AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Le Chatelier's principle can be used to predict the effects of changes in…
Full specification wording
Chemical equilibria and Le Chatelier's principle: Le Chatelier's principle can be used to predict the effects of changes in temperature, pressure and concentration on the position of equilibrium in homogeneous reactions.
Apply the principle one variable at a time.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 4 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Apply the principle one variable at a time. Raising the temperature shifts the equilibrium in the endothermic direction, absorbing energy to oppose the rise; cooling shifts it the exothermic way. So if the forward reaction is exothermic, a lower temperature raises the yield of product, and a question that reports a falling yield of a product on cooling is telling you the forward reaction is endothermic. Raising the pressure shifts the equilibrium towards the side with fewer moles of gas; if both sides have the same number, pressure has no effect. Increasing the concentration of a reactant shifts the equilibrium to the right, removing a product does the same. In a homogeneous system every species shares one phase, so all of them count.
What earns the marks
- Identifying whether the forward reaction is exothermic or endothermic when temperature is changed.
- Stating that the equilibrium shifts in the direction that opposes the change, quoting the direction.
- Counting the moles of gas on each side before predicting the effect of a pressure change.
- The consequence for the yield of the named substance.
Common mistakes and how to put them right
Shifting the equilibrium in the exothermic direction when the temperature is raised.
Counting all moles, including liquids and solids, when deciding the effect of pressure.
Predicting a pressure effect for a reaction with equal numbers of gaseous moles on each side.
Saying the yield increases without naming the direction the equilibrium moved.
Claiming that adding more of a reactant increases the equilibrium constant.
Exam tips
- Write the balanced equation and count the gas moles on each side before answering any pressure question.
- For temperature, state the sign of ∆H for the forward reaction first, then the direction of the shift.
- Give the effect and the explanation as two separate statements, since they are usually two separate marks.
Try the lesson
The start of the Chemical equilibria and Le Chatelier's principle: Le Chatelier's principle can be used to predict the effects of changes in temperature, pressure and concentration on the position of equilibrium in homogeneous reactions. lesson, made from checked teaching notes for this specification point.
Have a go first
In a homogeneous equilibrium, the concentration of a reactant is increased. What effect does Le Chatelier's principle predict for the position of equilibrium?
Listen
Apply the principle one variable at a time. Raising the temperature shifts the equilibrium in the endothermic direction, absorbing energy to oppose the rise; cooling shifts it the exothermic way. So if the forward reaction is exothermic, a lower temperature raises the yield of product, and a question that reports a falling yield of a product on cooling is telling you the forward reaction is endothermic. Raising the pressure shifts the equilibrium towards the side with fewer moles of gas; if both sides have the same number, pressure has no effect. Increasing the concentration of a reactant shifts the equilibrium to the right, removing a product does the same. In a homogeneous system every species shares one phase, so all of them count. A strong answer earns marks for these points. identifying whether the forward reaction is exothermic or endothermic when temperature is changed. stating that the equilibrium shifts in the direction that opposes the change, quoting the direction. counting the moles of gas on each side before predicting the effect of a pressure change. the consequence for the yield of the named substance. Exam tip: Write the balanced equation and count the gas moles on each side before answering any pressure question.
See it
Key words
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temperature and the endothermic direction
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pressure and moles of gas
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concentration changes
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homogeneous equilibria
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See every definition
- temperature and the endothermic direction
- Raising the temperature shifts the equilibrium in the endothermic direction, absorbing energy to oppose the rise, while cooling shifts it the exothermic way.
- pressure and moles of gas
- Raising the pressure shifts the equilibrium towards the side with fewer moles of gas; if both sides have the same number, pressure has no effect.
- concentration changes
- Increasing the concentration of a reactant shifts the equilibrium to the right, and removing a product does the same.
- homogeneous equilibria
- In a homogeneous system every species shares one phase, so all of them count.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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