Direction of Reversible Reactions
Use simultaneous forward/reverse events and stoichiometry to explain net direction.
AP Chemistry Topic 7.2 · CED printed p. 129 / PDF p. 136. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Dynamic equilibrium
- Stoichiometric coefficients
A reversible reaction can have a clear net direction while both directions are happening. If ten A particles become B while four B particles become A during the same interval, there are six more B particles afterward. The forward rate exceeds the reverse rate, so the mixture moves forward overall. Switch those event counts and the net direction reverses. Equal counts per interval give equilibrium. For a reaction such as 2A ⇌ B, tie the rate definition to stoichiometry: every net B formed consumes two A. The rate difference determines the net progress, while the coefficients determine each species’ change. Do not infer the event rates solely from which species has more particles; the rate constants and mechanism matter too.
Let's work through it
- In a model interval, 12 A → B events and 5 B → A events occur for A ⇌ B.
- Cancel five events in each direction when calculating the net change. Seven A become seven B overall.
- A decreases and B increases, even though the reverse reaction is active.
- When both event counts are eight in a later equal-length interval, the average composition is unchanged. Both rates are eight events per interval, not zero.
Make the picture move
Interactive concept model
Q tells you which way; K tells you the destination ratio.
Predict first. Predict net direction from Q compared with K. Check whether the final concentrations must be equal, and whether the total concentration changes.
A(aq) ⇌ B(aq); Q = [B]/[A]; x = (K[A]₀ − [B]₀)/(1 + K)
On a small screen, scroll inside the graph or diagram to read its labels. The values and explanation also appear below.
Q = 0.25, K = 3.98: net forward: form B. Equilibrium [A] = 0.201 and [B] = 0.799 mol L⁻¹.
Read the data and comparisons
| ICE row | [A] (mol L⁻¹) | [B] (mol L⁻¹) |
|---|---|---|
| Initial | 0.8 | 0.2 |
| Change | −0.599 | +0.599 |
| Equilibrium | 0.201 | 0.799 |
Macroscopic / data. Concentration bars describe initial and final mixtures on one common scale.
Particle view. Net interconversion redistributes A and B while conserving their total; both directions continue at equilibrium.
Symbolic view. [A]eq = [A]₀ − x; [B]eq = [B]₀ + x; x = 0.599 mol L⁻¹.
Why it works. The signed x is the forward change in concentration. A decreases by x while B increases by x, preserving [A] + [B]. Substitute both final concentrations into K. Equilibrium requires equal forward and reverse rates; it does not generally require equal concentrations.
Conditions and limits. Abstract 1:1 dissolved interconversion at fixed temperature (298 K) and volume; ideal dilute activities are approximated by concentration divided by a 1 mol L⁻¹ standard state, which cancels in this ratio. Selecting K chooses a different hypothetical system at that fixed temperature, not a temperature change. This simple linear ICE solution does not cover other stoichiometries, solubility, or coupled equilibria.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Which way does the mixture move if the reverse rate is larger?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Which way does the mixture move if the reverse rate is larger?
Worked answer and reason
Products are consumed overall and reactants form.
Net change equals the difference between competing progress rates.
Retrieval 2
If the rates become equal, must the molecule counts become equal?
Worked answer and reason
No. Equal event rates can occur with unequal populations.
Population affects a rate together with the rate constant; it is not itself a rate.
Catch the tempting mistake
- The more abundant side always reacts faster.
- Rates depend on both populations and the reaction pathway; use measured rates or a valid rate law.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 7 problem 2 · ap application · multiple
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For the teacher
- Require students to describe simultaneous events before using the word shift.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 7.2.A
- Explain the relationship between the direction in which a reversible reaction proceeds and the relative rates of the forward and reverse reactions.
- 7.2.A.1
- If the rate of the forward reaction is greater than the reverse reaction, then there is a net conversion of reactants to products. If the rate of the reverse reaction is greater than that of the forward reaction, then there is a net conversion of products to reactants. An equilibrium state is reached when these rates are equal.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 — Required topic-numbered learning objectives and essential knowledge, all continuations and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §13.2, Equilibrium Constants — Pressure/concentration quotients, pure-phase activities, and the distinction between composition and rate.
Glossary and deeper reading · 3 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Stoichiometry
Stoichiometry uses a balanced chemical equation to relate amounts of reactants and products. Equation coefficients provide mole ratios rather than direct mass ratios. For example, forming water from hydrogen and oxygen consumes two moles of hydrogen molecules per mole of oxygen molecules.
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Reversible reaction
A reversible reaction can proceed in both forward and reverse directions under the modeled conditions. Its net direction depends on the current composition and temperature. For example, a product-rich starting mixture can react backward toward equilibrium.
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Mixture
A mixture contains two or more substances whose proportions can vary. Its components retain their chemical identities until a chemical reaction changes them. For example, water and ethanol can form mixtures with many different compositions.