Reaction Quotient (Q)
Compare Q with K to predict shift
Definition
For aA + bB ⇌ cC + dD:
Q = ([C]^c [D]^d) / ([A]^a [B]^b)
Same form as Kc but using current (not equilibrium) activities/concentrations. For gases, use partial pressures to write Qp.
Direction Prediction
- Q < K: reaction shifts right (toward products)
- Q = K: at equilibrium
- Q > K: reaction shifts left (toward reactants)
Example
Given: A ⇌ B, K = 2.0; at some time [A]=0.50 M, [B]=0.60 M.
Q = [B]/[A] = 0.60/0.50 = 1.2; Q < K so reaction proceeds to the right.
Answer: Shifts toward products
FAQ
Do solids or liquids appear in Q?
No; activity is 1 for pure solids/liquids. Include only species with variable activity.
How does stoichiometry affect Q?
Exponents in Q come from stoichiometric coefficients in the balanced equation.
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