Chemical Equilibrium and Le Chatelier's Principle
Chemical Equilibrium
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Chemical Equilibrium
Open with the idea that many reactions do not go to completion. Frame the lesson around predicting what happens when conditions change.
Learning goal
I can explain dynamic equilibrium and use Le Chatelier's principle and equilibrium constants to predict how changing conditions affects equilibrium position and yield.
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What words do you link with chemical equilibrium?
Use this to activate prior knowledge after the learning goal. Likely responses: reversible, balance, rate, closed system, dynamic, yield, catalyst. Pick up misconceptions such as 'equal amounts' for later correction.
KS5 Chemistry lesson intro
This KS5 chemistry lesson explores Chemical Equilibrium and Le Chatelier's Principle. Students use equilibrium constants and qualitative reasoning to predict how changes in conditions affect equilibrium position and yield. It is suitable for A level classroom teaching, independent study or exam revision.
Keep this slide exactly as written to meet the user's requirement.
What equilibrium means
In a reversible reaction, reactants form products and products can react to reform reactants
At equilibrium, both directions are still happening
The amounts of reactants and products stay constant
Constant does not mean equal amounts
Students often confuse equilibrium with equal concentrations. Stress that the rates are equal, not necessarily the quantities.
Dynamic equilibrium
Happens in a closed system
Forward rate = reverse rate
No visible overall change
Microscopic change continues all the time
Link 'dynamic' to particle behaviour. A closed system is essential because substances must not escape if equilibrium is to be established.
How equilibrium is reached
At the start, the forward reaction is usually faster because only reactants are present. As reactants are used up, the forward rate falls. As products build up, the reverse rate rises. Equilibrium is reached when the two rates become equal.
If useful, sketch a rate–time graph while teaching this slide: forward starts high and falls; reverse starts at zero and rises.
A visible example
2NO₂(g) ⇌ N₂O₄(g)
NO₂ is brown; N₂O₄ is colourless
A change in conditions can make the mixture appear darker or paler
The colour change shows the equilibrium position has shifted
This is a useful visual example for later discussion of temperature and pressure changes.
Which statement about equilibrium is correct?
Reactants equal products
Rates become equal
Reaction has stopped
System is open
Checks the core idea from slides 3 to 6: dynamic equilibrium means equal forward and reverse rates in a closed system, not equal amounts and not a stopped reaction.
Complete the summary
In a reversible reaction, products can reform reactants. At dynamic equilibrium, the forward and reverse reactions occur at equal rates in a closed system. The amounts stay constant, but not necessarily equal.
Echoes the wording of the first subtopic and targets the common misconception that equilibrium means equal amounts.
Le Chatelier's principle
If a system at equilibrium is disturbed, the equilibrium position shifts in the direction that tends to oppose the change.
Keep the wording tight and reusable. Students need this as their core prediction rule.
Changing concentration
If concentration increases
The system shifts to use up what was added
Adding a reactant tends to favour products
Adding a product tends to favour reactants
If concentration decreases
The system shifts to replace what was removed
Removing a product tends to favour products
Removing a reactant tends to favour reactants
Remind students that only species in the equilibrium expression matter when later linking this to Kc calculations.
Changing pressure
Pressure changes matter only for equilibria involving gases
Increasing pressure favours the side with fewer moles of gas
Decreasing pressure favours the side with more moles of gas
If both sides have the same number of gaseous moles, there is no shift
Students often say pressure affects all equilibria. Restrict this to gaseous equilibria and compare total gas moles on each side.
Changing temperature
Treat heat as part of the equilibrium
Increasing temperature favours the endothermic direction
Decreasing temperature favours the exothermic direction
Temperature can change both equilibrium position and the value of the equilibrium constant
This is the most conceptually important condition change because, unlike concentration and pressure, it changes Kc.
Catalysts and equilibrium
A catalyst speeds up the forward and reverse reactions
Equilibrium is reached faster
The equilibrium position does not change
The value of Kc does not change
A common exam error is claiming that catalysts increase yield at equilibrium. Clarify that they only affect rate, not position.
If pressure increases, which side is favoured?
More gas moles
Endothermic side
Fewer gas moles
Catalysed side
Tests slide 9 specifically. Students often confuse pressure with temperature effects or think catalysts create a 'favoured side'.
Which change can alter the value of Kc?
Temperature
Catalyst
Pressure
Concentration
This is the key synthesis point from slides 10 and 11: only temperature changes Kc, while the others may change position or rate only.
Why does adding a reactant shift equilibrium to the right?
Expected elements: applying Le Chatelier's principle, students should say the system shifts to oppose the change by using up added reactant and therefore moves towards products. Accept precise references to 'shift to the right' if linked to adding a reactant. This supports the applied reasoning from slides 7 to 11.
