(c) Reversible reactions and equilibria
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
3.17Reversible-reaction symbolKnow that some reactions are reversible and this is indicated by the symbol ⇌ in equations3.18Examples of reversible reactionsDescribe reversible reactions such as the dehydration of hydrated copper(II) sulfate and the effect of heat on ammonium chloride319C Dynamic equilibrium in a sealed containerKnow that a reversible reaction can reach dynamic equilibrium in a sealed container320C Characteristics of dynamic equilibriumKnow that the characteristics of a reaction at dynamic equilibrium are:• the forward and reverse reactions occur at the same rate• the concentrations of reactants and products remain constant.321C Catalysts and equilibrium positionUnderstand why a catalyst does not affect the position of equilibrium in a reversible reaction322C Temperature, pressure and equilibrium positionKnow the effect of changing either temperature or pressure on the position of equilibrium in a reversible reaction:• an increase (or decrease) in temperature shifts the position of equilibrium in the direction of the endothermic (or exothermic) reaction• an increase (or decrease) in pressure shifts the position of equilibrium in the direction that produces fewer (or more) moles of gas References to Le Chatelier's principle are not required
Read the reversible-reaction symbol precisely
The symbol ⇌ shows that a reaction can proceed in both directions: reactants can form products, and the products can react to reform the reactants.
\ce{NH4Cl(s) <=> NH3(g) + HCl(g)}
Read the left-to-right process as the forward reaction and the right-to-left process as the reverse reaction. Conditions such as heating or cooling can favour one visible direction in a reversible change.
The symbol ⇌ does not by itself mean that the mixture is at equilibrium. It states that both directions are possible; equilibrium requires additional conditions and equal forward and reverse rates.
Explain two observable reversible reactions
A reversible reaction can be demonstrated by driving a chemical change in one direction and then changing the conditions so that the original substances reform.
| Example | Forward observation and change | How the reverse is produced |
|---|---|---|
| hydrated copper(II) sulfate | heating blue hydrated crystals removes water and leaves white anhydrous copper(II) sulfate | add water; the blue hydrated solid reforms |
| ammonium chloride | heating the white solid forms ammonia and hydrogen chloride gases | cool the gases; they recombine to form white ammonium chloride |
\ce{CuSO4.5H2O(s) <=> CuSO4(s) + 5H2O(g)}
\ce{NH4Cl(s) <=> NH3(g) + HCl(g)}
A colour change alone is not the explanation: identify which substances are formed and how changing the conditions restores the starting substance. These demonstrations show reversibility; they do not automatically show dynamic equilibrium.
Know when dynamic equilibrium can be reached
A reversible reaction can reach dynamic equilibrium when it occurs in a sealed container, so reacting substances cannot enter or escape.
| Stage | What happens in the sealed container |
|---|---|
| 1 | reactants form products, so the forward reaction is initially dominant |
| 2 | products accumulate and the reverse reaction becomes faster |
| 3 | forward and reverse rates become equal; dynamic equilibrium has been reached |
Sealing matters because the full reacting system is retained. If a gas escapes, its concentration changes and the reverse reaction may be unable to balance the forward reaction.
A sealed container is necessary for this equilibrium description, but sealing an irreversible reaction does not make it reversible. Both a reversible reaction and a sealed system are required.
Describe dynamic equilibrium without saying reactions stop
At dynamic equilibrium in a sealed container, the forward and reverse reactions continue at equal rates. Therefore reactant and product concentrations remain constant over time.
| At equilibrium | Correct interpretation |
|---|---|
| forward rate = reverse rate | equal amounts are converted in opposite directions per unit time |
| concentrations are constant | there is no overall composition change, although both reactions continue |
| concentrations need not be equal | the equilibrium mixture may contain different amounts of reactants and products |
Macroscopic properties such as colour and pressure can stay unchanged because there is no net change, while particles continue reacting in both directions at the microscopic level.
Equilibrium is dynamic, not static. Do not say that both reactions stop, and do not replace 'equal rates' with 'equal concentrations'.
Separate catalyst speed from equilibrium position
A catalyst increases both the forward and reverse reaction rates by providing lower-activation-energy pathways. It does not change the position of equilibrium.
| A catalyst changes | A catalyst does not change |
|---|---|
| how quickly equilibrium is reached | the equilibrium concentrations of reactants and products |
| both forward and reverse rates | the equilibrium yield or the direction in which equilibrium lies |
Because both directions are accelerated, their rates become equal sooner. The same equilibrium composition is reached at the same temperature and pressure, whether or not a catalyst is present.
A catalyst can improve production rate without improving equilibrium yield. It does not shift equilibrium toward products and is not a way to obtain a greater final equilibrium amount.
Predict temperature and pressure effects on equilibrium
Changing temperature or pressure can change the equilibrium composition. Predict the direction by identifying the endothermic direction for temperature and comparing gaseous mole ratios for pressure.
| Change | Direction favoured at the new equilibrium |
|---|---|
| increase temperature | the endothermic direction |
| decrease temperature | the exothermic direction |
| increase pressure | the side with fewer moles of gas |
| decrease pressure | the side with more moles of gas |
| change pressure when both sides have equal gaseous moles | no change in equilibrium position |
\ce{N2(g) + 3H2(g) <=> 2NH3(g)} \qquad \Delta H < 0
For ammonia formation, the forward direction is exothermic and changes four moles of gas into two. Lower temperature favours ammonia at equilibrium, while higher pressure favours ammonia because the product side has fewer gaseous moles.
Count only gaseous coefficients when predicting a pressure effect. A condition may increase equilibrium yield but slow the reaction, or decrease yield while speeding it up, so keep equilibrium position separate from reaction rate.