The effect of temperature changes on equilibrium (HT only) — AQA GCSE Combined Science
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The effect of temperature changes on equilibrium (HT only) explained
A system at equilibrium has forward and reverse reactions occurring at equal rates, so concentrations stay constant.
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If the temperature is increased, the position of equilibrium shifts to oppose the change. The endothermic direction is favoured because it takes in the added thermal energy, reducing the temperature. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward exothermic), raising the temperature makes the reverse, endothermic reaction more favourable, so the equilibrium yield of ammonia falls. Cooling does the opposite, favouring the exothermic direction. This statement is the starting point for predicting how heating changes the relative amount of products.
the relative amount of products at equilibrium increases for an endothermic reaction
When the forward reaction is endothermic, heating the equilibrium mixture favours that forward direction because it takes in the added thermal energy. More reactant is converted into product, so the relative amount of products at the new equilibrium increases and the yield rises. For example, in CaCO₃(s) ⇌ CaO(s) + CO₂(g), the forward reaction is endothermic, so raising the temperature increases the amount of CaO and CO₂ formed. The reverse applies on cooling: the exothermic direction is favoured and product amount falls. This statement is the specific case of the previous rule, so always check whether the forward reaction is endothermic before predicting an increase in products.
the relative amount of products at equilibrium decreases for an exothermic reaction.
According to Le Chatelier's principle, if a change is made to the conditions of a system at dynamic equilibrium, the system responds to counteract the change. For a reversible reaction where the forward direction is exothermic, the reverse path is inherently endothermic. Increasing the temperature of such a system forces the equilibrium to shift in the endothermic direction to absorb the excess thermal energy. Consequently, the equilibrium shifts to the left, and the relative amount of products decreases. For example, in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic. Heating the reaction mixture shifts the equilibrium to the left, lowering the yield of ammonia while increasing the amounts of nitrogen and hydrogen.
If the temperature of a system at equilibrium is decreased:
Le Chatelier's principle predicts that decreasing the temperature of a system at dynamic equilibrium causes the system to shift its position to generate heat and oppose the change. Therefore, cooling forces the system to favour the exothermic reaction path. If the forward reaction is exothermic, decreasing the temperature shifts the equilibrium to the right, increasing the yield of products. Conversely, if the forward reaction is endothermic, cooling shifts the equilibrium to the left, decreasing the product yield. For example, in the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic. Cooling the reaction mixture shifts the equilibrium to the right, raising the yield of ammonia. However, lower temperatures also reduce the rate of reaction.
the relative amount of products at equilibrium decreases for an endothermic reaction
Le Chatelier's principle predicts how a system at dynamic equilibrium responds to changes. If the forward reaction is endothermic, it absorbs heat energy. When the temperature of such a system is decreased, the system responds by opposing the change, shifting the position of equilibrium to release heat. This favours the exothermic reverse reaction. The reverse rate temporarily exceeds the forward rate until equilibrium is re-established. This shift to the left reduces the equilibrium yield, meaning less product is present. For example, in the endothermic forward reaction N₂O₄(g) ⇌ 2NO₂(g), cooling the mixture shifts the equilibrium to the left, producing a paler mixture as the proportion of brown NO₂ falls.
the relative amount of products at equilibrium increases for an exothermic reaction.
When a reversible reaction reaches dynamic equilibrium, the amounts of reactants and products stay constant. As Higher Tier only content, you must understand how altering temperature changes the equilibrium position. If the forward reaction releases heat, it is exothermic. Raising the temperature adds heat, so the system shifts in the endothermic direction to absorb heat, decreasing the product amount. Lowering the temperature removes heat, so equilibrium shifts in the exothermic forward direction to release heat and oppose the cooling. This increases the relative amount of product. For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), cooling increases the ammonia yield.
Students should be able to interpret appropriate given data to predict the effect of a change in temperature on given reactions at equilibrium.
This Higher Tier skill requires you to use supplied data, such as equilibrium yields or concentrations at different temperatures, to decide how heating or cooling shifts a reversible reaction at equilibrium. First, identify whether the forward reaction is exothermic or endothermic from a given ΔH value or yield data. If the product yield rises as temperature falls, the forward reaction is exothermic. Then apply Le Chatelier's principle: increasing temperature favours the endothermic direction, while decreasing temperature favours the exothermic direction. Use the data to justify the shift. For example, if product yield falls from 45% at 300 K to 20% at 500 K, the forward reaction is exothermic.
Your focus
- State that increasing temperature favours the endothermic direction in a system at equilibrium.
- Identify the endothermic direction from a given equation or energy information.
- Predict and explain how a temperature increase changes the relative amount of product at equilibrium.
