6.3Chemical Reactions
Reversible reactions and equilibrium — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
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Lesson summary
6.3 Reversible reactions and equilibrium — Notes — IGCSE Chemistry notes: Describe reversible reactions and dynamic equilibrium, and use Le Chatelier's principle to predict how changes shift the position of equilibrium.Reversible reaction — a reaction in which the products can re…
- Closed system
- Approach to equilibrium
- Approach from either side
- Open system
- Incomplete reaction
- Apply Le Chatelier
Section 1 of 9Reversible reactions
Reversible reaction
- Reversible reaction a reaction in which the products can react to re-form the reactants, shown by the symbol ⇌.
The forward reaction forms products and the reverse reaction forms reactants.
If the forward reaction is exothermic, the reverse reaction is endothermic with the same size of ΔH.
The direction can be changed by changing the conditions.
Section 2 of 9Hydrated and anhydrous salts
| Salt | Hydrated form | Anhydrous form | Colour change on adding water |
|---|---|---|---|
| Copper(II) sulfate | CuSO4·5H2O (blue) | CuSO4 (white) | White to blue |
| Cobalt(II) chloride | CoCl2·6H2O (pink) | CoCl2 (blue) | Blue to pink |
Hydrated and anhydrous salts
Heating removes the water of crystallisation, an endothermic change.
Adding water to the anhydrous salt is highly exothermic and reverses the change.
Anhydrous copper(II) sulfate and cobalt(II) chloride are used as chemical tests for water.
Section 3 of 9Dynamic equilibrium (Supplement)
Dynamic equilibrium (Supplement)
Closed system
- Closed system a reversible reaction reaches equilibrium only where nothing can enter or leave.
At equilibrium the rate of the forward reaction equals the rate of the reverse reaction.
The concentrations of reactants and products stay constant but are not necessarily equal.
The equilibrium is dynamic because both reactions continue at the same rate.
Approach to equilibrium
- Approach to equilibrium the forward rate starts highest and falls, and the reverse rate starts at zero and rises, until the two are equal.
Approach from either side
- Approach from either side starting with only reactants or only products gives the same mixture under the same conditions.
Open system
- Open system gases that escape prevent equilibrium from being reached.
Incomplete reaction
- Incomplete reaction in a closed system the reaction never goes to 100% completion, which is why ⇌ is used.
Section 4 of 9Changing the position of equilibrium
Le Chatelier's principle
- Le Chatelier's principle when a change is made to a system at equilibrium, the position of equilibrium moves to oppose the change.
Shifting right favours the forward reaction and gives more products, and shifting left favours the reverse reaction and gives more reactants.
| Change | Shift | Reason |
|---|---|---|
| Increase temperature | Towards the endothermic direction | Opposes the increase by absorbing heat |
| Decrease temperature | Towards the exothermic direction | Opposes the decrease by releasing heat |
| Increase pressure (gases) | Towards the side with fewer gas molecules | Reduces the number of gas molecules |
| Decrease pressure (gases) | Towards the side with more gas molecules | Increases the number of gas molecules |
| Increase concentration of a reactant | To the right | Uses up some of the added reactant |
| Remove a product | To the right | Replaces some of the removed product |
| Add a catalyst | No shift | Speeds up both directions equally, so equilibrium is reached sooner |
Section 5 of 9Predicting a shift
Predicting a shift
Method
Write the balanced equation with state symbols and the sign of ΔH.
Identify the change made (temperature, pressure, concentration or catalyst).
Apply Le Chatelier
- Apply Le Chatelier the system opposes the change.
For pressure, count the moles of gas on each side (gases only).
For temperature, identify the exothermic and endothermic directions.
State the direction of the shift and its effect on the yield of product.
If both sides have equal moles of gas, pressure has no effect on the position.
A catalyst raises the rates of the forward and reverse reactions equally, so the position is unchanged.
The industrial processes (Haber and Contact) apply these ideas, and their compromise conditions balance rate, yield, safety and cost.
Rate and position
- Rate and position they are separate ideas, because temperature and pressure change both, while a catalyst changes only the rate.
Section 6 of 9Examples of equilibria
Examples of equilibria
Hydrated copper(II) sulfate
- Hydrated copper(II) sulfate blue CuSO4·5H2O ⇌ white CuSO4 + 5H2O, where heating drives water off and adding water reverses it.
Nitrogen dioxide
- 2NO2 (brown) ⇌ N2O4 (colourless), where cooling makes the mixture paler.
A brown colour that deepens on heating shows that the position has moved towards NO2.
Section 7 of 9More from the specification
- Sulfur dioxide
- An acidic gas made when sulfur-containing fuels burn.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Reversible reaction | a reaction in which the products can react to re-form the reactants. |
| Dynamic equilibrium | the state in which the forward and reverse reactions occur at the same rate. |
| Closed system | a system in which nothing can enter or leave. |
| Le Chatelier's principle | a system at equilibrium opposes any change made to it. |
| Anhydrous | containing no water of crystallisation. |
| Hydrated | containing water of crystallisation. |
| Position of equilibrium | the relative amounts of reactants and products at equilibrium. |
| Yield | the amount of product obtained. |
Section 9 of 9Exam technique
1
Saying a catalyst moves the position of equilibrium — it only makes equilibrium be reached faster
2
Saying equilibrium means the reactions have stopped — both directions continue at equal rates
3
Saying that raising the temperature always increases the yield — it favours the endothermic direction
4
Saying equal concentrations at equilibrium — concentrations are constant, not necessarily equal
5
Counting solids or liquids when predicting the effect of pressure, or applying equilibrium to an open system — only gas moles count and escaping gases prevent equilibrium
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