HSC Exam Practice

Sample response. Le Chatelier’s Principle states that when a closed system at dynamic equilibrium is disturbed by a change in conditions, the system shifts in the direction that minimises the effect of the disturbance and re-establishes equilibrium.

Marking notes. 1 mark for “disturbed / change in conditions”; 1 mark for “shifts to minimise / partially counteract the disturbance” or equivalent. Award both marks for a complete correct statement. Do not award for “the system reverses the change” — LCP minimises, not eliminates.

Sample response. A concentration change shifts the equilibrium position (the relative amounts of products and reactants change) but does not alter the value of K_eq — because K_eq is defined as the ratio of equilibrium concentrations at a given temperature, and that temperature has not changed. A temperature change both shifts the equilibrium position and changes the value of K_eq, because it alters the activation energies of the forward and reverse reactions by different amounts, permanently changing the ratio of forward to reverse rates at equilibrium.

Marking notes. 1 mark — concentration change shifts equilibrium position but does not change K_eq. 1 mark — temperature change changes K_eq (not just shifts position). 1 mark — justification: temperature changes because it alters the thermodynamic basis (activation energies / Boltzmann distribution) of the reaction rates; concentration does not. Accept: “only temperature changes K_eq” for the second mark without full justification.

Sample response. The equilibrium shifts to the right (forward direction). The forward reaction is endothermic (ΔH = +57 kJ mol⁻¹), so increasing temperature adds thermal energy. By Le Chatelier’s Principle, the system shifts in the endothermic direction (right, toward NO₂) to absorb some of the added heat. K_eq increases, because the shift toward products means the ratio [NO₂]²/[N₂O₄] at the new equilibrium is larger than before.

Marking notes. 1 mark — shift right (forward). 1 mark — justified using endothermic forward / LCP correctly applied. 1 mark — K_eq increases (not just “changes”).

Sample response. Adding Fe(NO₃)₃ increases the concentration of Fe³⁺(aq) (a reactant). The solution turns darker red. By LCP, increasing the concentration of a reactant increases the forward reaction rate above the reverse rate, shifting equilibrium to the right (toward FeSCN²⁺). More FeSCN²⁺ (deep red) is formed, intensifying the red colour. K_eq is unchanged.

Marking notes. 1 mark — observation: solution turns darker/more intensely red. 1 mark — LCP: adding a reactant (Fe³⁺) shifts equilibrium right. 1 mark — more FeSCN²⁺ produced; K_eq unchanged (accept without K_eq statement if not explicitly asked).

Sample response. The carbonic acid equilibrium acts as a blood buffer: it maintains blood pH within the normal range (7.35–7.45) by responding to any change in H⁺ concentration. When [H⁺] rises, the system shifts left (removing H⁺); when [H⁺] falls, it shifts right (releasing H⁺). During hyperventilation, CO₂ is exhaled rapidly, reducing [CO₂(aq)] in blood plasma. This is a concentration disturbance (removing a reactant). LCP shifts the first equilibrium (CO₂ + H₂O ⇌ H₂CO₃) to the left, reducing [H₂CO₃]. The second equilibrium (H₂CO₃ ⇌ H⁺ + HCO₃⁻) also shifts left, consuming H⁺. Blood [H⁺] falls and pH rises (respiratory alkalosis).

Marking notes. 1 mark — equilibrium maintains blood pH by responding to [H⁺] changes. 1 mark — hyperventilation removes CO₂, a concentration disturbance. 1 mark — first equilibrium shifts left, reducing [H₂CO₃]. 1 mark — second equilibrium shifts left, reducing [H⁺]; pH rises.

Sample response. Ag⁺ ions (from AgNO₃) react with SCN⁻ to form a white precipitate of AgSCN(s), effectively removing SCN⁻ from solution. SCN⁻ is a reactant in the equilibrium Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq). According to LCP, removing a reactant shifts the equilibrium to the left (reverse), decomposing FeSCN²⁺ back to Fe³⁺ and SCN⁻. Less FeSCN²⁺ (deep red) remains in solution, so the colour fades. K_eq is unchanged — the disturbance is a concentration change, not a temperature change.

Marking notes. 1 mark — Ag⁺ removes SCN⁻ as AgSCN precipitate. 1 mark — LCP: removing reactant SCN⁻ shifts equilibrium left. 1 mark — FeSCN²⁺ decomposes → colour fades; K_eq unchanged. Award final mark for K_eq unchanged with correct justification.

