UNIT 2 CHEMISTRY · PEARSON AAQ · D1.4.1
Dynamic Equilibrium
FREE CHEMISTRY REVISION · TOPIC 43
Dynamic Equilibrium
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · D1.4.1
What you will learn
Define dynamic equilibrium, explain why a closed system is needed, and distinguish equal reaction rates from equal concentrations. Interpret original particle, concentration–time and rate–time representations of the same reversible process.
1. Reversible reactions and the closed-system condition
A reversible reaction can proceed in both directions under suitable conditions. Use the equilibrium arrow ⇌, for example N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction forms ammonia; the reverse reaction decomposes it. The arrow alone does not say how much of either side is present or how quickly equilibrium is reached.
A closed system does not exchange matter with its surroundings, although it may exchange energy. Retaining reactants and products allows both directions to continue and balance. An open vessel that loses a gaseous product may never reach the same equilibrium. Closing a vessel is necessary for the simple retained-mixture examples here, but does not guarantee an observable equilibrium within the available time.
Keep temperature and other imposed conditions unchanged when discussing a settled equilibrium. A reversible system can approach the same equilibrium composition from either side if the total material, volume and temperature are appropriate and identical. Reactants do not need to be completely used up first.
2. What dynamic equilibrium means
At dynamic equilibrium, the forward and reverse reaction rates are equal and nonzero, so reactant and product concentrations remain constant. Molecular reactions continue. Equal opposing changes cancel in the overall composition. The state is dynamic at particle level and unchanging in measurable bulk properties under unchanged conditions.
Equal rates do not require equal concentrations. A fast forward tendency at one concentration can balance a different reverse tendency at another. For an elementary teaching model A ⇌ B with forward rate 0.02[A] and reverse rate 0.06[B], [A] = 0.75 and [B] = 0.25 give equal rates of 0.015 mol dm⁻³ s⁻¹. The rate laws are stated for this model; do not infer them from balanced coefficients for an arbitrary reaction.
For a reaction with coefficients other than 1:1, compare forward and reverse reaction rates on the same stoichiometric basis. For N₂ + 3H₂ ⇌ 2NH₃, one forward reaction event consumes one N₂ and three H₂ and forms two NH₃. Equal event rates in opposite directions give zero net change of every species, even though their amounts and gross formation/consumption rates per species differ.
3. Concentration–time evidence
Starting with A only, [A] decreases as forward reaction initially dominates; [B] increases. As A becomes less abundant the forward rate falls, and as B builds up the reverse rate rises in this model. The concentration curves flatten when net change becomes negligible. They settle at different heights: the graph does not need to cross.
A horizontal concentration line by itself is not proof of dynamic equilibrium. A reaction may have stopped because a reactant ran out, or an observed quantity may lack sensitivity. Evidence should support reversibility, retention of material and continuing equal opposing reactions. Concentration is amount per volume; a volume change can alter it instantly even without an immediate chemical conversion.
4. Rate–time evidence
The forward and reverse rates meet at the same nonzero plateau. Zero net rate means forward minus reverse is zero; it does not mean each rate is zero. On a rate graph, equal heights establish rate equality. On a concentration graph, different horizontal heights can establish constant, unequal concentrations.
If the experiment begins with some product, the reverse rate need not start at zero. If it begins with products only, the reverse rate initially dominates instead. These are different initial conditions, not exceptions to the definition. A catalyst can shorten the time needed to approach equilibrium without changing its position at a fixed temperature.
5. Worked interpretations
Example 1 — identify the equilibrium region
A concentration graph shows [A] falling and [B] rising before both level off. Once both stay constant under unchanged conditions, the data are consistent with equilibrium. Explain the flattening through equal forward and reverse reaction rates, not cessation of reaction. The two plateau heights need not match.
Example 2 — calculate opposing rates
For the stated A ⇌ B model, [A] = 0.75 and [B] = 0.25 mol dm⁻³. Forward rate = 0.02 × 0.75 = 0.015 mol dm⁻³ s⁻¹. Reverse rate = 0.06 × 0.25 = 0.015 mol dm⁻³ s⁻¹. Equal rates with a 3:1 concentration ratio provide a direct counterexample to “equal concentrations”.
Example 3 — diagnose a non-equilibrium mixture
At one instant in the same model, [A] = 0.50 and [B] = 0.50. Forward rate = 0.010; reverse rate = 0.030 mol dm⁻³ s⁻¹. Reverse dominates, so B decreases and A increases. Equal concentrations do not make this mixture an equilibrium.
Example 4 — count reaction events
In a sealed A ⇌ B system, an average of 20 A → B events and 20 B → A events occur per second. Net changes are zero but 40 events occur altogether. If the counts become 25 forward and 20 reverse, there is a net conversion of five A to B per second; the system is no longer at equilibrium.
Example 5 — compare graph types
Two nonzero rate curves share a plateau at 0.015 mol dm⁻³ s⁻¹. Two concentration curves plateau at 0.75 and 0.25 mol dm⁻³. These graphs agree: one describes equal opposing rates; the other describes constant unequal concentrations. Do not compare the numerical heights across different axes or units.
Example 6 — evaluate a system boundary
In H₂O(l) ⇌ H₂O(g) at fixed temperature in a sealed container with both phases present, evaporation and condensation can balance. Removing the lid allows vapour to escape, so the original liquid–vapour equilibrium is disturbed. A sealed vessel can exchange heat with a thermostat and still be closed to matter.
Common misconceptions
- Dynamic equilibrium is static: forward and reverse reactions continue.
- Equal rates mean equal concentrations: rates depend on kinetic behaviour as well as concentrations.
- The forward reaction stops before the reverse begins: both can operate while composition is still changing.
- All reactants are used up at equilibrium: both sides can remain present.
- Any flat graph proves equilibrium: a completed or arrested process can also look flat.
- Closed means thermally insulated: closed describes matter exchange, not heat exchange.
Exam tips
- State both equal forward/reverse rates and constant concentrations.
- Use “nonzero” or “both reactions continue” to explain dynamic.
- Read the vertical axis before interpreting equal or unequal line heights.
- Identify the closed boundary and constant conditions.
- Say “no net change”, not “no reactions”.
- For coefficient ratios, distinguish species rates from the reaction rate.
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