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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 11

Lesson 2: Factors that affect chemical equilibrium

# Introduction to reaction quotient Qc

Introduction to the reaction quotient Qc, and comparing the reaction quotient with the equilibrium constant to predict how concentrations will change.

## Want to join the conversation?

• At around , the speaker states that having no reactants gives us a value of zero in the denominator, which makes the reaction quotient equal to infinity. Does that not violate a principle of mathematics: division by zero is undefined? Is this also how it is defined in chemistry, or was the infinity definition invented to make such a calculation possible in a chemistry context ?
• From the author:Great point, I think I misspoke in the video. We generally assume there is always at least some tiny bit (a molecule or two, perhaps) of the reactants left. That means the denominator is never equal to zero, it just gets very very very tiny so that the quotient approaches infinity but isn't undefined. I hope that helps!
• were did you get the concentrations from to get 4.3
• How did you get 4.3 for the first reaction?
• That value would've been determined through experimentation using varying concentrations of reactants.
(1 vote)
• Apart from acidic or basic medium, can we balance a redox reaction in a neutral medium?
• I really didn't get the difference between the formula for Kc and Qc
And how did you get Kc = 4.3?
• Consider the reaction: A + B <--> C.

Kc is the equilibrium constant and it equals [C]/[A][B] at equilibrium. But when you first mix A with B, the reaction won't be at equilibrium because there will be a lot of A and B, but very little C. If at this time you measure the concentrations of A, B and C and work out the value of [C]/[A][B] it won't equal Kc because the reaction is not at equilibrium - if fact, it will be smaller than Kc because the numerator will be small and the denominator will be big. We call the result of this calculation Qc.

As the reaction progresses towards equilibrium, the concentrations of A and B will decrease as more C is formed. If we keep measuring the concentrations of A, B and C, and calculating [C]/[A][B] (which we are calling Qc) then we will find that Qc gets closer and closer to Kc until, at equilibrium, Qc equals Kc.
• What will happen if inert gas is added at constant pressure, but increasing volume?
• Just to clarify, what if only one of the reactants gets completely used up?
Would the Keq be infinity? Or would we calculate it without taking the used up reactant into consideration?
(1 vote)
• If a reactant gets completely used up, then you don't have an equilibrium reaction!

This section is only relevant to reactions that don't go to "completion" — i.e. they reach a balance point where reactants and products both exist.

Does that help?