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SAChemistrySyllabus dot point

How does a system at equilibrium respond when conditions are changed?

Apply Le Chatelier's principle to predict the effect of changes in concentration, pressure and temperature on the position of equilibrium.

How Le Chatelier's principle predicts the direction an equilibrium shifts when concentration, pressure or temperature is changed, and why temperature alone changes Kc.

Generated by Claude Opus 4.78 min answer

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  1. What this dot point is asking
  2. The principle
  3. Changing concentration
  4. Changing pressure (gases)
  5. Changing temperature
  6. Catalysts

What this dot point is asking

You must predict the direction of an equilibrium shift for changes in concentration, pressure and temperature, and explain the reasoning.

The principle

The system never fully undoes the change - it only partly offsets it.

Changing concentration

  • Add a reactant β†’ equilibrium shifts toward the products to consume the added reactant.
  • Add a product β†’ shifts toward the reactants.
  • Remove a product (e.g. let a gas escape or precipitate it) β†’ shifts toward products to replace it.

KcK_c is unchanged; the system simply re-balances the concentrations.

Changing pressure (gases)

Increasing pressure (by reducing volume) favours the side with fewer moles of gas, because that reduces the total pressure.

Changing temperature

Temperature is the only factor that changes KcK_c. Treat heat as a reactant or product:

  • For an exothermic reaction (heat is a product), increasing temperature shifts equilibrium toward reactants (KcK_c decreases).
  • For an endothermic reaction (heat is a reactant), increasing temperature shifts toward products (KcK_c increases).

Catalysts

A catalyst speeds up the forward and reverse reactions equally, so it helps the system reach equilibrium faster but does not change the position of equilibrium or the value of KcK_c.

Exam-style practice questions

Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2023 SACE Stage 23 marksIn the methanol production process CO2(g) + 3H2(g) <=> CH3OH(g) + H2O(g), the CH3OH and H2O are removed from the equilibrium mixture as they form. In terms of Le Chatelier's principle, explain how this removal helps maximise profit for the manufacturer of methanol.
Show worked answer β†’

Apply Le Chatelier to a decrease in product concentration.

  1. Removing CH3OH and H2O lowers the concentration of the products.

  2. By Le Chatelier's principle, the system partially opposes this change by shifting the position of equilibrium to the right (the forward direction) to replace the removed products.

  3. This continually converts more reactants into methanol, increasing the yield of product that can be sold and thereby maximising profit. One mark each.

2022 SACE Stage 24 marksWhen bromine dissolves in water: Br2 + H2O <=> H+ + Br- + HOBr. Explain, in terms of the equilibrium, why the colour of this solution fades as the pH is increased.
Show worked answer β†’

Bromine (Br2) is the coloured species, so fading means Br2 is being consumed.

  1. Increasing the pH means decreasing the concentration of H+ ions (adding base, which removes H+).

  2. By Le Chatelier's principle, the equilibrium shifts to the right (the forward direction) to partially replace the H+ that was removed.

  3. This shift consumes Br2 (converting it to colourless products), so the concentration of coloured Br2 decreases.

  4. With less Br2 present, the orange/brown colour of the solution fades. One mark per linked step.

2024 SACE Stage 23 marksFor the water gas shift reaction H2O(g) + CO(g) <=> H2(g) + CO2(g), explain one advantage to the manufacturer of using increased pressure.
Show worked answer β†’

Note that there are 2 moles of gas on each side, so pressure does not shift this particular equilibrium. The advantage is therefore about rate, not yield.

  1. Increasing the pressure increases the concentration of the gaseous reactant particles per unit volume.

  2. More frequent collisions between reactant molecules means more successful collisions per second, so the rate of reaction increases.

  3. The manufacturer reaches equilibrium and produces the desired hydrogen faster, improving throughput and profit. One mark for increased concentration, one for more frequent collisions, one for the faster rate benefit.

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