HSC Chemistry · Year 12
HSC Chemistry Module 5: Equilibrium and Acid Reactions — Flashcards & Quiz
HSC Chemistry Module 5 covers chemical equilibrium — a core Year 12 topic. Revise reversible reactions, Le Chatelier's principle, the equilibrium constant (Kc), effects of concentration, temperature and pressure changes, and industrial applications like the Haber process. These 20 flashcards and 20 true/false questions target NESA syllabus dot-points, helping you master equilibrium calculations, ICE tables, and predict how systems respond to disturbances.
Key Terms
- Dynamic equilibrium
- The state in a closed system where the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of reactants and products remain constant over time. NESA HSC Chemistry Module 5 requires students to explain that equilibrium is dynamic (reactions continue) not static (reactions stopped).
- Equilibrium constant (Keq)
- A numerical value expressing the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients. HSC Chemistry exams assess students on writing Keq expressions, calculating values from concentration data, and interpreting whether products or reactants are favoured.
- Le Chatelier's principle
- The principle stating that if a system at equilibrium is subjected to a disturbance (change in concentration, pressure or temperature), the system will shift to partially counteract the disturbance and establish a new equilibrium. NESA expects HSC students to predict the direction of shift and explain its effect on concentrations for each type of change.
- ICE table
- A calculation method using Initial concentration, Change in concentration, and Equilibrium concentration to solve quantitative equilibrium problems. HSC Chemistry trial exams test this technique almost every year, requiring students to set up and solve ICE tables for given reaction data.
- Reaction quotient (Q)
- The ratio of product concentrations to reactant concentrations at any point during a reaction, calculated using the same expression as Keq. NESA HSC Chemistry requires students to compare Q with Keq to predict whether the system will shift forward (Q < Keq) or backward (Q > Keq) to reach equilibrium.
- Closed system
- A system that allows energy exchange with the surroundings but does not allow matter to enter or leave. NESA Module 5 outcomes emphasise that dynamic equilibrium can only be established in a closed system, and HSC exam questions often test whether students recognise this prerequisite condition.
Sample Flashcards
Q1: What is dynamic equilibrium?
Dynamic equilibrium occurs in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction. Reactants and products are constantly interconverting, but their concentrations remain constant over time. The system must be closed (no matter enters or leaves).
Q2: State Le Chatelier's principle.
If a system at equilibrium is subjected to a change in concentration, temperature or pressure, the system will shift to partially counteract the change and establish a new equilibrium position.
Q3: How does changing concentration affect equilibrium?
Increasing the concentration of a reactant shifts equilibrium toward the products (right). Increasing the concentration of a product shifts equilibrium toward the reactants (left). The system consumes the added substance to partially restore the original ratio.
Q4: How does changing temperature affect equilibrium?
Increasing temperature favours the endothermic direction. Decreasing temperature favours the exothermic direction. Temperature is the ONLY factor that changes the value of Kc.
Q5: How does changing pressure affect equilibrium in gaseous systems?
Increasing pressure shifts equilibrium toward the side with FEWER moles of gas (to reduce pressure). Decreasing pressure shifts toward the side with MORE moles of gas. Pressure changes only affect equilibria involving different moles of gas on each side.
Q6: Write the expression for the equilibrium constant Kc.
For aA + bB ⇌ cC + dD: Kc = [C]^c × [D]^d / [A]^a × [B]^b. Only include aqueous and gaseous species. Solids and pure liquids are excluded (their concentrations are constant). Square brackets represent molar concentration (mol/L).
Q7: What does the value of Kc tell you about a reaction?
Kc >> 1: products are favoured at equilibrium (reaction goes mostly to completion). Kc << 1: reactants are favoured (very little product at equilibrium). Kc ≈ 1: neither strongly favoured, significant amounts of both.
Q8: How does a catalyst affect equilibrium?
A catalyst increases the rate of BOTH forward and reverse reactions equally. It does NOT change the equilibrium position or the value of Kc. It only helps the system reach equilibrium faster.
Sample Quiz Questions
Q1: At dynamic equilibrium, both forward and reverse reactions have stopped.
Answer: FALSE
At dynamic equilibrium, both reactions CONTINUE at equal rates. Reactions do NOT stop — "dynamic" means ongoing activity.
Q2: At equilibrium, the concentrations of reactants and products must be equal.
Answer: FALSE
At equilibrium, concentrations are CONSTANT but not necessarily equal. The ratio depends on the value of Kc.
Q3: Adding more reactant to a system at equilibrium shifts the equilibrium to the right.
Answer: TRUE
Increasing reactant concentration shifts equilibrium toward the products (right) to partially counteract the increase.
Q4: Increasing temperature always shifts equilibrium to the right.
Answer: FALSE
Increasing temperature shifts equilibrium toward the ENDOTHERMIC direction — this could be right (if forward is endothermic) or left (if forward is exothermic).
