Chemistry • Year 12 • Module 5 • Lesson 11
Consolidation — ICE Table Mastery
Lock in the vocabulary, structure, and rules of the ICE table method before tackling harder problems.
1. Term–definition match
Match each term to its correct definition. Write the matching term from this list in the right-hand column: ICE table, initial concentration, change row, equilibrium concentration, stoichiometric ratio, simplifying assumption, 5% rule, reaction quotient (Q), Keq, verification step. 10 marks
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The concentration of each species at the exact moment of measurement after the system has reached equilibrium. | |
| 1.2 | A systematic layout of Initial, Change, and Equilibrium rows used to calculate equilibrium concentrations from initial conditions and Keq. | |
| 1.3 | The row in an ICE table that expresses how much each concentration changes, written in terms of x and using mole ratios from the balanced equation. | |
| 1.4 | The ratio of coefficients in the balanced chemical equation that determines how changes in one species relate to changes in all others. | |
| 1.5 | The approximation (Initial ± x) ≈ Initial, valid when x is very small relative to the initial concentration. | |
| 1.6 | The criterion that if x divided by the initial concentration is less than 5%, the simplifying assumption introduces acceptable error. | |
| 1.7 | A measure of the concentrations of products and reactants at any point that is compared to Keq to determine the direction of reaction. | |
| 1.8 | The concentration of each species before any reaction has occurred, placed in the first row of the ICE table. | |
| 1.9 | The equilibrium constant expressed in terms of molar concentrations of gaseous or aqueous species only. | |
| 1.10 | Substituting equilibrium concentrations back into the Keq expression to confirm the calculated answer is consistent with the given Keq. |
2. True or false — with correction
Circle T or F for each statement. If false, write the corrected version on the line provided. 10 marks (1 T/F + 1 correction where needed)
2.1 For the equilibrium N2(g) + 3H2(g) ⇌ 2NH3(g), the correct Change row is: N2: −x; H2: −x; NH3: +x. T / F
2.2 Pure solids and pure liquids are included in the Keq expression because their concentrations change during the reaction. T / F
2.3 If Q < Keq, the system must shift in the reverse direction to reach equilibrium. T / F
2.4 For the equilibrium 2HI(g) ⇌ H2(g) + I2(g), if x mol/L of H2 is produced, then 2x mol/L of HI is consumed.
T / F
2.5 The simplifying assumption is always valid when Keq < 1. T / F
3. Complete the ICE tables
Fill in every blank cell. The equilibrium for each problem is given below the table heading. 12 marks (2 per completed table row with correct stoichiometry)
Problem 3A — 1:1:1 ratio
PCl5(g) ⇌ PCl3(g) + Cl2(g). Initial: [PCl5] = 0.500 mol L−1; [PCl3] = [Cl2] = 0. Let x = change in [PCl3].
| PCl5 | PCl3 | Cl2 | |
|---|---|---|---|
| Initial (mol L−1) | 0.500 | 0 | 0 |
| Change (mol L−1) | +x | ||
| Equilibrium (mol L−1) | x |
Write the Keq expression using E-row values: Keq =
Problem 3B — 1:3:2 ratio (Haber process)
N2(g) + 3H2(g) ⇌ 2NH3(g). Initial: [N2] = 0.800, [H2] = 1.200, [NH3] = 0 mol L−1. Let x = decrease in [N2].
| N2 | H2 | NH3 | |
|---|---|---|---|
| Initial (mol L−1) | 0.800 | 1.200 | 0 |
| Change (mol L−1) | −x | ||
| Equilibrium (mol L−1) |
Write the Keq expression: Keq =
Problem 3C — Heterogeneous equilibrium
CaCO3(s) ⇌ CaO(s) + CO2(g). Initial: [CO2] = 0 mol L−1; CaCO3 and CaO are pure solids. Let x = increase in [CO2].
