Still changing. Staying the same.
A closed system can exchange energy with its surroundings, but not matter. In a reversible reaction, products can react to regenerate reactants. A sealed container allows both processes to continue.
At dynamic equilibrium the forward and reverse rates are equal and non-zero. Concentrations become constant, but they do not have to be equal. A flat concentration graph therefore does not mean the particles have stopped reacting.
The model below represents A ⇌ B with a fixed total particle count. Change the relative preference for B and follow the new balance. Particle motion is schematic; the curve uses a reversible first-order model.
A balance that keeps moving
Illustrative A ⇌ B model in a closed system; total concentration 1.00 mol L⁻¹. Ratio K = [B]/[A]. Not a model of any particular industrial reaction.
Explore: Watch both reactions continue while their rates settle to the same value.
A sealed flask has constant colour even though its reversible reaction continues. Explain this observation.
- Identify continuing forward and reverse reactions.
- Equal rates produce no net concentration change; colour stays constant.
Answer: The forward and reverse rates are equal, so the concentration of the coloured species remains constant.
Assumed knowledge
Concentration is amount per volume. A flat graph has zero gradient.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Distinguish open and closed systems.
- Explain equilibrium using rates and concentrations.
- Read concentration–time graphs.