(a) Trend (2 marks). K_eq decreases as temperature increases [1]. The magnitude of the decrease is substantial: K_eq falls from 4.3 × 10⁶ at 300°C to 6.2 × 10¹ at 600°C — a decrease of more than four orders of magnitude [1].

(b) Explanation (3 marks). The forward reaction is exothermic (ΔH = −92 kJ mol⁻¹) [1]. Increasing temperature adds thermal energy; Le Chatelier’s Principle shifts the equilibrium in the endothermic direction (reverse), producing more N₂ and H₂ and less NH₃ [1]. This shift toward reactants means the ratio [NH₃]²/([N₂][H₂]³) is smaller at higher temperatures — K_eq decreases. Temperature is the only variable that changes K_eq [1].

(c) Pressure and K_eq (2 marks). Increasing pressure at constant temperature shifts the equilibrium toward fewer moles of gas. The Haber process has 4 moles of gaseous reactants (1 N₂ + 3 H₂) and 2 moles of gaseous product (2 NH₃), so increased pressure shifts equilibrium right (toward NH₃), increasing yield [1]. However, K_eq does not change because temperature is unchanged. Pressure changes the equilibrium position (relative concentrations) but not the thermodynamic ratio that defines K_eq — K_eq only changes with temperature [1].

(a) Blue in oven (3 marks). The forward reaction is endothermic [1]. Heating to 110°C adds thermal energy; by LCP the system shifts in the endothermic direction (forward, right) to absorb some of the added heat [1]. Water is driven off as vapour, converting CoCl₂·6H₂O (pink hexahydrate) to CoCl₂ (anhydrous, blue) [1]. K_eq increases (endothermic forward; higher T = higher K_eq).

(b) Pink with steam (2 marks). Applying steam increases the concentration of H₂O(g), a product of the forward reaction [1]. LCP shifts the equilibrium to the left (reverse direction) to partially counteract the increased H₂O(g) concentration; CoCl₂ reabsorbs water to re-form the pink hexahydrate CoCl₂·6H₂O [1]. K_eq does NOT change — this is a concentration disturbance at constant temperature, not a temperature change.

Marking criteria.

• [1] States or implies LCP correctly: system shifts to minimise the disturbance.
• [1] Identifies that temperature changes K_eq, while concentration/pressure changes only alter equilibrium position — a key distinction the claim conflates.
• [1] Named industrial process correctly: Haber process (or Contact Process) with correct equation and ΔH sign.
• [1] Applies LCP: for the Haber process (exothermic), lower temperature shifts equilibrium right (toward product), increasing K_eq and yield, but also reduces rate.
• [1] Identifies the rate–yield compromise: choosing temperature also affects reaction rate; a temperature that maximises yield may give a commercially unacceptable rate.
• [1] Evaluates the claim: the claim is partially correct but oversimplified — industrial temperature choices are a compromise between equilibrium yield (favoured by lower T for exothermic forward reactions) and reaction rate (favoured by higher T), not a simple optimisation of equilibrium yield.
• [1] Integrative quality mark: a coherent, multi-criteria answer that correctly links LCP, K_eq, kinetics, and industrial practice; OR includes an additional nuanced point (e.g. pressure and catalyst also affect yield without changing K_eq; recycling of unreacted gases compensates for lower per-pass yield).

Sample response. Le Chatelier’s Principle states that when an equilibrium system is disturbed, it shifts to partially counteract the disturbance. The claim that LCP’s purpose in industrial chemistry is to maximise equilibrium yield by choosing temperature is partially correct but fundamentally incomplete. Temperature is unique among disturbances because it is the only one that changes K_eq; concentration and pressure changes alter the equilibrium position but leave K_eq unchanged. In the Haber process (N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92 kJ mol⁻¹) the forward reaction is exothermic. A lower temperature shifts equilibrium right (by LCP, toward the exothermic direction), increasing K_eq and the equilibrium yield of NH₃. At 300°C, K_eq is approximately 4.3 × 10⁶ and the equilibrium yield is ∼63% at 300 atm. However, at 300°C the reaction rate is too slow for commercial production even with an iron catalyst; the high activation energy means few collisions are successful. Incitec Pivot operates at 400–450°C where K_eq is lower (∼10³–10⁴) and yield is lower (26–36%), but the rate is commercially viable and equilibrium can be reached within hours. The lower per-pass yield is compensated by recycling unreacted gases. The claim is therefore an oversimplification: LCP and K_eq tell us that lower temperature increases yield for an exothermic forward reaction, but industrial temperature choice must also account for reaction rate, catalyst activity, and process economics — it is a compromise, not a yield-maximisation exercise.