How the graphs are made
Each problem takes a real reversible reaction, such as N₂O₄ ⇌ 2NO₂, the Haber process or the iron(III) thiocyanate equilibrium, and works out the graph exactly. Every flat section fits one equilibrium constant. When the mixture shifts, each line moves in the mole ratio of the equation, and they all level out at the same time. Added or removed substances and volume changes jump at once, while temperature changes curve away with no jump. Values sit on gridlines wherever a question asks you to read them. A set code makes the same problems on any device, so a class can share one set or print up to 30 different ones.
Choose the course and its rules follow: Kc with units in M for VCE, Keq with no units for the HSC, a decimal comma and two decimal places for the NSC Physical Sciences paper, and Kp in kPa from partial pressures for A level. The graph has no data table under it, because a table would give the readings away.
An example from set EQ5K24
H₂(g) + I₂(g) ⇌ 2HI(g). Before t₁ the graph is flat at [H₂] = 0.14, [I₂] = 0.06 and [HI] = 0.18 mol L⁻¹. Kc = [HI]² ÷ ([H₂] × [I₂]) = 0.18² ÷ (0.14 × 0.06) = 0.0324 ÷ 0.0084 = 3.857…, which rounds to 3.86, with no units because both sides have 2 mol of gas. At t₁ more HI is added and only the HI line jumps, to 0.28, so Q = 0.28² ÷ (0.14 × 0.06) = 0.0784 ÷ 0.0084 = 9.33. Q > Kc, so the reverse reaction is favoured until Q falls back to 3.86. Open set EQ5K24
What each course asks for
VCE Chemistry Units 3 and 4 covers Le Chatelier's principle with concentration-time graphs, and calculations with equilibrium expressions including units. HSC Chemistry Module 5 covers Le Chatelier's principle, Keq and the reaction quotient Q. The NSC Physical Sciences Paper 2 adds rate-time graphs, catalysts and the Kc table with n(initial), n(change), n(equilibrium) and c(equilibrium) rows; its exam guidelines ask for answers to at least two decimal places. Cambridge 9701 and AQA 7405 A level cover Kc and Kp, and state that a catalyst does not change K.
Questions
How can you tell what change was made from a concentration-time graph?
Look at what happens at the moment of the change. If one line jumps and the others do not, that substance was added or removed. If every line jumps together, the volume changed (the pressure of a gas mixture, or water added to a solution). If nothing jumps and the lines curve away gradually, the temperature changed. In VCAA's 2025 report, 67% chose the graph with no instantaneous changes for a temperature rise.
Why do the lines change in the mole ratio?
As the mixture shifts, every substance reacts in proportion to its coefficient in the equation. For N₂ + 3H₂ ⇌ 2NH₃, H₂ falls three times as far as N₂, and NH₃ rises twice as far. VCAA's 2016 report marked graphs on both points: relative concentration changes proportional to the mole ratios, and all substances reaching equilibrium at the same time.
When does the value of K change?
Only when the temperature changes. Adding or removing a substance, changing the volume or pressure, adding an inert gas and adding a catalyst all leave K as it was. A rise in temperature increases K for an endothermic forward reaction (ΔH > 0) and decreases it for an exothermic one.
How do you compare Q with K?
Q uses the same expression as K, with the concentrations at that moment. If Q < K, the forward reaction is favoured until Q rises to K. If Q > K, the reverse reaction is favoured. Just after more reactant is added, Q < K. In VCAA's 2024 multiple-choice question on this, 49% chose the right answer.
Does adding an inert gas shift the equilibrium?
At constant volume, no. The total pressure rises, but no concentration or partial pressure changes, so nothing shifts. VCAA's 2025 report did not award a mark for just saying the pressure increased, because an inert gas increases the pressure without causing a shift. At constant pressure the volume grows, which acts like lowering the pressure.
How do you work out Kc from a graph?
Read each concentration on a flat section, where the system is at equilibrium. Put them into the Kc expression with each one raised to the power of its coefficient, and round only at the end. From a moles-time graph, divide each amount by the volume first: South Africa's chief markers' reports remind learners that equilibrium concentrations, not moles, go into Kc.
Sources, checked 29 October 2026: VCAA VCE Chemistry Study Design (Word); VCAA Chemistry examination reports for 2016, 2024 and 2025; NESA HSC Chemistry examinations for 2024 and 2025; DBE Physical Sciences Grade 12 Examination Guidelines 2021 and the Eastern Cape Physical Sciences Paper 2 chief markers' reports for 2024 and 2025; Cambridge International AS and A Level Chemistry 9701 syllabus 2025 to 2027; AQA AS and A level Chemistry specification.