Equilibrium
Reversible Reactions
Some reactions go to completion: all the reactants are converted into products and the reaction stops. In reversible reactions, the products can react with each other to re-form the original reactants.
A reversible reaction can proceed in both directions:
- The forward reaction forms the products.
- The reverse reaction re-forms the reactants from the products.
Reversible reactions are shown using the symbol ⇌ (two opposing half-headed arrows):
A + B ⇌ C + D
The direction a reversible reaction proceeds can be changed by altering the conditions.
Dehydration of Hydrated Copper(II) Sulfate
Hydrated copper(II) sulfate (blue crystals, CuSO4.5H2O) can be dehydrated by heating to form anhydrous copper(II) sulfate (white powder, CuSO4) and water:
CuSO4.5H2O(s) ⇌ CuSO4(s) + 5H2O(l/g)
- Forward reaction (heating): Blue crystals lose water and become white powder; endothermic.
- Reverse reaction (adding water): White powder absorbs water and turns blue; exothermic.
This reversible reaction is also used as the chemical test for water: anhydrous copper(II) sulfate turns from white to blue in the presence of water.
Thermal Decomposition of Ammonium Chloride
Heating white solid ammonium chloride (NH4Cl) causes it to decompose into ammonia gas (NH3) and hydrogen chloride gas (HCl). On cooling, the gases recombine to form ammonium chloride again:
NH4Cl(s) ⇌ NH3(g) + HCl(g)
- The white solid appears to sublime, but in fact decomposes; the gases reform the solid further up the cooler tube.
Dynamic Equilibrium
Higher Tier
Spec points 3.19C to 3.22C cover dynamic equilibrium and the effects of changing conditions.
A reversible reaction can reach dynamic equilibrium only in a closed system (a sealed container where no substances can enter or leave).
As the forward reaction proceeds, the concentration of reactants decreases and the concentration of products increases. The rate of the forward reaction therefore decreases. At the same time, the rate of the reverse reaction increases as more product is present. Eventually, the rate of the forward reaction equals the rate of the reverse reaction and equilibrium is reached.
Characteristics of Dynamic Equilibrium
At dynamic equilibrium:
- The forward and reverse reactions occur at the same rate.
- The concentrations of reactants and products remain constant (but they are not necessarily equal).
- The reaction is still occurring in both directions; it is dynamic, not static.
This can only be achieved in a closed system.
Position of Equilibrium
The position of equilibrium describes the relative amounts of reactants and products present at equilibrium:
- If the equilibrium lies to the right, more products are present (the forward reaction is favoured).
- If the equilibrium lies to the left, more reactants are present (the reverse reaction is favoured).
Changing the conditions can shift the position of equilibrium.
Effect of Temperature on Equilibrium
When the temperature is changed, the equilibrium shifts to oppose the change.
- Increasing temperature shifts the position of equilibrium in the direction of the endothermic reaction (the reaction that absorbs heat, counteracting the temperature increase).
- Decreasing temperature shifts the position of equilibrium in the direction of the exothermic reaction (the reaction that releases heat, counteracting the temperature decrease).
| Temperature change | Equilibrium shifts toward | Effect on yield of products (if forward is exothermic) |
|---|---|---|
| Increase | Endothermic direction | Decreased |
| Decrease | Exothermic direction | Increased |
Example: In N2(g) + 3H2(g) ⇌ 2NH3(g), the forward reaction is exothermic.
- Increasing temperature shifts equilibrium to the left (toward endothermic, reverse direction), decreasing the yield of ammonia.
- Decreasing temperature shifts equilibrium to the right, increasing the yield of ammonia.
Effect of Pressure on Equilibrium
Changing pressure only affects reactions involving gases.
- Increasing pressure shifts the position of equilibrium in the direction that produces fewer moles of gas (fewer gas molecules). This opposes the pressure increase.
- Decreasing pressure shifts the position of equilibrium in the direction that produces more moles of gas.
- If the total number of moles of gas is the same on both sides, changing pressure has no effect on the position of equilibrium.
Example: N2(g) + 3H2(g) ⇌ 2NH3(g)
- Left side: 1 + 3 = 4 moles of gas
- Right side: 2 moles of gas
- Increasing pressure shifts equilibrium to the right (toward fewer gas moles), increasing the yield of ammonia.
- Decreasing pressure shifts equilibrium to the left, decreasing the yield.
| Number of moles of gas | Equilibrium shifts on increasing pressure | Equilibrium shifts on decreasing pressure |
|---|---|---|
| More on left (reactants) | Right (toward products) | Left (toward reactants) |
| More on right (products) | Left (toward reactants) | Right (toward products) |
| Equal on both sides | No change | No change |
Effect of a Catalyst on Equilibrium
A catalyst does not change the position of equilibrium. It increases the rate of both the forward and reverse reactions by the same amount (by providing an alternative pathway with lower activation energy). Therefore it does not change the relative amounts of reactants and products at equilibrium; it only helps the system reach equilibrium more quickly.
Exam Questions
Q: What is meant by the term “dynamic equilibrium”?
Dynamic equilibrium is a state reached in a closed system when the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products remain constant. The reactions are still taking place in both directions continuously.
Q: For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), the forward reaction is exothermic. State and explain the effect of increasing the temperature on the yield of SO3.
Increasing the temperature shifts the position of equilibrium in the direction of the endothermic reaction. Since the forward reaction is exothermic, the reverse reaction is endothermic. Therefore, increasing temperature shifts equilibrium to the left, reducing the yield of SO3.
Q: For the same reaction, state and explain the effect of increasing the pressure on the yield of SO3.
Left side: 2 + 1 = 3 moles of gas. Right side: 2 moles of gas. Increasing pressure shifts equilibrium in the direction that produces fewer moles of gas, which is the right (forward) direction. So the yield of SO3 increases.
Q: Why does a catalyst not change the position of equilibrium?
A catalyst increases the rate of the forward and reverse reactions by equal amounts (by providing a lower activation energy pathway for both). Since both rates increase equally, the relative concentrations of reactants and products at equilibrium remain unchanged. The catalyst only speeds up how quickly equilibrium is reached.
Q: Describe the reversible reaction involving hydrated copper(II) sulfate.
Hydrated copper(II) sulfate (blue crystals) can be heated to form anhydrous copper(II) sulfate (white powder) and water. This is the forward reaction and is endothermic. Adding water to the white anhydrous copper(II) sulfate reverses the reaction, re-forming the blue hydrated salt. This reverse reaction is exothermic.
Exam Tip
When asked about the effect of changing conditions on equilibrium: (1) state which direction the equilibrium shifts; (2) explain why (oppose the change using the endo/exothermic direction for temperature, or fewer/more gas moles for pressure); (3) state the effect on yield of product. Note: references to Le Chatelier’s principle are not required in this specification.