Reaction Mechanism and Rate Law
Test slow-first-step predictions against observed rate laws under stated supply conditions.
AP Chemistry Topic 5.8 · CED printed p. 102 / PDF p. 109. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Balanced equations and stoichiometric ratios
- Molarity and concentration versus amount
- Bond rearrangement and energy conservation
- Slopes, logarithms and reciprocal graphs
When an explicitly elementary first step is much slower than the following steps under the stated conditions, it controls how rapidly the sequence supplies intermediates and products. Write its rate law from its reactant molecularity. Fast later steps consume what the slow step supplies, so a reactant used only later can be absent from the observed overall law. This approximation assumes those later steps remain fast and adequately supplied; depleting their reactants can break it. Test the predicted law against experimental data. Agreement makes the mechanism consistent, not uniquely proven. Do not use coefficients of the summed net equation in place of the slow elementary step. If a later slow step uses an intermediate, its concentration cannot automatically be replaced by a net reactant; that is the next topic’s approximation problem.
Let's work through it
Step 1: 2NO2 → NO3 + NO, slow. Step 2: NO3 + CO → NO2 + CO2, fast.
- The sum is NO2 + CO → NO + CO2.
- The first elementary step involves two NO2 particles, so r =k[NO2]2 under slow-first-step conditions.
- The prediction is zero order in CO while step 2 remains fast and CO sufficient. If measured initial rate doubles when CO doubles in that same regime, the stated slow-first model does not fit those data.
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
Why may a net reactant be missing from the rate law?
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
Why may a net reactant be missing from the rate law?
Worked answer and reason
It can be used only in a fast step after the rate-controlling event, within the conditions supporting that approximation.
Net consumption and control of reaction frequency differ.
Retrieval 2
Does a matching rate law prove this mechanism?
Worked answer and reason
No. It removes a contradiction but other mechanisms can predict the same dependence.
Consistency is weaker than uniqueness.
Catch the tempting mistake
- The slowest step is chosen from the largest coefficient.
- Step speed comes from kinetics and conditions, not coefficient size.
- A zero-order species is irrelevant to forming product.
- It may still be required downstream even if its tested concentration does not control rate.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 5 problem 15 · ap synthesis · data
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
- Ask students to compare the predicted and observed laws explicitly.
- State fast-following-step assumptions instead of treating a mechanism cartoon as universal.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 5.8.A
- Identify the rate law for a reaction from a mechanism in which the first step is rate limiting.
- 5.8.A.1
- For reaction mechanisms in which each elementary step is irreversible, or in which the first step is rate limiting, the rate law of the reaction is set by the molecularity of the slowest elementary step (i.e., the rate-limiting step).
- Collection of data pertaining to detection of a reaction intermediate will not be assessed on the AP Exam.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Reaction Mechanisms — Scientific check: intermediates, elementary molecularity, slow steps and rapid-equilibrium substitution.
Glossary and deeper reading · 4 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.
-
Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
-
Reaction intermediate
A reaction intermediate is produced in one mechanism step and consumed in another. It cancels from the overall equation and differs from a catalyst, which is present before it is consumed and then regenerated. For example, a short-lived species formed in step one and used in step two can be an intermediate.
-
Rate law
A rate law relates a reaction rate to reactant concentrations and a rate constant under stated conditions. For an overall reaction, its concentration exponents are established from evidence rather than copied from the balanced equation. For example, doubling one reactant concentration doubles the rate when the reaction is first order in that reactant and other conditions are fixed.