When the reaction is not at equilibrium, deltaG = deltaG(naught) + RTlnQ. There is a difference between deltaG and deltaG(naught). When the system is at equilibrium then delta G would equal 0 and the equation would be deltaG(naught)=-RTlnK. So depending on what state the system is, depends on what equation would be used.

AKhanna_3H
Posts: 104
Joined: Wed Sep 18, 2019 12:19 am

### Re: K and Q and G

If the system is at equilibrium, you can replace Q with K in the equation.

AronCainBayot2K
Posts: 101
Joined: Fri Aug 30, 2019 12:17 am

### Re: K and Q and G

Q refers to when the reaction given is not at an equilibrium where as K would mean the reaction is at equilibrium. Usually, Q can = K such that delta g would be 0 and delta g naught would be -RT ln K at equilibrium.

Ruth Glauber 1C
Posts: 100
Joined: Wed Sep 18, 2019 12:20 am

### Re: K and Q and G

Only if the system is at equilibrium can replace Q with K in this equation.

Ying Yan 1F
Posts: 101
Joined: Fri Aug 02, 2019 12:16 am

### Re: K and Q and G

In equilibrium, the activities constant Q is equivalent to the equilibrium constant K, that is why, when a reaction is at equilibrium, when can state that Q=K and replace Q in the equation delta G= -RTlnQ with K: delta G=-RTlnK. I hope this helps!