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# Standard change in free energy and the equilibrium constant

The relationship between standard Gibbs free energy change and the equilibrium constant K. Calculating K when you know the standard free energy of reaction.

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• I'm confused by the word "equilibrium". From the previous video it was said that at (delta)G = 0, the reaction is in equilibrium, But in this video, eventhought it question is asking for equilibrium constance, (delta)G = -33 kJ.
Well this is what I understand,
The "equilibrium" indicated by (delta)G = 0 is the equilibrium of spontaneity. It means by the energy and entropy of that environment, the reaction rate will be constant both forward and backward.
The "equilibrium" indicated by equilibrium constant K however, is the equilibrium of the concentration. It determines whether the reaction goes forward or backward depending on the qoutient (concentration of the products and reactants)
For instance,
ex1. the reaction has (delta)G = 0, the reaction still goes forward, if Q<K.
ex2. if Q < Kp (the reaction favours products) but (delta)G > 0, the reaction still doesn't occurs, since the reaction is not spontaneous. (unless, energy is added into the system)
ex3 if Q> Kp (the reaction favours reactants, but (delta)g<0 , the reaction occurs (spontaneous), but there are more reactants than products,

Am i understanding it right? please correct me if i'm wrong.
• Equilibrium means that there is no observed change in the concentrations of reactants or products. Think about it as when the reaction has completed, even though it isn't exactly like that. For Example, for the equation:
N2(g) + 3H2(g) --> 2NH3(g)
at Equilibrium, the Reaction might not stop occurring, but the concentrations of N2, H2, and NH3 stay relatively the same.
The K value tells how much the concentrations of products and reactants there are.
If K >1, there are more Products than Reactants in Equilibrium.
If K=1, there are the same amount of Products as Reactants in Equilibrium.
If K <1, there are more Reactants than Products in Equilibrium.
Q is basically what the value of the the ratio of Products to Reactants before Equilibrium is established. so if
Q<K, the Reaction shifts to the Right,
Q>K, the Reaction shifts to the Left,
Q=K, the Reaction is at Equilibrium.
Delta G comes into Play when figuring out if the Reaction is Spontaneous.
when delta G>0, the Reaction is non- Spontaneous, but if
delta G <0, the reaction is spontaneous.
When K<1, the reaction favors the Reactants, so the Reaction is not Spontaneous, making delta G >0. but when K >1, the Reaction favors the Products, so it is Spontaneous, making delta G< 0.
delta G = -RTlnK
R=8.31 Joules/(mole*Kelvin)
Hope that clears up some of your confusion!
• I thought dG0 was a constant for the reaction at 25C. To then find the new equilibrium at a new temperature you take e^(-dg0/(RT))... Why does dg0 change in the different examples for the same reaction?
• Why the ΔG0 has different values if the temperature is always T=298 K ?
• Thats because, ΔG = ΔH - TΔS
Even if T of environment is constant, but the change in the entropy is dependent on change in disorder. Hence the term TΔS is different for every reaction.
• What is Standard change in free energy?
• Is ΔG only applicable to reversible chemical reactions?
• ΔG applies to every reaction, but ΔG = 0 only for a reaction at equilibrium.
• I’m a bit confused about Kp and Kc, how can they be equal since isn’t the formula the Kp is Kp=Kc(RT)^delta N(N is the coefficient of the elements) and N is clearly not 0 here
• At he said "since we're dealing with gases, if you wanted to put in a Kp here you could." But because Kp and Kc aren't interchangeable (related by a factor of (RT)^∆n), how do you know if the value you got is Kp or Kc? I know the example given is with gases, but if I had denoted the K as Kc instead, it would supposedly be wrong, right? How is the K in the larger expression -RTlnK defined?
• Over here, the teacher uses the value of Qp instead of Qc to find the value of change in gibbs free energy, isnt that incorrect because Qp and Qc have different values here?
• You can use either Qp or Qc to find free energy. But you would only use Qp for reactions involving gases and Qc for reactions involving solutes dissolved in solution. It would only be incorrect if they used Qp to find free energy in a reaction where all the states were aqueous because, as you pointed out, Qp and Qc can have different values.

Hope that helps.
• Why would increased T cause delta G naught to increase? I figured it would be more - at greater T