OCR Specification focus:
‘Use le Chatelier’s principle to predict qualitatively how temperature, pressure or concentration changes shift the position of homogeneous equilibria.’
Introduction
Le Chatelier’s principle predicts how a homogeneous equilibrium responds to external changes. Understanding qualitative shifts helps explain industrial decisions, laboratory observations, and the behaviour of dynamic systems.
Le Chatelier’s Principle: Core Ideas
Le Chatelier’s principle states that when a system at dynamic equilibrium experiences a change in conditions, it responds to oppose the change and restore equilibrium balance. Any prediction made using this principle must remain qualitative, focusing on the direction of the equilibrium shift, not numerical magnitudes.
Dynamic equilibrium: A state in a closed system where forward and reverse reaction rates are equal and concentrations remain constant over time.
A homogeneous equilibrium involves reactants and products in the same physical state, typically gaseous or aqueous. Because every species shares the same state, changes in concentration, pressure or temperature affect the entire system uniformly.
Normal sentence: These foundational ideas allow us to explore how different types of external changes influence equilibrium position.
Concentration Changes
Altering concentration affects the relative amounts of reactants and products available for collision. The system then shifts to reduce the imposed change.
Increasing Concentration of a Reactant
The system reduces the added reactant by favouring the forward reaction, producing more product.
Bullet points for predicted behaviour:
Added reactant increases reaction quotient Q.
The reaction proceeds in the direction that consumes added species.
Equilibrium shifts right.
Decreasing Concentration of a Reactant
The system compensates by producing more of the removed species, shifting equilibrium left.
Increasing Concentration of a Product
Predicted effects:
System favours reverse reaction.
Concentrations adjust until equilibrium is re-established.
Shift occurs left, reducing product concentration.
Normal sentence: These qualitative predictions must always be tied to the idea that the system counteracts the imposed change.
Three test tubes demonstrate how altering thiocyanate concentration shifts the Fe³⁺/SCN⁻ equilibrium, increasing or decreasing the red Fe(SCN)²⁺ complex. Extra procedural details exceed OCR requirements but support qualitative understanding. Source
Pressure Changes in Gaseous Equilibria
Pressure affects only equilibria involving gases, because gas volume and particle number are directly linked.
Changing Total Pressure
The system responds by shifting to the side with fewer or more moles of gas.
Mole ratio in gaseous equilibrium: The relative number of gaseous moles on each side of a balanced equation, determining how pressure changes influence equilibrium position.
If pressure is increased, the system shifts to the side with fewer moles of gas, reducing overall pressure.
If pressure is decreased, the shift occurs towards the side with more moles of gas.
If both sides contain equal moles of gas, pressure changes do not affect equilibrium position.
Normal sentence: Pressure changes must always be analysed by comparing gaseous mole totals on each side of the equilibrium.
A piston compressing an equilibrium mixture of NO₂ and N₂O₄ illustrates how increased pressure shifts the equilibrium towards fewer gas molecules. Numerical pressure values exceed OCR needs but reinforce qualitative predictions. Source
Temperature Changes
Temperature affects equilibrium by changing the relative rate of the forward and reverse reactions according to their enthalpy changes. Reactions can be exothermic (heat released) or endothermic (heat absorbed).
Exothermic reaction: A reaction in which heat is released; the forward direction has a negative enthalpy change.
Normal sentence: Recognising whether heat behaves like a product or reactant is essential when predicting shifts.
Endothermic reaction: A reaction in which heat is absorbed; the forward direction has a positive enthalpy change.
Increasing Temperature
If the forward reaction is exothermic, increasing temperature shifts equilibrium left (towards reactants).
If the forward reaction is endothermic, increasing temperature shifts equilibrium right (towards products).
Temperature changes alter Kc, unlike concentration or pressure changes.
Decreasing Temperature
Exothermic forward reaction → equilibrium shifts right.
