Chemistry · Year 12 · Module 5 · Lesson 10
Calculating Keq & ICE Tables
Recall the ICE framework, the stoichiometric change-row rules, and the steps for direct-substitution Keq calculations.
1. Term–definition match
Match each term in the left column to the correct definition by writing the term letter in the blank. Terms: A. ICE table B. Initial concentration C. Change row D. Equilibrium concentration E. Stoichiometric ratio F. Assumption (small-x) G. Verification H. Direct substitution 8 marks
| # | Definition | Term (letter) |
|---|---|---|
| 1.1 | The starting molar concentration of each species before any reaction proceeds toward equilibrium. | |
| 1.2 | The tabular method that tracks Initial, Change, and Equilibrium concentrations for every species in a reversible reaction. | |
| 1.3 | The molar concentration of each species at equilibrium; these values are substituted into the Keq expression. | |
| 1.4 | The row that records how much each concentration changes as the reaction moves to equilibrium, expressed using stoichiometric multiples of a variable x. | |
| 1.5 | The proportional relationship between the coefficients of reactants and products in a balanced equation — determines the multiples of x in the Change row. | |
| 1.6 | A simplification valid when Keq is very small: x is treated as negligible compared to the initial concentration, checked by the 5% rule. | |
| 1.7 | Calculating Keq by plugging known equilibrium concentrations directly into the Keq expression — no algebra required. | |
| 1.8 | Substituting equilibrium concentrations back into the Keq expression to confirm the calculated value matches the given Keq. |
2. True or false — with correction
Circle T or F. If false, write the corrected version on the line. 8 marks (1 T/F + 1 correction where false)
2.1 In an ICE table for H2(g) + I2(g) ⇌ 2HI(g), if H2 decreases by x, then HI increases by x. T / F
2.2 In a direct substitution Keq calculation, you must always write the Keq expression before substituting numbers. T / F
2.3 Adding more reactant to a system at equilibrium changes the value of Keq. T / F
2.4 In an ICE table for N2(g) + 3H2(g) ⇌ 2NH3(g), the Change row for H2 is −3x when N2 changes by −x. T / F
3. Fill-in-the-blank paragraph
Complete the paragraph by selecting the correct word from the word bank. Each word is used once. 8 marks
Word bank: stoichiometric | Equilibrium | Initial | Change | temperature | x | verification | products
An ICE table has three rows. The row records the starting concentrations before any shift toward equilibrium. The row shows how concentrations alter as the reaction proceeds; all entries are expressed as multiples of the variable . Reactants decrease, while increase (or vice versa if starting from pure products). The row is calculated as Initial + Change. Once the Keq expression is solved, a step confirms the answer. Keq itself only changes when changes.
4. Function recall
Answer each question in 1–2 sentences using precise chemical vocabulary. 8 marks, 2 each)
4.1 What is the purpose of the Change row in an ICE table, and why must its entries reflect stoichiometric ratios rather than all being ±x?
4.2 Why is it important to write the Keq expression before substituting equilibrium concentrations into it?
4.3 What does the 5% rule test in an ICE table calculation, and what must you do if the assumption fails?
4.4 A Keq value of 6.5 × 10−4 is calculated at 300°C. Describe the function of Keq in telling a chemist about the relative amounts of reactants and products at equilibrium.
5. Concept map — ICE framework
Draw labelled arrows between the six terms below to show how they connect in a Keq calculation. Each arrow must carry a linking phrase (e.g. "is used to find", "requires", "leads to"). Aim for at least 6 labelled arrows. 6 marks
Terms: ICE table · stoichiometric ratio · equilibrium concentration · Keq expression · Keq value · verification
6. Complete the ICE table
The balanced equation is: N2O4(g) ⇌ 2NO2(g). Initial concentration of N2O4 = 0.500 mol/L; [NO2]initial = 0. At equilibrium, [NO2] = 0.300 mol/L. Fill in every blank cell in the ICE table. 6 marks
| Row | [N2O4] (mol/L) | [NO2] (mol/L) |
|---|---|---|
| Initial (I) | ||
| Change (C) | ||
| Equilibrium (E) | 0.300 |
Using your completed ICE table, calculate Keq for this reaction. Show all working.
Q1 — Term–definition match
1.1 B · 1.2 A · 1.3 D · 1.4 C · 1.5 E · 1.6 F · 1.7 H · 1.8 G
Q2 — True / False with correction
2.1 False. Correction: if H2 decreases by x, then HI increases by 2x — the stoichiometric coefficient of HI is 2 in the balanced equation.
2.2 True.
2.3 False. Correction: adding more reactant shifts the equilibrium position but does not change Keq. Keq depends only on temperature.
2.4 True. The coefficient ratio N2 : H2 : NH3 = 1 : 3 : 2, so if N2 changes by −x, H2 changes by −3x.
Q3 — Cloze
Initial row (starting concentrations) → Change row (expressed as stoichiometric multiples of variable x; reactants decrease while products increase) → Equilibrium row (Initial + Change). After solving, a verification step confirms correctness. Keq changes only when temperature changes.
Q4 — Function recall
4.1 The Change row records how much each concentration changes as equilibrium is established. Entries must reflect stoichiometric ratios because each mole of reaction consumed/produced follows the balanced equation coefficients — using ±x for every species ignores these ratios and gives incorrect equilibrium concentrations.
4.2 Writing the expression first forces correct placement of each species (numerator = products, denominator = reactants) with correct stoichiometric powers before any arithmetic. Students who substitute first frequently use wrong powers or invert the expression.
4.3 The 5% rule checks whether x is small enough to neglect compared to the initial concentration (x/[initial] × 100% < 5%). If the assumption fails, the quadratic formula must be used instead.
4.4 Keq = 6.5 × 10−4 is much less than 1, meaning at equilibrium reactants are strongly favoured — the reaction proceeds only slightly toward products, and the mixture at equilibrium consists predominantly of reactants.
Q5 — Sample concept map
Award 1 mark per valid labelled arrow (minimum 6). Correct connections include: ICE table — uses → stoichiometric ratio (in Change row); stoichiometric ratio — determines → equilibrium concentration; equilibrium concentration — substituted into → Keq expression; Keq expression — evaluated to give → Keq value; Keq value — confirmed by → verification; verification — checks → Keq expression.
Q6 — ICE table and Keq calculation
ICE table:
| Row | [N2O4] (mol/L) | [NO2] (mol/L) |
|---|---|---|
| I | 0.500 | 0 |
| C | −0.150 | +0.300 |
| E | 0.350 | 0.300 |
Working: [NO2]eq = 0.300 mol/L. Stoichiometric ratio N2O4 : NO2 = 1 : 2, so Δ[N2O4] = −0.300/2 = −0.150 mol/L. [N2O4]eq = 0.500 − 0.150 = 0.350 mol/L.
Keq = [NO2]2 / [N2O4] = (0.300)2 / (0.350) = 0.0900 / 0.350 = 0.257