Q5: Increasing pressure shifts equilibrium toward the side with more moles of gas.
Answer: FALSE
Increasing pressure shifts equilibrium toward the side with FEWER moles of gas, to reduce the total pressure.
Why It Matters
Equilibrium and Acid Reactions is a pivotal Year 12 module that introduces the concept of dynamic balance in chemical systems — a theme that runs through every remaining HSC Chemistry module. Le Chatelier's principle is one of the most versatile tools in your exam toolkit, applicable to acid-base, industrial and organic chemistry questions. Equilibrium constant calculations also test your mathematical skills under pressure. Strong students treat this module as the keystone that connects theory to real-world applications like the Haber process and industrial manufacturing. Equilibrium principles are revisited in Module 6 (Acid-Base Reactions) when analysing weak acid dissociation and buffer systems, and in Module 8 when evaluating industrial process conditions. ICE table calculations and Le Chatelier's principle application questions appear in nearly every HSC Chemistry exam, often in both the multiple-choice section and as structured calculation questions worth 4-6 marks.
Key Concepts
Dynamic Equilibrium
At equilibrium, the rates of forward and reverse reactions are equal and concentrations remain constant (not necessarily equal). Understanding that equilibrium is dynamic — reactions still occur in both directions — is essential for avoiding common misconceptions tested in multiple-choice questions.
Le Chatelier's Principle
When a system at equilibrium is disturbed (change in concentration, pressure, temperature), it shifts to partially counteract the change. Practise predicting shift direction for each type of disturbance, and remember that catalysts do NOT shift equilibrium — they only speed up both rates equally.
Equilibrium Constant (Keq)
The equilibrium expression Keq = [products]/[reactants] quantifies the position of equilibrium. A large Keq means products are favoured; a small Keq means reactants are favoured. Practise ICE table calculations — these appear in nearly every HSC Chemistry exam.
Industrial Applications
The Haber process (ammonia synthesis) and Contact process (sulfuric acid) are classic examples of applying Le Chatelier's principle to optimise yield. Know the compromises between equilibrium position and reaction rate that determine industrial operating conditions.
Common Mistakes to Avoid
- Stating that equal concentrations of reactants and products exist at equilibrium — NESA HSC Chemistry Module 5 requires students to understand that equilibrium means constant concentrations, not necessarily equal concentrations. The Keq value determines the relative amounts.
- Including solids and pure liquids in equilibrium constant expressions — HSC Chemistry marking guidelines penalise students who include concentrations of solids or pure liquids in Keq expressions, as their concentrations are constant and incorporated into the Keq value itself.
- Claiming that a catalyst changes the position of equilibrium or the value of Keq — NESA expects HSC students to explain that catalysts increase the rates of both forward and reverse reactions equally, reaching equilibrium faster without changing Keq or product yield.
- Confusing the effect of adding an inert gas at constant volume with changing pressure — HSC Chemistry trial exams test whether students recognise that adding an inert gas at constant volume does not change the partial pressures of reactants or products and therefore does not shift equilibrium.
- Incorrectly predicting the effect of temperature changes on Keq — NESA requires HSC students to explain that increasing temperature shifts equilibrium in the endothermic direction and actually changes the value of Keq, unlike concentration or pressure changes which shift position but do not alter Keq.
Study Tips
- Master ICE (Initial, Change, Equilibrium) tables by solving at least 10 practice problems — this calculation method is tested almost every year.
- For Le Chatelier questions, always state three things: the disturbance, the direction of shift, and the effect on concentrations.
- Remember that catalysts do NOT appear in equilibrium expressions and do NOT change Keq — this is a frequently tested trap.
- Link equilibrium to industrial chemistry by preparing a case study on the Haber process (temperature, pressure, catalyst trade-offs).
- Use spaced-repetition flashcards to memorise equilibrium rules and exception cases — consistent short sessions prevent the confusion that arises from cramming this conceptually dense module.
- Before your exam, work through the practice questions in this set at least twice using spaced repetition. Testing yourself repeatedly is the most effective revision strategy for long-term retention.
Related Topics
Frequently Asked Questions
What is chemical equilibrium?
Chemical equilibrium is a dynamic state where the rate of the forward reaction equals the rate of the reverse reaction, so concentrations of reactants and products remain constant (but not necessarily equal).
What is Le Chatelier's principle?
Le Chatelier's principle states that if a system at equilibrium is disturbed, the system will shift to partially counteract the disturbance and establish a new equilibrium.
What is the Haber process?
The Haber process is the industrial synthesis of ammonia: N₂ + 3H₂ ⇌ 2NH₃. It uses high pressure (~200 atm), moderate temperature (~450°C) and an iron catalyst to maximise yield economically.
Last updated: March 2026 · 20 flashcards · 20 quiz questions · Content aligned to the NESA Syllabus