Endothermic forward reaction → equilibrium shifts left.
Normal sentence: These predictions hinge on treating heat as a species within the equilibrium system.
Four sealed tubes display the NO₂/N₂O₄ equilibrium shifting with temperature, with darker brown NO₂ at higher temperatures and paler N₂O₄ at lower temperatures. Thermodynamic data on the page exceed OCR requirements but are unnecessary for interpreting the visual change. Source
Combined Effects and Qualitative Reasoning
Le Chatelier’s principle must be applied systematically, considering each change separately before combining them in multi-step predictions.
Layered Considerations
Identify the type of change: temperature, pressure, or concentration.
Determine how the system will oppose the imposed change.
Predict the direction of equilibrium shift only, not the new equilibrium composition.
Recognise limitations: the principle provides qualitative guidance, not quantitative outcomes.
Homogeneous Equilibria Context
Homogeneous equilibria are particularly suitable for application of Le Chatelier’s principle because the uniform phase ensures consistent responses across species. In solution equilibria, concentration changes are most significant, whereas in gaseous systems, pressure plays a central role.
Key Points for OCR
Predictions must be qualitative.
Focus on direction of shift, not numerical data.
Use terminology such as right/left shift, forward/reverse reaction, and oppose the change accurately.
Normal sentence: Mastering these ideas ensures strong performance in both analysis and written explanation tasks related to equilibrium behaviour.
FAQ
Le Chatelier’s principle offers a qualitative prediction, focusing on how a system opposes an external change.
The reaction quotient method compares Q with K to determine the direction of net change.
Le Chatelier’s principle does not involve numerical values, whereas Q and K allow a more quantitative, calculation-based prediction.
The principle predicts the initial direction of shift, not continuous changes.
Once equilibrium is re-established, the system no longer responds to the original disturbance.
Further predictions require a new change in conditions because the system treats each disturbance independently.
No, it predicts only the direction.
Magnitude depends on factors such as:
numerical values of K
reaction stoichiometry
extent of concentration or pressure change
Le Chatelier’s principle cannot quantify how far the new equilibrium lies from the original position.
At constant volume, adding an inert gas does not change partial pressures, so equilibrium position remains unchanged.
At constant pressure, total volume must increase when inert gas is added, reducing partial pressures of gaseous species.
This mimics a decrease in pressure and shifts equilibrium towards the side with more moles of gas.
The principle simplifies heat as a reactant or product and does not address energy distribution or molecular behaviour.
It does not explain why temperature changes alter equilibrium constants.
For deeper analysis, thermodynamic quantities such as enthalpy, entropy and Gibbs free energy are needed, which extend beyond the qualitative scope of this subsubtopic.
Practice Questions
A homogeneous equilibrium mixture is disturbed by adding more of one of the reactants.
Using Le Chatelier’s principle, state and explain the effect this change has on the position of equilibrium.
(2 marks)
Mark points:
States that the equilibrium shifts to the right / towards products. (1 mark)
Explains that this shift occurs to reduce the concentration of the added reactant and oppose the change. (1 mark)
The equilibrium below is established in a sealed container:
N2O4(g) ⇌ 2NO2(g)
A student increases the temperature and decreases the pressure of the system.
Using Le Chatelier’s principle, predict and explain how:
a) increasing the temperature affects the position of equilibrium
b) decreasing the pressure affects the position of equilibrium
c) comment on whether the colour of the mixture becomes darker or paler
(5 marks)
a) Temperature increase:
Identifies that forward reaction is endothermic / temperature increase favours the endothermic direction. (1 mark)
States that equilibrium shifts to the right / towards NO2. (1 mark)
b) Pressure decrease:
Recognises that the side with more moles of gas is favoured. (1 mark)
States that equilibrium shifts to the right / towards 2NO2. (1 mark)
c) Colour observation:
States that the mixture becomes darker brown because NO2 is brown and its concentration increases. (1 mark)
