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 tableuses → stoichiometric ratio (in Change row); stoichiometric ratiodetermines → equilibrium concentration; equilibrium concentrationsubstituted into → Keq expression; Keq expressionevaluated to give → Keq value; Keq valueconfirmed by → verification; verificationchecks → 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