QCAA 2026 Unit 3 & 4 Chemistry Flashcards

What is the difference between an open and a closed chemical system?

A closed system allows energy transfer but not matter transfer with its surroundings; an open system allows both matter and energy to transfer.

Why can equilibrium only be reached in a closed system?

Because matter cannot escape or enter, reactants and products remain trapped so their concentrations can stabilise into a dynamic balance; in an open system, products/reactants can be lost and equilibrium cannot be maintained.

Are all physical changes reversible? Are all chemical reactions reversible?

Physical changes are usually reversible (e.g. melting/freezing). Only some chemical reactions are reversible.

What symbol is used to represent an equilibrium reaction in a balanced chemical equation?

⇋ (a double half-arrow)

What defines the 'position of equilibrium' in a reversible reaction?

The relative concentrations of products and reactants at equilibrium.

Define dynamic equilibrium.

The state reached in a closed system where the forward and reverse reaction rates are equal, so the macroscopic concentrations of reactants and products remain constant over time, even though the forward and reverse reactions continue to occur.

Why is equilibrium described as 'dynamic' rather than 'static'?

Because the forward and reverse reactions are still occurring continuously at equal rates — nothing has stopped, but net concentrations don't change.

In terms of activation energy, explain why some reactions are reversible and others are not.

A reaction is reversible when the reverse reaction has an activation energy low enough to be overcome under the reaction conditions; if the reverse activation energy is too high, the reverse reaction is negligible and the reaction is effectively irreversible.

On a concentration-vs-time graph for a reaction reaching equilibrium, what indicates that equilibrium has been reached?

The concentration lines for reactants and products become horizontal (flatten out) and no longer change with time.

State Le Châtelier's principle.

If a system at equilibrium is subjected to a change (in concentration, temperature or pressure), the system will shift/respond in the direction that partially opposes (counteracts) that change, to re-establish equilibrium.

Using collision theory, explain why increasing concentration of a reactant shifts equilibrium toward the products.

More reactant particles increase the frequency of successful collisions between reactant particles, increasing the forward reaction rate temporarily above the reverse rate, so the position of equilibrium shifts toward products until a new equilibrium is established.

How does increasing pressure affect an equilibrium involving gases with unequal moles on each side?

The equilibrium shifts toward the side with fewer moles of gas, since that reduces the total number of gas particles and opposes the pressure increase.

Does adding a catalyst change the position of equilibrium? Explain.

No. A catalyst speeds up the forward and reverse reactions equally (by lowering activation energy for both), so equilibrium is reached faster but the position of equilibrium (and K꜀) is unchanged.

Common misconception: does a catalyst increase the yield of an equilibrium reaction?

No — a catalyst only increases the rate at which equilibrium is reached, not the amount of product at equilibrium (yield is unchanged).

How does increasing temperature affect the position of equilibrium for an exothermic forward reaction?

The equilibrium shifts toward the reactants (favours the endothermic reverse reaction), because the system opposes the temperature increase by favouring the direction that absorbs heat.

What is the effect of temperature change on the value of K꜀, compared with the effect of concentration or pressure change?

Temperature change is the ONLY factor that changes the actual value of K꜀ (increasing T increases K꜀ for endothermic forward reactions, decreases K꜀ for exothermic forward reactions). Concentration and pressure changes shift the position of equilibrium but leave K꜀ unchanged.

Students often confuse the effect of a catalyst and a temperature increase on equilibrium — what is the key difference?

A catalyst changes the rate of reaching equilibrium but not the position of equilibrium or K꜀. A temperature change alters both the position of equilibrium and the value of K꜀.

What does the equilibrium constant K꜀ represent?

The relationship between product and reactant concentrations at equilibrium for a given reversible reaction at a given temperature.

Write the general equilibrium law expression for the reaction aA + bB ⇋ cC + dD.

K꜀ = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ)

What does the solubility product Kₛₚ measure?

