5.3.1Additional lessons
The Characteristics of Dynamic Equilibrium — IB Chemistry SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 73 of 98
Part 1 of 4Reversible Reactions and Closed Systems
Equilibrium can only be established when a reversible reaction takes place without loss of any species.
Reversible reaction
- Products can react to re-form the reactants, so the reaction can proceed in both directions and is shown with the ⇌ sign.
Reaction to completion
- In a one-way reaction the limiting reactant is used up and the reaction stops, shown with a single arrow →.
Closed system
- No matter enters or leaves, so gaseous reactants and products cannot escape and equilibrium can be reached.
Open system
- Matter can be lost to the surroundings; if a gas escapes, a reversible reaction may be driven to completion.
| Example | System | Outcome |
|---|---|---|
| CaCO₃(s) ⇌ CaO(s) + CO₂(g) | Sealed vessel (closed) | Equilibrium established |
| CaCO₃(s) → CaO(s) + CO₂(g) | Open crucible | CO₂ escapes, so all CaCO₃ decomposes |
Part 2 of 4How Equilibrium Is Reached
A concentration–time graph shows both curves levelling off at the point where the two rates match.
1
Start with only reactants, for example H₂(g) and I₂(g) in a sealed flask, so the forward rate is at its highest.
2
As reactants are used up, their concentrations fall and the forward reaction slows down steadily.
3
Product molecules form and begin reacting back, so the rate of the reverse reaction rises from zero.
4
Eventually the forward and reverse rates become equal, and from this moment dynamic equilibrium exists.
5
The same equilibrium mixture is reached at a given temperature if you start with only the products instead.
Part 3 of 4Features of Dynamic Equilibrium
Features of Dynamic Equilibrium
Equal rates
- The rate of the forward reaction equals the rate of the reverse reaction, so there is no overall change.
Constant concentrations
- The concentrations of all reactants and products stay constant, but they are not necessarily equal to each other.
Dynamic
- Both reactions continue at the molecular level, so bonds keep breaking and forming even though nothing seems to change.
Macroscopic properties
- Colour, density, pressure and pH remain constant because they depend on the unchanging concentrations.
Physical equilibria
- Changes of state such as C₂H₅OH(l) ⇌ C₂H₅OH(g) in a sealed bottle also reach equilibrium when evaporation rate equals condensation rate.
Part 4 of 4Exam Checklist & Tips
5.3.1 – Key Points for the Exam
- Dynamic equilibrium A state in a closed system where forward and reverse reaction rates are equal and concentrations are constant.
- Reversible reaction A reaction in which products can react to form the reactants again.
- Closed system A system that exchanges no matter with its surroundings, so no reactant or product can escape.
- Macroscopic property An observable property, such as colour or density, that does not change at equilibrium.
- Position of equilibrium The relative amounts of reactants and products present in the equilibrium mixture.
Common Mistakes
- Stating that concentrations of reactants and products are equal at equilibrium; they are constant, not equal.
- Saying the reactions stop at equilibrium; both directions continue at equal rates.
- Forgetting that equilibrium involving a gas needs a closed system.
- Describing the forward rate as zero at the start;
- it is the reverse rate that begins at zero when only reactants are present.
Last updated:
3 & IGCSE —
Kc
- : Kc! (ΔH < 0) Kc. (ΔH > 0) Kc.
- : ( ).
- : Kc .