Show all 21 objectives
- Explain why heating an endothermic equilibrium increases the relative amount of products.
- Apply the endothermic-direction rule to a given reversible equation.
- Describe the effect of cooling on the same endothermic equilibrium.
- Apply Le Chatelier's principle to predict the effect of temperature changes on equilibrium.
- Explain why heating an exothermic reaction shifts the equilibrium position to the left.
- Predict that the relative amount of product decreases for an exothermic reaction upon heating.
- Explain the effect of cooling on a system at dynamic equilibrium using Le Chatelier's principle.
- Identify the exothermic direction of a reversible reaction from given data.
- Predict how the relative amount of products changes on cooling for any given equilibrium system.
- Identify the forward reaction as endothermic from a positive enthalpy change or given information.
- Predict that decreasing the temperature shifts the equilibrium of an endothermic reaction in the reverse direction.
- Explain why the equilibrium yield of product falls when an endothermic reaction is cooled.
- Identify the forward reaction as exothermic from the information given.
- Predict the direction of the equilibrium shift when the temperature is lowered.
- State that the relative amount of product at equilibrium increases when an exothermic reaction is cooled.
- Interpret given equilibrium data to identify whether the forward reaction is exothermic or endothermic.
- Predict the direction of equilibrium shift when temperature is increased or decreased.
- Justify a prediction using specific data values and Le Chatelier's principle.
The effect of temperature changes on equilibrium (HT only) exam tips
Marking Points
- Equilibrium means forward and reverse reaction rates are equal, so the amounts of reactants and products remain constant.
- Increasing temperature adds thermal energy, and the system responds by favouring the direction that takes in energy.
- The endothermic direction is favoured when temperature is increased; the exothermic direction is favoured when temperature is decreased.
- The shift changes the relative amounts of substances present at the new equilibrium, not the rates being unequal at that new position.
- A catalyst speeds up both directions equally and does not change the position of equilibrium, unlike a temperature change.
- For an endothermic forward reaction, increasing temperature shifts equilibrium to the right, towards products.
- The shift occurs because the forward reaction takes in the added thermal energy, opposing the temperature rise.
- The relative amount of products at the new equilibrium is greater than before the temperature increase.
- Cooling the same system favours the reverse, exothermic reaction and decreases the relative amount of products.
- The prediction depends on whether the forward reaction is endothermic or exothermic, so the equation or energy data must be checked first.
- Identify that the forward reaction is exothermic and the reverse reaction is endothermic.
- State that increasing the temperature causes the system to favour the endothermic reaction to absorb heat.
- Conclude that the equilibrium position shifts to the left (towards the reactants).
- Deduce that the relative amount of products decreases, lowering the overall yield.
- State that decreasing the temperature causes the system to favour the exothermic reaction to release heat.
- Identify whether the forward or reverse reaction is exothermic based on the given enthalpy change.
- Conclude that the equilibrium shifts to the right if the forward reaction is exothermic, or to the left if it is endothermic.
- Deduce the final effect on the relative amount of products, such as an increased yield of ammonia in the Haber process.
- State that decreasing the temperature of an equilibrium mixture causes the system to shift to oppose the change by releasing heat.
- Identify that for a forward endothermic reaction, the reverse reaction must be exothermic.
- Explain that a temperature decrease therefore favours the reverse exothermic reaction.
- Conclude that the position of equilibrium shifts to the left, reducing the equilibrium yield of products.
- Apply this principle to a specific equation by naming the product whose equilibrium yield will fall upon cooling.
- State that the forward reaction is exothermic, meaning it releases heat to the surroundings.
- Explain that lowering the temperature removes heat energy from the equilibrium system.
- Apply Le Chatelier's principle: the equilibrium shifts in the direction that releases heat, which is the forward reaction.
- Conclude that the forward reaction is favoured, so the relative amount of product at equilibrium increases.
- Recognise that the reverse reaction must be endothermic because the forward reaction is exothermic.
- Describe the effect of heating as the opposite change, shifting equilibrium towards the endothermic reverse reaction and decreasing the product amount.
- Identify from the given data whether the forward reaction is exothermic or endothermic, for example by comparing product yield or concentration at two temperatures.
- State the direction of shift caused by raising or lowering temperature, linking it to the endothermic or exothermic direction respectively.
- Use specific numerical values from the data to support the predicted shift rather than making a general statement.
- Explain the prediction in terms of Le Chatelier's principle, noting that the system opposes the temperature change by favouring the reaction that absorbs or releases heat.
- Apply the prediction to a named reaction or context, such as the Haber process, and comment on the yield of product or position of equilibrium.
Examiner Tips
- 💡State clearly that the endothermic direction is favoured when temperature increases, then apply it to the equation given.