A measure of the solubility of a sparingly soluble ionic compound — the equilibrium constant for its dissolution.

Write the Kₛₚ expression for the reaction aA(s) ⇋ cC(aq) + dD(aq).

Kₛₚ = [C]ᶜ[D]ᵈ (the solid A is omitted because its 'concentration' is constant).

Why is the solid reactant omitted from a heterogeneous equilibrium constant expression (e.g. Kₛₚ)?

Because the concentration of a pure solid (or liquid) is essentially constant and is incorporated into the value of the equilibrium constant itself, rather than varying with the amount present.

What is the reaction quotient Q, and how does it differ from K꜀?

Q has the same mathematical form as K꜀, but is calculated using concentrations at ANY point in time (not necessarily at equilibrium). K꜀ is Q evaluated specifically at equilibrium.

If Q < K꜀, in which direction will the reaction proceed to reach equilibrium?

Forward (toward products) — because there are relatively too few products/too many reactants, so the reaction will produce more products until Q = K꜀.

If Q > K꜀, in which direction will the reaction shift?

Reverse (toward reactants), since there are proportionally too many products relative to equilibrium.

If Q = K꜀, what can be concluded about the system?

The system is already at equilibrium — no net shift will occur.

When calculating K꜀ for a reaction with a very small K꜀ value, what simplifying assumption is commonly made, and why?

Assume [reactants]initial ≈ [reactants]equilibrium, because a very small K꜀ means the reaction barely proceeds forward, so the change in reactant concentration is negligible. This assumption must be explicitly stated when used.

What does a very large K꜀ value (K꜀ >> 1) tell you about the extent of a reaction?

The reaction proceeds nearly to completion — at equilibrium, products are strongly favoured over reactants.

What does a very small K꜀ value (K꜀ << 1) tell you about the extent of a reaction?

The reaction barely proceeds — at equilibrium, reactants are strongly favoured and very little product forms.

Define an acid using the Brønsted-Lowry model.

A substance that acts as a proton (H⁺ ion) donor.

What is the difference between a monoprotic, diprotic and polyprotic acid?

Monoprotic acids donate one proton per molecule; diprotic donate two; polyprotic donate more than one (general term including diprotic and beyond).

Name three strong acids and identify how many protons each donates.

Hydrochloric acid HCl (monoprotic), nitric acid HNO₃ (monoprotic), sulfuric acid H₂SO₄ (diprotic).

Name the strong bases studied in this syllabus.

Group 1 hydroxides (e.g. NaOH, KOH) and barium hydroxide Ba(OH)₂.

Name examples of weak acids and weak bases studied in this syllabus.

Weak acids: carboxylic acids, carbonic acid. Weak bases: ammonia, amines.

What is the difference between 'strong/weak' and 'concentrated/dilute' when describing an acid?

Strong/weak describes the extent of dissociation (ionisation) of the acid in water. Concentrated/dilute describes the amount of acid dissolved per litre of solution — independent of strength.

Students often confuse 'strong acid' with 'concentrated acid' — why is this wrong?

A strong acid dissociates almost completely regardless of its concentration; a dilute solution of a strong acid is still 'strong' (fully ionised) but has a low concentration. Strength and concentration are independent properties.

How do strong and weak acids of equal concentration differ in pH, rate of reaction, and electrical conductivity?

A strong acid has a lower pH, faster rate of reaction (with metals/carbonates) and higher electrical conductivity than a weak acid of the same concentration, because it produces more free H⁺ ions in solution.

What does K_w represent and what is its formula?

K_w is the self-ionisation (autoionisation) constant of water: K_w = [H⁺][OH⁻].

At 25°C, what is the accepted value of K_w?

1.0 × 10⁻¹⁴

Given [OH⁻] in a solution, how do you calculate [H⁺] using K_w?

