The reaction quotient Q tells chemists how a reacting mixture compares to equilibrium at a particular moment. It uses the same concentration or pressure pattern as the equilibrium constant K, but it can be calculated before the reaction has reached equilibrium. This makes Q useful for predicting whether the reaction will shift toward products, shift toward reactants, or already be at equilibrium.
Comparing Q and K is like using a chemical decision scale.
Key Facts
- For aA + bB ⇌ cC + dD, Qc = [C]^c[D]^d / ([A]^a[B]^b).
- For gases, Qp = (PC)^c(PD)^d / ((PA)^a(PB)^b), where P means partial pressure.
- If Q < K, the reaction shifts forward toward products.
- If Q > K, the reaction shifts backward toward reactants.
- If Q = K, the system is at equilibrium and there is no net shift.
- Pure solids and pure liquids are not included in Q or K expressions.
Vocabulary
- Reaction quotient
- The reaction quotient Q is a ratio of product amounts to reactant amounts at any moment during a reaction.
- Equilibrium constant
- The equilibrium constant K is the value of the reaction quotient when the forward and reverse reaction rates are equal.
- Equilibrium
- Equilibrium is the state in which reactants and products are still reacting but their concentrations no longer change.
- Shift
- A shift is the direction a reaction proceeds to reduce a disturbance and move toward equilibrium.
- Partial pressure
- Partial pressure is the pressure a single gas in a mixture would exert if it occupied the container alone.
Common Mistakes to Avoid
- Using reactants over products in the Q expression is wrong because Q follows products over reactants, each raised to its coefficient.
- Including pure solids or pure liquids in Q is wrong because their activities are effectively constant and are left out of the expression.
- Assuming Q is always equal to K is wrong because Q can be calculated at any time, while K applies only at equilibrium.
- Reversing the shift prediction is wrong because Q < K means too few products, so the reaction shifts forward, while Q > K means too many products, so it shifts backward.
Practice Questions
- 1 For N2(g) + 3H2(g) ⇌ 2NH3(g), write the expression for Qp and calculate Qp if PN2 = 2.0 atm, PH2 = 1.5 atm, and PNH3 = 0.60 atm.
- 2 For H2(g) + I2(g) ⇌ 2HI(g), Kc = 50.0 at a certain temperature. If [H2] = 0.20 M, [I2] = 0.10 M, and [HI] = 0.50 M, calculate Qc and predict the direction of shift.
- 3 A reaction mixture has Q greater than K. Explain what this says about the relative amounts of products and reactants, and describe how the system will change as it moves toward equilibrium.