How does Q predict the direction?

Calculate Q from the starting concentrations: below Kc goes forward, above Kc goes backward.

Why omit solids from Kc?

A pure solid has a fixed concentration, so it merges into the constant and disappears from the expression.

S6 Chemistry Chemical Equilibrium, Paper 2: meaning of Kc, reaction quotient and calculations

2026-11-02

Ten graded items from the size of Kc through solids being omitted and Q against Kc to worked equilibrium calculations, each with an answer.

Focus　A large Kc lies to the right; pure solids and liquids are omitted; compare Q with Kc to find the direction; subtract the change first in an ICE table.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S6 Chemistry: Chemical Equilibrium
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationWhat does a Kc of about 1 x 10^5 tell you about a reaction?
FoundationAre pure solids and pure liquids included in a Kc expression?
FoundationH2 + I2 <=> 2HI has equal moles of gas on both sides. What does raising the pressure do?
FoundationHow does a catalyst affect the time needed to reach equilibrium?
IntermediateFor N2O4 <=> 2NO2 the equilibrium concentrations are 0.050 and 0.20 mol dm^-3. Find Kc.
IntermediateFor A + B <=> 2C with Kc = 9.0 and both reactants at 0.10 mol dm^-3, find [C].
IntermediateIf the reaction quotient Q is smaller than Kc, which way does the reaction proceed?
Challenge0.50 mol each of CO and H2O are sealed in 1.00 dm^3. For CO + H2O <=> CO2 + H2 with Kc = 1.0, find [CO2] at equilibrium.
Challenge0.200 mol of N2 and 0.600 mol of H2 are sealed in 1.00 dm^3. At equilibrium [NH3] = 0.100 mol dm^-3. Find Kc for N2 + 3H2 <=> 2NH3.
Error redoError repair: a student claims heating always increases Kc. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	The products dominate at equilibrium, so the reaction is nearly complete	Kc is the ratio of products to reactants, so a large value puts the position far to the right.
2	No, their concentrations are taken as constant	Pure solids and liquids have fixed concentrations folded into the constant, so they are left out.
3	Nothing; the position does not shift	Both sides have the same number of gas molecules, so pressure affects them equally.
4	The time shortens and Kc is unchanged	A catalyst speeds both directions, so equilibrium arrives sooner at the same position.
5	Kc = 0.80	The top is 0.20 squared, or 0.040, which divides by 0.050 to give 0.80.
6	0.30 mol dm^-3	Kc times the reactant concentrations gives 0.090, whose square root is 0.30.
7	To the right, making more product	Q below Kc means too little product, so the forward reaction dominates until Q reaches Kc.
8	0.25 mol dm^-3	Let x react; x squared over (0.50 – x) squared equals 1.0, so x is 0.25.
9	Kc = 0.732	Making 0.100 mol of ammonia uses 0.050 mol of nitrogen and 0.150 mol of hydrogen, leaving 0.150 and 0.450; 0.0100 over that product gives 0.732.
10	Only endothermic reactions gain; for exothermic ones Kc falls on heating	Heating favours the endothermic direction; for an exothermic reaction the reverse is endothermic, so Kc falls.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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