- 💡Use the equation's energy label or data to identify which direction is endothermic before predicting the shift.
- 💡Link your prediction to relative amounts of products, for example 'the yield of product decreases', rather than only saying 'shifts left'.
- 💡Quote the endothermic direction explicitly, then state that the relative amount of products increases.
- 💡Use the equation to name the products whose amount increases, showing the link between theory and the specific reaction.
- 💡If the question gives an energy profile or states the energy change, use it to justify why the forward direction is favoured on heating.
- 💡Always identify which direction is exothermic and which is endothermic before predicting the effect of a temperature change.
- 💡Explicitly state 'shifts to the left' or 'shifts to the reactants' and clearly conclude that the 'amount of product decreases'.
- 💡Check the sign of the enthalpy change (ΔH) to determine if the forward reaction is exothermic (negative) or endothermic (positive).
- 💡Link the phrase 'shifts to the right/left' directly to the change in the amount of products to secure full marks.
- 💡Always check the sign of the enthalpy change to determine if the forward reaction is endothermic before predicting the effect of a temperature change.
- 💡When explaining a shift, explicitly state that the system opposes the temperature decrease by favouring the exothermic direction, leading to a lower yield of products.
- 💡Check whether the forward reaction is exothermic or endothermic before deciding the direction of the shift.
- 💡Link the shift to the temperature change by saying the system opposes the change, for example cooling shifts equilibrium to the right for an exothermic forward reaction.
- 💡Remember this is Higher Tier content; you may need to apply Le Chatelier's principle to unfamiliar reactions.
- 💡Quote at least two numerical values from the table or graph to justify your prediction.
- 💡Write the direction of shift clearly, for example 'equilibrium shifts to the left' or 'towards the reactants'.
- 💡As this is Higher Tier only, expect to apply these principles to unfamiliar data sets.
Common Mistakes
- Saying the forward reaction always speeds up more when heated: correct this by stating that the endothermic direction is favoured, which may be the reverse reaction.
- Confusing rate with position of equilibrium: correct this by explaining that a new equilibrium is reached where both rates are equal again but amounts differ.
- Thinking a catalyst shifts equilibrium to raise yield: correct this by stating that a catalyst only reduces the time taken to reach equilibrium.
- Assuming all forward reactions are endothermic: correct this by checking the energy change given for the specific reaction.
- Saying the equilibrium shifts left when heated for an endothermic forward reaction: correct this by stating it shifts right, increasing products.
- Writing that heating increases the rate of the forward reaction only: correct this by explaining that both rates change and a new equilibrium position is established.
- Assuming a faster reaction rate means a higher yield: rate and yield are distinct; heating increases the rate of reaching equilibrium but decreases the yield of an exothermic reaction.
- Treating exothermic reactions as favoured by heat: heating always favours the endothermic direction to oppose the temperature increase.
- Confusing equilibrium shift with reaction speed: a shift to the left alters the relative amounts of reactants and products, not just how fast they form.
- Assuming cooling always increases product yield: the effect depends entirely on whether the forward reaction is exothermic or endothermic.
- Confusing shift direction with reaction rate: cooling shifts the position towards the exothermic side to increase yield, but it simultaneously slows down the rate of reaction.
- Omitting the link to the exothermic direction: students often state the shift without explaining that the system is trying to release thermal energy.
- Stating that cooling an endothermic reaction increases the product yield: correct this by applying Le Chatelier's principle, which shows cooling favours the exothermic reverse reaction, decreasing the product.
- Confusing the rate of reaction with the position of equilibrium: correct this by noting that while cooling decreases both forward and reverse rates, it decreases the endothermic forward rate more, shifting equilibrium to the left.
- Assuming the forward reaction is always exothermic: correct this by carefully checking the provided enthalpy change sign; a positive value indicates an endothermic forward reaction.
- Assuming that cooling always slows the reaction and therefore lowers the product amount: correct this by separating rate from equilibrium position, because cooling shifts the position towards the exothermic direction.
- Stating that heating an exothermic reaction increases the product amount: correct this by explaining that heating favours the endothermic reverse reaction, so product decreases.
- Mixing up the labels exothermic and endothermic for the forward and reverse reactions: correct this by noting that if the forward reaction is exothermic, the reverse reaction is endothermic.
- Assuming that increasing temperature always increases product yield: correct this by checking the data and identifying the exothermic direction, which is favoured by cooling.
- Confusing the direction of shift with the rate of reaction: correct this by separating equilibrium position from reaction rate, since temperature affects both but in different ways.
- Ignoring the sign of ΔH or misreading it: correct this by using the given data to infer whether the forward reaction is exothermic or endothermic before predicting the shift.