| CaCO3(s) | CaO(s) | CO2(g) | |
|---|---|---|---|
| Initial | solid (excluded) | solid (excluded) | 0 |
| Change | excluded | excluded | |
| Equilibrium | excluded | excluded |
Explain in one sentence why CaCO3 and CaO are excluded from the Keq expression:
Write the Keq expression: Keq =
Stuck? Revisit the exclusion rule for heterogeneous equilibria from Lesson 9.4. Fill-in-the-blank paragraph
Complete the paragraph using the word bank below. Each word is used exactly once. 8 marks
Word bank: quadratic, five, stoichiometric, positive, reverse, verify, equilibrium, quotient
When setting up an ICE table, the Change row must use ________ ratios from the balanced equation. If the initial concentrations of all species are non-zero, the reaction ________ (Q) must be calculated first to determine whether the system will shift in the forward or ________ direction. Once x is solved, all ________ concentrations (the E row) must be ________—that is, physically meaningful. The simplifying assumption is valid only if x / [initial] × 100 is less than ________ percent. When this threshold is exceeded, the ________ formula must be used. Finally, always ________ by substituting the E-row values back into the Keq expression.
Stuck? Revisit the Formula Panel and Lesson 11 Cards 1, 3 and 4.5. Short recall questions
Answer each question in 1–2 sentences using precise chemical terms. 8 marks, 2 each
5.1 What does the value of Q compared to Keq tell you about the direction a system will shift?
5.2 Why must you check the 5% rule after solving for x, not just before applying the simplifying assumption?
5.3 A student obtains a negative equilibrium concentration in the E row. What does this indicate and what should they do?
5.4 State the most common error students make in the Change row of an ICE table and explain how to avoid it.
Q1 — Term–definition matches
1.1 equilibrium concentration • 1.2 ICE table • 1.3 change row • 1.4 stoichiometric ratio • 1.5 simplifying assumption • 1.6 5% rule • 1.7 reaction quotient (Q) • 1.8 initial concentration • 1.9 Keq • 1.10 verification step
Q2 — True / false with correction
2.1 False. Correct Change row: N2: −x; H2: −3x; NH3: +2x. (Stoichiometric coefficients must be used.)
2.2 False. Pure solids and pure liquids are excluded from the Keq expression because their concentrations are constant and are incorporated into the value of Keq.
2.3 False. If Q < Keq, the system shifts forward (toward products) to increase Q toward Keq.
2.4 True. The ratio 2HI : 1H2 means if H2 increases by x, HI decreases by 2x. Correct.
2.5 False. The simplifying assumption is valid only when Keq / [initial] < 5%. A small Keq does not guarantee this — e.g., Keq = 0.4 with [initial] = 0.5 gives 80%, which is invalid.
Q3A — PCl5 decomposition ICE table
Change: PCl5: −x; PCl3: +x; Cl2: +x. Equilibrium: PCl5: 0.500 − x; PCl3: x; Cl2: x. Keq = x × x / (0.500 − x) = x² / (0.500 − x).
Q3B — Haber process ICE table
Change: N2: −x; H2: −3x; NH3: +2x. Equilibrium: N2: 0.800 − x; H2: 1.200 − 3x; NH3: 2x. Keq = (2x)² / [(0.800 − x)(1.200 − 3x)³].
Q3C — Heterogeneous equilibrium
Change: CO2: +x. Equilibrium: CO2: x. Exclusion reason: Pure solids have a constant concentration that does not change during the reaction; their activity is defined as 1 and is incorporated into the numerical value of Keq. Keq = [CO2].
Q4 — Cloze paragraph
stoichiometric • quotient • reverse • equilibrium • positive • five • quadratic • verify
Q5.1
If Q < Keq, the system shifts forward (producing more products). If Q > Keq, the system shifts in reverse (consuming products). If Q = Keq, the system is already at equilibrium and no net shift occurs.
Q5.2
The pre-check (Keq / [initial]) is an estimate of whether x will be small, but it is not exact. After solving, you must confirm that the actual x satisfies x / [initial] < 5%; if it does not, the assumption was invalid and the problem must be re-solved using the quadratic formula.
Q5.3
A negative equilibrium concentration is physically impossible. It indicates an error—most likely the wrong direction of shift (wrong sign in the Change row) or a stoichiometric ratio error. The student should check Q vs Keq to confirm the direction, then redo the Change row.
Q5.4
The most common error is writing ±x for every species regardless of coefficients. To avoid it: always write out the balanced equation first and use the stoichiometric coefficients as multipliers for x in every Change row entry.