## Expression for Q (Homework 14.37)

$E_{cell} = E_{cell}^{\circ}-\frac{RT}{nF}\ln Q$

Lily Guo 1D
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### Expression for Q (Homework 14.37)

I'm a little confused about how to calculate Q. I thought that the expression for Q was [product A]^a/[reactant B]^b using either the concentrations of the reactant and product or the partial pressures of the reactant and product. However, when doing 14.37a and c, the solutions manual combines the concentrations and pressures. In 14.37a, the concentrations and pressures are multiplied by each other in the numerator and in the denominator. In 14.37c, the pressures are multiplied by each other in the numerator and the concentrations are multiplied by each other in the denominator (hopefully that made sense). Why is Q not calculated using the given concentrations alone? How do you calculate Q for the Nernst equation?

Chem_Mod
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### Re: Expression for Q (Homework 14.37)

I would ask your TA or Dr. Lavelle to be sure, but this is what was answered in a previous post:

Chem_Mod wrote:The exact reasons for this are quite technical and relate to the concept of standard states. In the end, if you want to use a "mixed" equilibrium expression, containing both concentrations and pressures, then the pressures must be in bars, and concentrations in molarity.

Return to “Appications of the Nernst Equation (e.g., Concentration Cells, Non-Standard Cell Potentials, Calculating Equilibrium Constants and pH)”

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