5.3.5Additional lessons
The Reaction Quotient (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 113 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5What Q Is
Q tells you how far a mixture is from equilibrium and in which direction it must move.
Definition
- Definition The reaction quotient Q has the same form as the Kc expression but uses the concentrations present at any moment.
Any time
- Any time It can be calculated at any stage of a reaction, whether or not the system is at equilibrium.
Same equation
- Same equation Q uses the same powers and the same layout as K, with products in the numerator.
At equilibrium
- At equilibrium When the mixture is at equilibrium Q is equal to K.
Section 2 of 5Comparing Q with K
Comparing Q with K
Q less than 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
- Q equal to K The system is at equilibrium and there is no net change in concentrations.
Q greater than K
- 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
- Link to Le Chatelier Adding a reactant makes Q smaller than K, so the position shifts right until Q rises back to K.
| Comparison | Direction of net reaction |
|---|---|
| Q < K | Forward (to the right) |
| Q = K | None, at equilibrium |
| Q > K | Reverse (to the left) |
Section 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.
Section 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?
- Q = [HI]² / ([H₂][I₂]) = (2.0)² / (0.50 × 0.50).
- Q = 4.0 / 0.25 = 16.
- 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
Section 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.
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