Reversible Reactions and Equilibrium
This topic explores reversible chemical reactions which don't fully convert reactants to products. Instead, they reach a dynamic equilibrium in a closed system, a state of balance which can be shifted by changing reaction conditions.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- In a closed system, a reversible reaction reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction.
- At equilibrium, the concentrations of reactants and products remain constant, even though both reactions are still occurring.
- Le Chatelier's Principle states that if a change is applied to a system at equilibrium, the equilibrium position will shift to counteract that change.
- Increasing the concentration of a reactant or decreasing the concentration of a product shifts the equilibrium to the right (product side).
- Increasing the temperature shifts the equilibrium in the direction of the endothermic reaction (the one that absorbs heat, ΔH > 0).
- Increasing the overall pressure shifts the equilibrium to the side with the fewer total moles of gaseous molecules.
Formulae
aA + bB ⇌ cC + dD This is the general representation for any reversible reaction at equilibrium. The letters a, b, c, and d represent the stoichiometric coefficients for substances A, B, C, and D.
Definitions
- Reversible Reaction
- A chemical reaction that can proceed in both the forward (reactants to products) and reverse (products to reactants) directions, indicated by the '⇌' symbol.
- Dynamic Equilibrium
- The state reached in a closed system where the forward and reverse reactions occur at the same rate, resulting in no net change in the concentrations of reactants and products.
- Closed System
- A system from which no matter can escape and into which no matter can enter. Energy, however, can be exchanged with the surroundings.
Worked example
The synthesis of methanol is a reversible exothermic reaction that takes place in a sealed container: CO(g) + 2H2(g) ⇌ CH3OH(g). State and explain the effect on the yield of methanol (CH3OH) at equilibrium if (a) the concentration of CO is increased, and (b) the pressure is decreased.
- 1
For (a), identify the change:
concentration of a reactant, CO, is increased.
- 2
Apply Le Chatelier's Principle:
the system will try to decrease the concentration of CO.
- 3
To do this, the equilibrium must shift to the right, favouring the forward reaction which consumes CO.
- 4
Conclusion for (a):
A shift to the right increases the amount of product, so the yield of methanol increases.
- 5
For (b), identify the change:
the overall pressure is decreased.
- 6
Apply Le Chatelier's Principle:
the system will try to increase the pressure.
- 7
Count the moles of gas on each side.
Left side:
1 (CO) + 2 (H2) = 3 moles.
Right side:
1 (CH3OH) = 1 mole - 8
To increase pressure, the equilibrium must shift to the side with more moles of gas, which is the left side.
- 9
Conclusion for (b):
A shift to the left decreases the amount of product, so the yield of methanol decreases.
Answer: (a) The yield of methanol increases because the equilibrium shifts to the right to use up the added reactant CO. (b) The yield of methanol decreases because the equilibrium shifts to the left, the side with more moles of gas (3 vs 1), to counteract the decrease in pressure.
Common mistakes
- ×Forgetting that changes in pressure only affect the equilibrium position for reactions involving gases.
- ×Miscounting or ignoring the moles of gas. Pressure changes have no effect if the total number of gaseous moles is identical on both sides of the equation.
- ×Overlooking the requirement of a closed system for equilibrium to be established and maintained.
- ×Confusing the rate of reaction with the position of equilibrium. A catalyst increases the rate at which equilibrium is reached but does not change the final equilibrium position or yields.
No-calculator tips
- ✓Think of temperature change as adding or removing 'heat' as a product or reactant. For an exothermic reaction (heat is a product), increasing temperature 'adds a product', shifting equilibrium left.
- ✓To quickly determine the effect of pressure, just sum the stoichiometric coefficients for ONLY the gaseous (g) species on the left and right. The equilibrium will shift towards the side with the smaller sum if pressure is increased.