Unit 3: Equilibrium, acids and redox reactions
8 dot points across 2 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.
Topic 1: Chemical equilibrium systems
- Describe acids and bases using the Bronsted-Lowry model, including the identification of conjugate acid-base pairs, amphiprotic species, and the distinction between strong and weak acids and bases
A focused answer to the QCE Chemistry Unit 3 dot point on Bronsted-Lowry acids and bases. Defines proton donors and acceptors, walks through conjugate acid-base pairs, identifies amphiprotic species (water, hydrogencarbonate, dihydrogenphosphate), and contrasts strong with weak acids and bases using Ka and Kb in equilibrium terms.
9 min answer → - Describe the composition and action of buffer systems, and explain qualitatively how a buffer resists changes in pH on the addition of small amounts of strong acid or strong base
A focused answer to the QCE Chemistry Unit 3 dot point on buffers. Defines a buffer as a weak acid plus its conjugate base in comparable amounts, walks through how each component consumes added strong acid or base, and applies the reasoning to the carbonic acid/hydrogencarbonate buffer in blood. Includes the buffer-question types that appear in IA1 stimulus.
8 min answer → - Explain dynamic equilibrium in terms of rates of forward and reverse reactions, and recognise that equilibrium can only be established in a closed system
A focused answer to the QCE Chemistry Unit 3 dot point on dynamic equilibrium. Defines reversible reactions, contrasts dynamic with static equilibrium, explains why equilibrium requires a closed system, and works through a sample concentration-vs-time graph with the kind of stimulus QCAA uses in IA1.
8 min answer → - Derive and apply the equilibrium law expression (Kc) for homogeneous reactions, including calculating Kc from equilibrium concentrations and predicting the position of equilibrium from the value of Kc
A focused answer to the QCE Chemistry Unit 3 dot point on the equilibrium constant. Sets out the equilibrium law expression, works through Kc calculation from a stimulus ICE table (the dominant IA1 question type), interprets the value of Kc in terms of extent, and addresses why Kc is temperature-dependent but pressure-independent.
9 min answer → - Predict, using Le Chatelier's principle, the qualitative effects of changes in concentration, temperature, pressure and volume on the equilibrium position of homogeneous reactions
A focused answer to the QCE Chemistry Unit 3 dot point on Le Chatelier's principle. Defines the principle, works through how concentration, temperature, pressure and volume changes shift equilibrium position, explains why catalysts do not shift equilibrium, and applies the reasoning to the Haber process and the iron(III) thiocyanate system used in IA1 and IA2.
9 min answer → - Use Kw and the relationship pH = -log10[H3O+] to calculate the pH of strong acid and strong base solutions, and to relate [H3O+] and [OH-] in any aqueous solution
A focused answer to the QCE Chemistry Unit 3 dot point on pH and Kw. Derives Kw from the self-ionisation of water, uses pH = -log10[H3O+] to calculate pH of strong acids and bases (including dilution and mixed solutions), and connects Kw temperature-dependence to the limits of "neutral pH = 7".
9 min answer →
Topic 2: Oxidation and reduction
- Describe the construction and operation of a galvanic cell, including the role of the salt bridge, the conventions of anode and cathode, and the calculation of standard cell potentials from the standard reduction potential table
A focused answer to the QCE Chemistry Unit 3 dot point on galvanic cells. Identifies anode and cathode by sign and process, explains the role of the salt bridge, and calculates standard cell potentials from the reduction potential table. Includes worked Zn/Cu and Cu/Ag cells, cell-diagram notation, and the spontaneity criterion frequently examined in IA2 and EA Paper 2.
9 min answer → - Determine oxidation numbers and use them to identify oxidation and reduction in chemical reactions, and construct balanced half-equations and overall ionic equations for redox reactions in aqueous solution
A focused answer to the QCE Chemistry Unit 3 dot point on assigning oxidation numbers, identifying oxidising and reducing agents, and constructing balanced half-equations and overall ionic equations for redox reactions in aqueous solution. Includes the half-equation balancing protocol QCAA expects in IA1 short response and EA Paper 2.
9 min answer →