QCE Chemistry - Unit 3 - Chemical equilibrium systems
Chemical Equilibrium | QCE Chemistry
Learn how reversible reactions reach dynamic equilibrium in QCE Chemistry, including closed systems, forward and reverse reaction rates, and equilibrium position.
Updated 2026-05-18 - 3 min read
QCAA official coverage - Chemistry 2025 v1.3
Exact syllabus points covered
- Discriminate between open or closed chemical systems.
- Identify that physical changes are usually reversible, whereas only some chemical reactions are reversible.
- Symbolise equilibrium equations using ⇋ in balanced chemical equations.
- Explain observable properties and the characteristics of physical and chemical systems in a state of equilibrium.
- Explain that, over time, physical change and reversible chemical reactions reach a state of dynamic equilibrium in a closed system, with the relative concentrations of products and reactants defining the position of equilibrium.
- Explain the reversibility of chemical reactions by considering the activation energies of the forward and reverse reactions.
- Analyse data and interpret graphical representations of relative changes in the concentration of reactants and product against time, to determine the position of equilibrium.
Chemical equilibrium is what a reversible reaction settles into when the forward and reverse reactions are happening at the same rate. The reaction has not stopped. Particles are still colliding, bonds are still breaking and forming, but the visible amounts of each substance stay constant.
Open, closed and reversible systems
An open system can exchange matter and energy with its surroundings. A campfire is open: oxygen enters, and gases and heat leave.
A closed system can exchange energy, but not matter. A stoppered reaction flask is a useful model because particles stay available for both the forward and reverse reactions.
Original Sylligence diagram for open closed systems.
Reversible reactions are shown with a double arrow:
$ \mathrm{N_2O_4(g)} \rightleftharpoons 2\mathrm{NO_2(g)} $
The forward reaction forms products. The reverse reaction reforms reactants. In a closed system, both can continue for long enough to reach equilibrium.
Original Sylligence diagram for activation energy profile.
Dynamic equilibrium
At the start, the direction with more available reactant particles is usually faster. If a flask starts with only $\mathrm{N_2O_4}$, the forward reaction is fast because there are many $\mathrm{N_2O_4}$ particles and no $\mathrm{NO_2}$ particles yet. As $\mathrm{NO_2}$ builds up, the reverse reaction becomes more frequent.
Eventually:
- forward rate = reverse rate
- concentrations are constant
- reactants and products are both still reacting
- concentrations are not necessarily equal
Original Sylligence diagram for dynamic equilibrium rates.
The word dynamic matters. It means "still moving at the particle level". A flat concentration graph does not mean nothing is happening; it means there is no net change.
Equilibrium position
The equilibrium position describes which side is favoured at equilibrium.
- Products favoured: the equilibrium mixture contains relatively more products.
- Reactants favoured: the equilibrium mixture contains relatively more reactants.
- Neither strongly favoured: both sides are present in comparable amounts.
This is a qualitative idea. Later, $K_c$ gives a numerical way to describe the same balance.
Reading equilibrium graphs
On a concentration-time graph, equilibrium is reached when the concentration curves become horizontal. The curves do not need to meet. Equal concentrations are not required.
On a rate-time graph, equilibrium is reached when the forward and reverse rate lines meet and then remain equal.
Exam traps
Other traps:
- writing the arrow as one-way when the reaction is reversible
- saying particles stop reacting at equilibrium
- calling an open beaker a closed equilibrium system when gas can escape
- assuming a flat graph means rate is zero