[H⁺] = K_w / [OH⁻]

Write the formula for calculating pH from [H⁺].

pH = -log₁₀[H⁺]

Write the formula for calculating pOH from [OH⁻].

pOH = -log₁₀[OH⁻]

At 25°C, what is the relationship between pH and pOH for any aqueous solution?

pH + pOH = 14

Why is the pH scale described as logarithmic, and what does this mean practically?

Each whole-number change in pH represents a 10-fold change in [H⁺] concentration — e.g. pH 3 has 10× the [H⁺] of pH 4.

Why is water classified as a weak electrolyte?

Because only a very small fraction of water molecules self-ionise into H⁺ and OH⁻ ions at any given time (K_w is very small), so pure water conducts electricity only weakly.

Explain the Brønsted-Lowry model of acid-base reactions.

Acids and bases react by transferring a proton (H⁺) between them: the acid donates a proton to the base, forming a conjugate base (from the acid) and a conjugate acid (from the base).

What is a conjugate acid-base pair?

Two species that differ by exactly one proton (H⁺) — e.g. HA (acid) and A⁻ (its conjugate base).

What is an amphiprotic species? Give an example.

A species that can act as either a Brønsted-Lowry acid or base depending on the reaction, e.g. HCO₃⁻ or H₂O.

What is the conjugate base of H₂PO₄⁻?

HPO₄²⁻ (loses one H⁺).

What is the conjugate acid of NH₃?

NH₄⁺ (gains one H⁺).

What is a buffer solution, in terms of its composition?

A solution containing a conjugate acid-base pair (e.g. a weak acid and its conjugate base, or a weak base and its conjugate acid) that resists changes in pH when small amounts of acid or base are added.

Using Le Châtelier's principle, explain how a buffer resists a small addition of H⁺ ions.

The added H⁺ reacts with the conjugate base component of the buffer, shifting the buffer equilibrium to consume the extra H⁺ and minimise the pH change.

Using Le Châtelier's principle, explain how a buffer resists a small addition of OH⁻ ions.

The added OH⁻ is neutralised by the weak acid component of the buffer (or reacts with H⁺ produced from the buffer equilibrium), shifting the equilibrium to replace the consumed H⁺ and minimise the pH change.

What determines the 'strength' of a weak acid in terms of equilibrium?

The degree (extent) of ionisation of the acid at equilibrium in aqueous solution — a larger degree of ionisation means a stronger acid.

Write the K_a expression for the weak acid dissociation HA + H₂O ⇋ H₃O⁺ + A⁻.

K_a = [H₃O⁺][A⁻] / [HA]

Write the K_b expression for the weak base dissociation B + H₂O ⇋ BH⁺ + OH⁻.

K_b = [BH⁺][OH⁻] / [B]

What is the relationship between K_a, K_b and K_w for a conjugate acid-base pair?

K_w = K_a × K_b

What do pK_a and pK_b represent, and how are they calculated?

pK_a = -log₁₀(K_a) and pK_b = -log₁₀(K_b); they express acid/base dissociation constants on a logarithmic scale, similar to pH.

A weak acid has a larger K_a than another weak acid. What does this tell you about their relative strengths?

The acid with the larger K_a is the stronger acid (it ionises to a greater extent at equilibrium).

What type of chemical species is an acid-base indicator?

A weak acid or weak base whose conjugate acid-base pair have different colours, represented as HIn(aq) ⇋ H⁺(aq) + In⁻(aq) (or BOH ⇋ B⁺ + OH⁻).

At what point does an indicator change colour?

When pH = pK_a of the indicator (the point where [HIn] = [In⁻], i.e. equal amounts of both colour forms are present).

Over what pH range does an indicator typically change colour, relative to its pK_a?

Approximately pK_a ± 1

How do you select an appropriate indicator for a titration?

Choose an indicator whose colour-change pH range (pK_a ± 1) overlaps with the equivalence point pH of the titration.

What is the difference between the 'end point' and the 'equivalence point' of a titration?

