When to Use Q

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When to Use Q

Postby KDang_1D » Tue Jan 21, 2020 7:59 pm

A reaction mixture is prepared by mixing 0.100 mol SO2, 0.200 mol NO2, 0.100 mol NO, and 0.150 mol SO3 in a reaction vessel of volume 5.00 L. The reaction SO2(g) 1 NO2(g) ∆ NO(g) 1 SO3(g) is allowed to reach equilibrium at 460 8C, when Kc 5 85.0. What is the equilibrium concentration of each substance?

Why do we need to calculate Q to determine the direction of the reaction, but we didn't for the problem in lecture, where we had a solution with 0.100 M nitrous acid and 0.150 potassium nitrate? (Week 2 Friday)

Hannah Lee 2F
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Re: When to Use Q

Postby Hannah Lee 2F » Tue Jan 21, 2020 8:38 pm

In the case of the lecture problem, I think it was assumed that the reaction would just proceed forward. However, unless you're working with acid-base equilibria related reactions, I think it's always safest to calculate Q to make sure that the direction of the reaction is correct, even if the problem doesn't explicitly state to.

Sally Qiu 2E
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Re: When to Use Q

Postby Sally Qiu 2E » Tue Jan 21, 2020 8:39 pm

wait so for this problem we had to make sure which direction it went in?
Last edited by Sally Qiu 2E on Tue Jan 21, 2020 8:54 pm, edited 1 time in total.

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Re: When to Use Q

Postby BNgo_2L » Tue Jan 21, 2020 8:40 pm

The quotient, Q, shows us the concentration of the reactants and products at any time while K shows us the unique concentrations for each when at equilibrium. Because reactions are in constant motion to reach equilibrium, you can use Q and compare to K to determine if the reaction will move forward or reverse.

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