Chemistry • Year 12 • Module 5 • Lesson 12
Reaction Quotient Q
Lock in the definition of Q, the Q vs Keq decision rule, and the key vocabulary before moving to calculation and application.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: reaction quotient (Q), equilibrium constant (Keq), current concentrations, equilibrium concentrations, Q < Keq, Q > Keq, Q = Keq, snapshot, shift right, shift left. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The ratio of product concentrations raised to stoichiometric powers divided by reactant concentrations raised to stoichiometric powers, calculated using the concentrations present at any given moment—not necessarily at equilibrium. | |
| 1.2 | The same expression as Q but evaluated exclusively using concentrations measured after the system has reached equilibrium; it is temperature-dependent and constant for a given reaction at a given temperature. | |
| 1.3 | The concentrations of reactants and products at the particular instant when Q is calculated; they change as the reaction proceeds. | |
| 1.4 | The concentrations of reactants and products once the forward and reverse reaction rates have become equal and no further net change in composition occurs. | |
| 1.5 | The Q vs Keq relationship that means too few products exist relative to equilibrium—the system must produce more products. | |
| 1.6 | The Q vs Keq relationship that means too many products exist relative to equilibrium—the system must convert products back to reactants. | |
| 1.7 | The Q vs Keq relationship that means the system is already at equilibrium and no net composition change will occur. | |
| 1.8 | The informal name given to Q to emphasise that it describes the instantaneous composition of a mixture rather than the equilibrium destination. | |
| 1.9 | The direction of net reaction when Q < Keq: the forward reaction is favoured so product concentrations increase. | |
| 1.10 | The direction of net reaction when Q > Keq: the reverse reaction is favoured so reactant concentrations increase. |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line below. 10 marks (1 for T/F, 1 for correction where needed)
2.1 Q and Keq use different algebraic expressions—Q uses stoichiometric powers of concentrations, while Keq uses mole fractions. T / F
2.2 When a system is at equilibrium, Q equals Keq by definition. T / F
2.3 If Q < Keq, the reaction will shift to the left to reach equilibrium. T / F
2.4 Solids and pure liquids are excluded from the expression for Q for the same reason they are excluded from Keq. T / F
2.5 When a reaction begins from pure reactants only, Q = ∞ and the system shifts left. T / F
3. Fill in the blanks — the Q story
Complete the passage using the word bank below. Each word is used once. 8 marks
The reaction quotient Q is calculated using the concentrations of all species in a reaction mixture—whatever they happen to be at that moment. This is why Q is sometimes called a of the system: it captures the composition right now, not at the end point. By contrast, Keq is the that the system is travelling toward. When Q < Keq, there are too few products, so the reaction shifts to the . As it does so, product concentrations increase and reactant concentrations decrease, which means Q toward Keq. When Q > Keq, there are too many products, so the reaction shifts to the . Q then until it equals Keq. When Q = Keq, the system is at and no further net change in concentrations occurs.
4. Function recall
Answer each question in 1–2 sentences using precise lesson terms. 8 marks (2 each)
4.1 What is the function of Q in predicting the direction of an equilibrium shift?
4.2 What does it mean, in terms of concentrations in the Q expression, when Q > Keq?
4.3 Why does adding a reactant to an equilibrium system cause Q to decrease?
4.4 Engineers at Incitec Pivot’s fertiliser plant monitor Q in real time during the Haber process (N2 + 3H2 ⇌ 2NH3). Why is Q more useful than Keq for moment-to-moment reactor control?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a brief linking phrase (e.g. “is compared to”, “determines”, “approaches”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: Q • Keq • current concentrations • equilibrium concentrations • direction of shift • equilibrium
Q1 — Term–definition matches
1.1 reaction quotient (Q) • 1.2 equilibrium constant (Keq) • 1.3 current concentrations • 1.4 equilibrium concentrations • 1.5 Q < Keq • 1.6 Q > Keq • 1.7 Q = Keq • 1.8 snapshot • 1.9 shift right • 1.10 shift left
Q2 — True / false with correction
2.1 False. Q and Keq use exactly the same algebraic expression (products to the power of stoichiometric coefficients divided by reactants to the power of their coefficients). The only difference is which concentrations are substituted: current concentrations for Q; equilibrium concentrations for Keq.
2.2 True.
2.3 False. Q < Keq means the reaction shifts to the right (forward direction) to produce more products and increase Q toward Keq.
2.4 True. Pure solids and pure liquids have fixed activities equal to 1 and are excluded from both Q and Keq expressions by the same convention.
2.5 False. When starting from pure reactants only, [products] = 0, so the numerator of Q is 0, giving Q = 0 (not infinity). Q = 0 < any positive Keq, so the system shifts right. Q = ∞ applies when starting from pure products (no reactants → denominator = 0).
Q3 — Cloze paragraph
current • snapshot • destination • right • increases • left • decreases • equilibrium
Q4.1 — Function of Q
Q is used to predict which direction a reaction must shift to reach equilibrium. By comparing Q to Keq, a chemist can determine whether the current mixture has too few products (Q < Keq → shift right), too many products (Q > Keq → shift left), or is already at equilibrium (Q = Keq).
Q4.2 — Q > Keq meaning
Q > Keq means the numerator of the Q expression (product concentrations raised to stoichiometric powers) is currently too large relative to the denominator (reactant concentrations raised to stoichiometric powers) compared with the equilibrium ratio. There are too many products relative to equilibrium, so the system shifts left (reverse direction) to reduce product concentrations and increase reactant concentrations until Q = Keq.
Q4.3 — Adding a reactant decreases Q
Reactants appear in the denominator of the Q expression. Adding a reactant increases its concentration, which increases the denominator, so Q (= numerator ÷ denominator) decreases. This gives Q < Keq, so the system shifts right.
Q4.4 — Q vs Keq for reactor control
Keq is a fixed constant at a given temperature and only tells engineers what the final equilibrium composition will be—it gives no information about where the current reactor mixture stands. Q, calculated from the actual current concentrations in the reactor at any moment, tells engineers whether the mixture is product-deficient (Q < Keq, need to push right) or product-rich (Q > Keq). This real-time information allows them to adjust feed rates, temperature, or pressure to keep the reactor mixture as close to Keq as possible for maximum ammonia yield.
Q5 — Sample concept map
A correct map should include arrows such as:
- Q —is calculated using→ current concentrations
- Keq —is calculated using→ equilibrium concentrations
- Q —is compared to→ Keq
- Q vs Keq comparison —determines→ direction of shift
- direction of shift —moves Q toward→ Keq
- Q —equals Keq at→ equilibrium
Any biologically/chemically valid linking phrases are accepted. Award full marks for at least 6 correctly labelled arrows that respect causal direction.