The equivalence point is where the moles of acid and base have reacted in exact stoichiometric proportion (theoretical). The end point is the point observed experimentally (e.g. indicator colour change), which should closely approximate the equivalence point.

On an acid-base titration curve, what features can be read at the equivalence point?

The volume of titrant added, and (for weak acid/base titrations) that pH ≈ 7 is NOT guaranteed — the equivalence point pH depends on the strength of acid/base being titrated.

On a titration curve of a weak acid with a strong base, what does the 'buffer region' correspond to, and where is pK_a located?

The buffer region is the relatively flat section before the equivalence point where pH changes slowly; the pK_a of the weak acid is found at the half-equivalence point, where pH = pK_a (this is also the point of maximum buffering capacity).

What does a conductometric titration curve measure, and how is the equivalence point identified?

It measures the electrical conductivity of the solution as titrant is added; the equivalence point is identified from the intercept/change in gradient (minimum or inflection point) of the conductivity vs volume graph.

Why does conductivity typically decrease then increase during a strong acid-strong base titration to excess?

Initially, H⁺ ions (very high mobility) are replaced by less mobile Na⁺ ions as neutralisation occurs, decreasing conductivity; after the equivalence point, excess OH⁻ ions (also highly mobile) are added, increasing conductivity again.

BackgroundKnowledge What formula relates moles, concentration and volume of a solution?

Concentration (c) = moles of solute (n) / volume of solution (V), i.e. n = c × V

How do you calculate the concentration of an unknown solution from titration data against a standard solution?

Use the titration volume and known concentration of the standard solution to find moles reacted at the equivalence point, apply the mole ratio from the balanced equation to find moles of the unknown, then divide by its volume: c = n/V.

Give a real-world example of controlled equilibrium in food/beverage production.

Wine production relies on controlling reversible reactions (e.g. fermentation equilibria) to maintain the correct chemical balance in the final product.

How do oceans help stabilise atmospheric CO₂ concentrations?

CO₂ dissolves in seawater through reversible equilibrium processes, allowing oceans to absorb and release CO₂ in response to changing atmospheric levels — acting as a buffer/sink for atmospheric CO₂.

Who contributed early evidence (1856) for the heat-absorbing effect of CO₂ and water vapour relevant to climate change?

Eunice Newton Foote

What makes 'superacids' such as carborane acids far stronger than sulfuric acid?

Their molecular structure results in an extremely stable, non-basic conjugate base after deprotonation, which strongly favours the forward (ionisation) equilibrium — extrapolated to aqueous solution, their effective acid strength is estimated to be a million times that of sulfuric acid.

In QCAA Science syllabuses, what does the command word 'Explain' require in a response?

A response that makes relationships, reasons, or causes evident — going beyond describing WHAT happens to state WHY it happens.

What is the difference between 'Describe' and 'Explain' as command words?

Describe requires giving a detailed account of characteristics/features. Explain requires making relationships, reasons or causes evident — i.e. justifying why something occurs, not just what occurs.

What does the command word 'Analyse' typically require?

Identifying the essential elements, features, trends, patterns, relationships, limitations or uncertainty within data, often using mathematical processes for quantitative data.

What does the command word 'Interpret' require, and how does it differ from 'Analyse'?

Interpret requires using understanding to draw a conclusion or make a prediction/justification based on evidence. Analyse is the step of identifying trends/patterns in the data; Interpret uses that analysis to reach a conclusion.

What does the command word 'Evaluate' require in a chemistry response?

Critically judging the quality, validity or reliability of evidence, processes or claims — often including scrutinising evidence, extrapolating findings, and suggesting improvements or extensions.

What is the key difference between 'Analyse' and 'Evaluate'?

Analyse identifies trends, patterns and relationships in evidence. Evaluate makes a judgment about the quality/validity/reliability of that evidence or the conclusions drawn from it.

What does 'Apply' require students to do?

Use scientific concepts, theories, models or systems (within their limitations) to determine unknown quantities, explain phenomena, or predict outcomes — usually involves calculation or use of representations.