Equilibrium constants
An equilibrium constant compares the amounts of products and reactants at equilibrium
For a given reaction, the expression depends on the balanced equation
At A level, Kc uses equilibrium concentrations
Solids are not included in a Kc expression
Keep the focus on what Kc represents before moving to calculation structure.
Writing a Kc expression
For aA + bB ⇌ cC + dD:
Kc = [C]^c[D]^d / [A]^a[B]^b
Use equilibrium concentrations
Powers come from the stoichiometric coefficients
Point out that Kc has a fixed form for a given balanced equation, so students should always start by balancing correctly.
Interpreting Kc
Products are favoured at equilibrium
Equilibrium lies to the right
Reaction gives a high equilibrium yield of products
Reactants are favoured at equilibrium
Equilibrium lies to the left
Reaction gives a low equilibrium yield of products
Large Kc
Small Kc
Avoid implying that Kc says anything directly about rate. A reaction can have a large Kc and still reach equilibrium slowly.
Temperature and Kc
Only temperature changes the value of Kc
Concentration changes shift the position until the same Kc is re-established
Pressure changes can shift gaseous equilibria, but Kc stays the same if temperature is unchanged
Catalysts do not change Kc
This is a key synthesis slide linking qualitative reasoning to equilibrium constants.
What does a large Kc show?
Products favoured
Fast reaction rate
Reactants favoured
Catalyst present
Targets the misconception from slides 12 to 15 that Kc tells you about rate. It indicates equilibrium position, not speed.
Complete the Kc summary
For a given balanced equation, Kc uses equilibrium concentrations. The products go in the numerator and the reactants go in the denominator. The powers come from the stoichiometric coefficients. Only temperature changes the value of Kc.
Reinforces the structure and interpretation of Kc using the exact language from the surrounding slides.
Applied example: Haber process
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Forward reaction is exothermic
Fewer moles of gas on the product side
Conditions are chosen to balance yield, rate and cost
This is the standard UK A level industrial example. Use it to integrate pressure, temperature and catalyst ideas.
Why these conditions are used
Higher pressure increases ammonia yield because equilibrium shifts to the side with fewer gaseous moles
Lower temperature increases ammonia yield because the forward reaction is exothermic
In practice, industry uses a compromise temperature because very low temperatures make the reaction too slow
An iron catalyst speeds up the reaction without changing equilibrium position
Emphasise the idea of compromise conditions, which is central in UK exam questions.
Predicting shifts step by step
Identify the change in conditions
Decide which rule applies: concentration, pressure, temperature or catalyst
Compare the two sides of the equation if gases are involved
Check whether the forward reaction is exothermic or endothermic
State the shift and the effect on equilibrium yield
This gives students a reliable exam method. Encourage precise language: 'equilibrium shifts to the right' rather than 'reaction moves forwards'.
For the Haber process, which change increases ammonia yield?
Higher temperature
Lower pressure
Higher pressure
Adding catalyst
Applied question based on slides 16 to 18. Higher pressure favours the side with fewer gas moles, so ammonia yield increases. Catalyst increases rate only.
Why is a compromise temperature used in the Haber process?
Expected elements: low temperature would improve yield because the forward reaction is exothermic, but the rate would become too slow; industry therefore uses compromise conditions balancing yield, rate and cost.
Which part of this lesson feels most secure now?
Dynamic equilibrium
Condition changes
Kc
Haber process
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What is one idea from today that you will remember?
Reflection only. Invite students to identify one durable takeaway from the lesson, such as equal rates, closed system, fewer gas moles, only temperature changes Kc, or compromise conditions in the Haber process.
Which part still feels hardest, and why?
Reflection only. Use responses to diagnose where reteaching or exam-practice support is needed; likely areas are pressure vs temperature effects and distinguishing equilibrium position from Kc.
Glossary
reversible reaction - a reaction that can proceed in both the forward and reverse directions
dynamic equilibrium - a state in which forward and reverse reactions occur at equal rates in a closed system
closed system - a system in which no substances enter or leave during the reaction
equilibrium position - the relative amounts of reactants and products present at equilibrium
Le Chatelier's principle - the rule that a system at equilibrium shifts to oppose an imposed change
Kc - the equilibrium constant expressed using equilibrium concentrations
yield - the amount of product formed
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Glossary
exothermic - describing a reaction that releases heat to the surroundings
endothermic - describing a reaction that takes in heat from the surroundings
forward reaction - the reaction that converts reactants into products
reverse reaction - the reaction that converts products back into reactants
catalyst - a substance that increases reaction rate without being used up
stoichiometric coefficient - the number in front of a substance in a balanced chemical equation
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