Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

5.3.5Additional lessons

The Reaction Quotient (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 113 of 172

📚 IB Chemistry HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 5What Q Is

5.3.5 The Reaction Quotient (HL)

Q tells you how far a mixture is from equilibrium and in which direction it must move.

Definition
  • The reaction quotient Q has the same form as the Kc expression but uses the concentrations present at any moment.
Any time
  • It can be calculated at any stage of a reaction, whether or not the system is at equilibrium.
Same equation
  • Q uses the same powers and the same layout as K, with products in the numerator.
At equilibrium
  • When the mixture is at equilibrium Q is equal to K.

Part 2 of 5Comparing Q with K

Comparing Q with K
Q less than K
  • There are too many reactants compared with products, so the net reaction goes forward (to the right).
Q equal to K
  • The system is at equilibrium and there is no net change in concentrations.
Q greater than K
  • There are too many products compared with reactants, so the net reaction goes backward (to the left).
Link to Le Chatelier
  • Adding a reactant makes Q smaller than K, so the position shifts right until Q rises back to K.
ComparisonDirection of net reaction
Q < KForward (to the right)
Q = KNone, at equilibrium
Q > KReverse (to the left)

Part 3 of 5Using Q

1
Write the equilibrium expression for the reaction and substitute the concentrations of the mixture to calculate Q.
2
Compare the numerical value of Q with K at the same temperature.
3
Decide the direction: if Q is below K the mixture needs more products, so the forward reaction dominates.
4
As the reaction proceeds the value of Q moves towards K until the two are equal.
5
Remember that Q can change with concentration at constant temperature, while K stays fixed.

Part 4 of 5Worked Example

Question

  • For H₂(g) + I₂(g) ⇌ 2HI(g), Kc = 50 at a certain temperature.
  • A mixture contains [H₂] = 0.50, [I₂] = 0.50 and [HI] = 2.0 mol dm⁻³.
  • In which direction will it move?

  1. Q = [HI]² / ([H₂][I₂]) = (2.0)² / (0.50 × 0.50).
  2. Q = 4.0 / 0.25 = 16.
  3. 16 is less than K = 50, so there are too many reactants compared with equilibrium.

Answer Q < K, so the net reaction goes forward to form more HI

Part 5 of 5Exam Checklist & Tips

5.3.5 – Key Points for the Exam

  • Reaction quotient (Q) The ratio of product to reactant concentrations at any point, calculated with the K expression.
  • Equilibrium constant (K) The value of Q when the system is at equilibrium.
  • Net reaction The overall direction in which a reaction mixture moves, forward or reverse.
  • Disturbance A change in concentration, pressure or temperature applied to a system at equilibrium.

Common Mistakes

  • Believing that Q must always equal K.
  • Using Q > K to predict that the forward reaction proceeds.
  • Using concentrations of pure solids in the Q expression.
  • Comparing Q and K at different temperatures.

Last updated:

\n