Chemistry • Year 12 • Module 5 • Lesson 5
Le Chatelier’s Principle: Concentration & Temperature
Secure the principle statement, the four concentration rules, the temperature–Keq relationship, and the key investigation observations before moving to harder application tasks.
1. Term–definition match
Write the matching term from this list in the right-hand column: Le Chatelier’s Principle, equilibrium position, Keq (equilibrium constant), concentration disturbance, temperature disturbance, exothermic reaction, endothermic reaction, forward reaction, reverse reaction, dynamic equilibrium. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The statement that a system at equilibrium shifts to minimise the effect of any disturbance. | |
| 1.2 | A qualitative description of whether products or reactants are favoured at equilibrium. | |
| 1.3 | The ratio of product to reactant concentrations raised to their stoichiometric powers at equilibrium; unchanged by concentration changes. | |
| 1.4 | Adding or removing a dissolved or gaseous species while keeping temperature constant. | |
| 1.5 | The only type of disturbance that changes the numerical value of Keq. | |
| 1.6 | A reaction that releases heat to the surroundings (ΔH < 0). | |
| 1.7 | A reaction that absorbs heat from the surroundings (ΔH > 0). | |
| 1.8 | The reaction written left to right as the equation is given. | |
| 1.9 | The reaction written right to left; opposite of the forward direction. | |
| 1.10 | A state where the forward and reverse rates are equal and concentrations are constant, though both reactions still proceed. |
2. True or false — with correction
Circle T or F. If false, write the corrected statement on the line. 12 marks (1 T/F + 1 correction each)
2.1 Adding more reactant to an equilibrium system increases the value of Keq. T / F
2.2 Increasing temperature always shifts an equilibrium to the right. T / F
2.3 Le Chatelier’s Principle predicts the direction of equilibrium shift, not the magnitude or new equilibrium concentrations. T / F
2.4 For an exothermic forward reaction, decreasing temperature shifts the equilibrium to the left and decreases Keq. T / F
2.5 Removing a product from an equilibrium system causes a shift in the forward (right) direction. T / F
2.6 A catalyst shifts the equilibrium position to the right by speeding up the forward reaction more than the reverse. T / F
3. Fill-in-the-blank paragraph
Complete the passage using the word bank below. Each word is used once only. 10 marks (1 each)
Le Chatelier’s Principle states that when a system at equilibrium is disturbed, it shifts to (1) the effect of the disturbance. If a reactant is added ( (2) disturbance), the system shifts to the (3) to consume some of the added species. In this case, the value of Keq remains (4). However, if (5) is increased, Keq does change. For a forward reaction that is (6) (ΔH < 0), increasing temperature shifts the equilibrium to the (7), and Keq (8). For a forward reaction that is (9) (ΔH > 0), increasing temperature shifts the equilibrium to the right, and Keq (10).
4. Function recall — short prose
Answer each in 1–2 precise sentences using lesson vocabulary. 8 marks (2 each)
4.1 What is the function of Le Chatelier’s Principle in predicting the behaviour of an equilibrium system?
4.2 Why does a concentration change shift the equilibrium position but not change Keq?
4.3 Why does a temperature increase cause Keq to change, unlike a concentration change?
4.4 What is the function of adding silver nitrate (AgNO3) to an iron(III) thiocyanate equilibrium mixture in a laboratory demonstration?
5. Build a concept map
Draw labelled arrows between the six terms to show how they connect. Each arrow must carry a brief linking phrase (e.g. “shifts equilibrium to”, “changes value of”, “does not change”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: Le Chatelier’s Principle · concentration change · temperature change · equilibrium position · Keq · direction of shift.
Q1 — Term–definition match
1.1 Le Chatelier’s Principle • 1.2 equilibrium position • 1.3 Keq (equilibrium constant) • 1.4 concentration disturbance • 1.5 temperature disturbance • 1.6 exothermic reaction • 1.7 endothermic reaction • 1.8 forward reaction • 1.9 reverse reaction • 1.10 dynamic equilibrium.
Q2 — True / false with correction
2.1 False. Adding more reactant does not change Keq. Keq is only changed by temperature. The equilibrium position shifts right (more product), but the ratio of concentrations at the new equilibrium is the same Keq value.
2.2 False. Increasing temperature shifts the equilibrium in the endothermic direction. For an exothermic forward reaction (ΔH < 0) this means a shift to the left; for an endothermic forward reaction (ΔH > 0) this means a shift to the right.
2.3 True.
2.4 False. For an exothermic forward reaction, decreasing temperature shifts the equilibrium to the right (forward, exothermic direction) and increases Keq.
2.5 True.
2.6 False. A catalyst speeds up both the forward and reverse reactions equally. It does not shift the equilibrium position and does not change Keq. It only allows equilibrium to be reached more quickly.
Q3 — Fill-in-the-blank
(1) minimise • (2) concentration • (3) right • (4) unchanged • (5) temperature • (6) exothermic • (7) left • (8) decreases • (9) endothermic • (10) increases.
Q4.1 — Function of Le Chatelier’s Principle
Le Chatelier’s Principle predicts the direction in which an equilibrium system will shift when disturbed by a change in concentration, temperature or pressure. It does not predict the magnitude of the new concentrations — only the direction of shift toward the new equilibrium.
Q4.2 — Concentration change and Keq
A concentration change alters the relative rates of the forward and reverse reactions, causing the system to shift to a new equilibrium position where concentrations are different. However, the ratio of product to reactant concentrations (raised to stoichiometric powers) remains the same — Keq is only altered by temperature because temperature changes the thermodynamic landscape (activation energies) of both reactions by different amounts.
Q4.3 — Why temperature changes Keq
Temperature changes the Boltzmann distribution of molecular energies. Because the forward and reverse reactions have different activation energies, increasing temperature increases the rate of the endothermic reaction proportionally more than the exothermic reaction. This permanently alters the ratio of forward to reverse rates, so the equilibrium is re-established at a new Keq value — unlike a concentration change, which does not alter the ratio of activation energies.
Q4.4 — Function of adding AgNO3
AgNO3 provides Ag+ ions, which react with SCN− ions to form a white precipitate of AgSCN(s). This effectively removes SCN− (a reactant) from the equilibrium Fe3+(aq) + SCN−(aq) ⇌ FeSCN2+(aq), causing the equilibrium to shift left. The solution becomes paler as FeSCN2+ decomposes. Keq is unchanged.
Q5 — Concept map sample
Correct arrows include (any valid linking phrase accepted):
- Le Chatelier’s Principle → predicts → direction of shift
- direction of shift → determines new → equilibrium position
- concentration change → shifts → equilibrium position
- concentration change → does NOT change → Keq
- temperature change → shifts → equilibrium position
- temperature change → changes value of → Keq
Award 1 mark per correctly labelled arrow that respects causal direction (max 6).