Chemical equilibria
Section: 7 Equilibria | Syllabus: Cambridge AS Level Physics 9702
Reversible Reactions Reversible Reaction A reaction in which the products can themselves react to reform the original reactants. Represented using the reversible arrow symbol (⇌). In a reversible reaction, two processes occur simultaneously: Forward reaction: reactants → products Reverse reaction: products → reactants Classic Example The thermal decomposition of ammonium chloride: NH₄Cl(s) ⇌ NH₃(g) + HCl(g) Heating decomposes it into gases; on cooling the gases recombine to reform the solid.
Other common reversible reactions: CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g) - blue hydrated ⇌ white anhydrous copper(II) sulfate N₂(g) + 3H₂(g) ⇌ 2NH₃(g) - the Haber process 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) - the Contact process Dynamic Equilibrium Dynamic Equilibrium The state reached in a closed system when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant over time.
Two essential features of dynamic equilibrium: Constant concentrations: the amounts of reactants and products do not change over time Dynamic - not static: both forward and reverse reactions continue to occur; they simply proceed at the same rate Fig 7.1 - Rate vs Time Graph at Equilibrium Diagram: Graph of reaction rate on the y-axis against time on the x-axis.
The forward reaction rate starts high and decreases; the reverse reaction rate starts at zero and increases. The two lines meet and become equal at the point of dynamic equilibrium, marked with a vertical dashed line.
Both rates remain constant after that point. Closed System A system in which no matter can enter or leave, although energy can be exchanged with the surroundings. Dynamic equilibrium can only be established in a closed system - if the system is open, products or reactants escape before equilibrium is reached.
Common Misconception Equilibrium does not mean equal concentrations of reactants and products. It means concentrations are constant (no longer changing). The position of equilibrium - whether it favours reactants or products - depends on the specific reaction.
Le Chatelier's Principle Le Chatelier's Principle If a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise that change. The "position of equilibrium" describes the relative amounts of reactants and products in the equilibrium mixture: Shifts right (→): more products form (forward reaction favoured) Shifts left (←): more reactants form (reverse reaction favoured) Effect of Changing Concentration Change Shift Direction Reason Increase [reactant] Right (→) System consumes the added reactant by producing more product Decrease [reactant] Left (←) System replaces lost reactant by decomposing product Increase [product] Left (←) System removes the added product by reforming reactant Decrease [product] Right (→) System replaces lost product by producing more Effect of Changing Temperature For an equilibrium where the forward reaction is exothermic (releases heat), the reverse reaction is endothermic (absorbs heat).
Change Shift Direction Effect on K Increase temperature Endothermic direction - absorbs heat to minimise the increase K changes Decrease temperature Exothermic direction - releases heat to minimise the decrease K changes Key Rule Temperature is the only factor that changes the value of the equilibrium constant K.
Changes in concentration, pressure, or presence of a catalyst shift the position of equilibrium but leave K unchanged. Effect of Changing Pressure (gas-phase equilibria) Pressure changes affect equilibria only when there are different numbers of moles of gas on each side (Δn gas ≠ 0).
Fig 7.2 - Effect of Pressure on 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) Diagram: Two containers side by side. Left container (low pressure): 3 moles of gas total on the left side, 2 moles on the right. Right container (high pressure): arrows show equilibrium shifting right (towards fewer gas moles, i.e.
2 moles of SO₃). Caption: "Increasing pressure shifts equilibrium towards the side with fewer moles of gas." Change Shift Direction Condition Increase pressure Towards fewer moles of gas Only if Δn(gas) ≠ 0 Decrease pressure Towards more moles of gas Only if Δn(gas) ≠ 0 Change pressure No shift If equal moles of gas on both sides Effect of a Catalyst Catalyst and Equilibrium A catalyst speeds up both the forward and reverse reactions equally.
It does not shift the position of equilibrium and does not change the value of K. Its only role is to allow equilibrium to be reached more quickly. Summary - Effect on the Equilibrium Constant K Condition Changed Position of Equilibrium Shifts?
Value of K Changes? Temperature increased Yes - towards endothermic direction Yes Temperature decreased Yes - towards exothermic direction Yes Concentration changed Yes No Pressure changed Yes (if Δn gas ≠ 0) No Catalyst added No No Industrial Applications The Haber Process The Haber process synthesises ammonia (NH₃) from nitrogen (from the air) …
Interactive revision notes, videos and practice questions load below.