Reaction Rates
Name the species and distinguish its disappearance rate from a stoichiometrically normalized reaction rate.
AP Chemistry Topic 5.1 · CED printed p. 94 / PDF p. 101. 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
A rate tells us how quickly chemical amount changes, not how much product will eventually form. At constant volume, concentration change per time is convenient: a reactant’s concentration slope is negative, so its disappearance rate is written with a minus sign. An average rate uses a finite interval; an instantaneous rate is the tangent slope at one time. If volume changes, a concentration change can include dilution, so do not mistake it automatically for chemical conversion. A measured mass, gas pressure, or absorbance signal must be related to the species amount before it becomes a reaction rate.
Species do not all change at the same numerical rate. For 2A → B, two A units disappear for each B formed. The reaction rate is . Dividing by the coefficient creates a common rate. A problem asking for A’s disappearance rate wants −Δ[A]/Δt instead. State which you use; otherwise a factor of two can quietly change the answer.
Higher reactant concentration often increases collision frequency; higher temperature usually increases the fraction of sufficiently energetic collisions as well as collision frequency. For a solid, exposing more surface can increase contact with another reactant without changing the total mass. A catalyst changes the route and the number of effective reaction events. Mixing, light, solvent, and other conditions can matter too. These are explanations to test under controlled conditions, not universal promises: a zero-order regime can be insensitive to one concentration, and a surface can become saturated.
Let's work through it
For 2NO2(g) → 2NO(g) + O2(g) in a constant-volume container, [NO2] falls from 0.0800 M to 0.0680 M in 30.0 s.
- Δ[NO2] =−0.0120 M. Its average disappearance rate is 0.0120/30.0 =4.00×10−4 M s−1.
- Divide by its coefficient two for the common reaction rate: r =2.00×10−4 M s−1.
- O2 forms at 2.00×10−4 M s−1; NO forms at twice r, 4.00×10−4 M s−1. These are interval averages, not necessarily the initial rates.
A supplied comparison uses equal solid mass and identical solution at 25 °C, with only solid particle size changed. Powder produces the same final product sooner.
- Equal mass and net stoichiometry fix the maximum product; shorter time changes rate rather than maximum yield.
- Smaller particles expose more surface, increasing accessible reacting contacts.
- A second comparison at different temperature would change the energetic collision distribution too; it cannot isolate surface area.
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 3A → 2B, A disappears at 0.060 M s−1. What is B’s formation rate?
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 3A → 2B, A disappears at 0.060 M s−1. What is B’s formation rate?
Worked answer and reason
0.040 M s−1. The common rate is 0.060/3 =0.020, then multiply by two for B.
Coefficient ratios relate species rates.
Retrieval 2
Why might powdered solid react faster than the same mass of one chunk?
Worked answer and reason
More exposed surface permits more reactant contact at accessible solid sites, provided other conditions are comparable.
Surface accessibility affects heterogeneous collision events.
Catch the tempting mistake
- Every species has the same concentration-change rate.
- Their rates are related by stoichiometric coefficients; the normalized reaction rate is common.
- Rate is the fraction of reactant eventually converted.
- Rate includes time; yield/conversion alone does not describe speed.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 5 problem 1 · ap application · data
- Unit 5 problem 2 · ap synthesis · macroscopic
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 name the species and distinguish slope, disappearance rate and normalized rate.
- For a variable-rate experiment, request the tangent or a specified time interval.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 5.1.A
- Explain the relationship between the rate of a chemical reaction and experimental parameters.
- 5.1.A.1
- The kinetics of a chemical reaction is defined as the rate at which an amount of reactants is converted to products per unit of time.
- 5.1.A.2
- The rates of change of reactant and product concentrations are determined by the stoichiometry in the balanced chemical equation.
- 5.1.A.3
- The rate of a reaction is influenced by reactant concentrations, temperature, surface area, catalysts, and other environmental factors.
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: Chemical Reaction Rates — Scientific check: reaction-rate conventions, concentration slopes and stoichiometric rates.
Glossary and deeper reading · 7 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.
-
Dilution
Dilution lowers solute concentration by adding solvent while conserving the solute amount when no reaction or loss occurs. Concentration calculations use the final solution volume rather than simply the volume of solvent added. For example, increasing a solution's volume from 100 to 200 milliliters halves its concentration if its solute amount is unchanged.
-
Absorbance
Absorbance is a logarithmic measure of how much a sample reduces transmitted light at a specified wavelength. It depends on the ratio of incident to transmitted intensity rather than simply being the fraction absorbed. For example, absorbance 1 corresponds to transmission of one-tenth of the incident intensity.
-
Reaction rate
Reaction rate describes how quickly a reactant or product quantity changes with time. Rates expressed using different species must account for their stoichiometric coefficients. For example, a product's concentration–time slope can be used to determine its rate of formation.
-
Catalyst
A catalyst participates in a reaction pathway and is regenerated overall. It can increase the reaction rate by providing a different pathway without changing the reaction's equilibrium constant at fixed temperature. For example, a catalyst can help a mixture reach equilibrium sooner without changing the final equilibrium composition.
-
Solvent
A solvent is the component used as the dissolving medium in a solution. It is often present in the greater amount, though the designation also depends on the mixture's context. For example, water is the solvent in a dilute aqueous sugar solution.