What does 'Discriminate between' require?

Identifying and explaining the differences between two related terms or concepts (e.g. strength vs concentration, validity vs reliability).

What is the difference between 'validity' and 'reliability' of an experiment?

Validity refers to whether the experiment actually tests what it claims to test (controlled variables, appropriate method). Reliability refers to whether repeated trials/measurements give consistent, reproducible results.

Define oxidation and reduction in terms of electron transfer.

Oxidation = loss of electrons from a chemical species. Reduction = gain of electrons by a chemical species. (Mnemonic: OIL RIG — Oxidation Is Loss, Reduction Is Gain.)

In a redox reaction, what is the difference between the species 'oxidised' and the 'oxidising agent'?

The species oxidised is the substance that loses electrons (undergoes oxidation). The oxidising agent is the substance that CAUSES oxidation by accepting those electrons — it is itself reduced.

Common misconception: is the oxidising agent the species that gets oxidised?

No — the oxidising agent is REDUCED (it gains electrons and causes another species to be oxidised). This reversal is a very common exam trap.

Name four types of reactions that can be modelled as redox reactions.

Displacement reactions of metals, combustion, corrosion, and electrochemical processes.

How is the oxidation state (number) of an atom represented in notation?

As a sign given BEFORE the number, e.g. +2, -1 (distinct from ionic charge notation which is written after, e.g. Fe²⁺).

How are oxidation numbers used in naming transition metal compounds?

Roman numerals are used after the metal name to indicate its oxidation state, e.g. iron(III) chloride, copper(II) sulfate.

How do you balance a redox equation using half-equations under acidic conditions?

Write separate oxidation and reduction half-equations, balance atoms and charge in each (using H⁺ and H₂O for acidic conditions), then scale and combine so electrons lost equal electrons gained, cancelling electrons from both sides.

What do all electrochemical cells consist of, structurally?

Oxidation and reduction half-reactions connected via an external circuit, allowing electrons to move from the anode (oxidation) to the cathode (reduction).

What is the key difference between a galvanic cell and an electrolytic cell?

A galvanic cell generates electrical energy from a spontaneous redox reaction. An electrolytic cell uses an external electrical energy source to drive a non-spontaneous redox reaction.

In BOTH galvanic and electrolytic cells, at which electrode does oxidation occur, and which does reduction occur?

Oxidation always occurs at the anode; reduction always occurs at the cathode — this is true for both cell types (only the electrode polarity/sign differs between them).

What energy conversion occurs in a galvanic cell?

Chemical energy (from a spontaneous redox reaction) is converted into electrical energy.

In a galvanic cell, which electrode is the anode (positive or negative) and which is the cathode?

The anode is negative (oxidation, electron source) and the cathode is positive (reduction, electron sink) in a galvanic cell.

What is the purpose of the salt bridge in a galvanic cell?

It allows ions to migrate between the two half-cells to maintain electrical neutrality (balance charge) as the reaction proceeds, completing the internal circuit without letting the bulk solutions mix.

List the essential components of a galvanic cell.

Two half-cells (oxidation and reduction), their electrodes (positive and negative), their electrolyte solutions containing the relevant ions, an external wire for electron flow, and a salt bridge for ion movement.

In a Zn/Cu galvanic cell, which metal is the anode and why?

Zinc is the anode, because zinc is more reactive (more easily oxidised, has a more negative standard reduction potential) than copper, so Zn loses electrons (is oxidised) while Cu²⁺ gains electrons (is reduced) at the cathode.

What is the standard hydrogen electrode (SHE), and why is it important?

A reference half-cell (H₂ gas over a Pt electrode in 1 mol L⁻¹ H⁺ solution) assigned a standard reduction potential of exactly 0 V, used as the reference point against which all other standard electrode potentials are measured.

Write the formula for calculating standard cell potential, E°cell.

E°cell = E°(reduction half-cell) − E°(oxidation half-cell)