Representations of Equilibrium
Construct a scaled equilibrium particle representation with atom conservation and ongoing opposite events.
AP Chemistry Topic 7.8 · CED printed p. 136 / PDF p. 143. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Stoichiometry
- Q expressions
A particle picture should obey the same equation as your calculation. For an isomerization A ⇌ B, each A and B symbol contains the same atoms, so ten initial symbols remain ten afterward. For 2A ⇌ A2, however, two separate A atoms become one bonded pair; conserve atoms rather than the number of drawn objects. In equal-volume diagrams, symbol counts are proportional to concentrations if every symbol has the same scale. Then a 1:1 quotient is a simple count ratio. With a squared term, counts alone are not an absolute concentration unless the drawing’s particle-to-mole and volume scale is specified. You can still compare Q values across identically scaled boxes. An equilibrium picture may contain mostly products or mostly reactants. Draw a sequence or use arrows to communicate ongoing forward and reverse events; a static drawing cannot itself establish a rate balance. Real macroscopic equilibrium is an average over enormous populations, so tiny illustrative boxes may hide statistical fluctuations.
Let's work through it
- Use equal-volume diagrams with a common scale for A ⇌ B. The first has 6 A and 2 B; the second has 2 A and 6 B.
- The respective Q values are 2/6 and 6/2. If K = 3, only the second depicts the equilibrium average.
- Starting from the first, four more A become B overall. All eight symbols still represent the same total atoms.
- Show two simultaneous opposite event arrows in the second. Keeping its average composition does not freeze each particle’s identity.
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
For 2A ⇌ A2, what must a diagram conserve?
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
For 2A ⇌ A2, what must a diagram conserve?
Worked answer and reason
A atoms, including both atoms in every A2 pair.
Object counts change when particles associate.
Retrieval 2
Can any count ratio be used directly for a squared quotient?
Worked answer and reason
Only with appropriate concentration and volume scaling; an unscaled count is not molarity.
Different powers leave an uncanceled scale factor.
Catch the tempting mistake
- The same molecules must remain reactants at equilibrium.
- Species populations stay constant on average while individual molecules keep interconverting.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 7 problem 9 · ap application · particle
Scientific calculator workspace
Use this workspace for arithmetic, logarithms and elementary functions. Keep intermediate precision and report final values with appropriate units and significant figures. Graphing and general equation solving need a suitable approved tool.
Scientific calculator
This calculator requires JavaScript. Use the physics formula sheets and show your working manually.
+ − * / ^ ! · pi · e · sqrt · sin, cos, tan · asin, acos, atan · atan2(y,x) · ln, log · abs · exp · hypot(x,y). Use * between factors. Keep units in your written working.
For the teacher
- Require a legend and volume statement; ask students to count atoms before judging equilibrium.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 7.8.A
- Represent a system undergoing a reversible reaction with a particulate model.
- 7.8.A.1
- Particulate representations can be used to describe the relative numbers of reactant and product particles present prior to and at equilibrium, and the value of the equilibrium constant.
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.
-
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.
-
Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
-
Chemical equilibrium
Chemical equilibrium is a dynamic state in which forward and reverse reaction rates are equal. Macroscopic composition remains constant in a closed system under fixed conditions, but reactant and product concentrations need not be equal. For example, a reversible reaction can reach equilibrium with much more reactant than product.