Reaction Rate Examples: 46 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Reaction Rate.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The speed at which reactants are converted into products in a chemical reaction, quantified as the change in concentration of a reactant or product per unit time.

How quickly the reaction happens—from instant explosion to years of rusting.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Reaction Rate starts by naming reactants and products, then checks conservation with a balanced equation.

Common stuck point: Students often know a formula related to reaction rate but skip the recognition step: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?

Worked Examples

Example 1

easy

List four factors that affect the rate of a chemical reaction and explain how each works.

Answer

Temperature, concentration, surface area, catalyst

First step

Temperature: higher temperature → faster molecular motion → more frequent and energetic collisions → faster rate.

Full solution

2
Concentration: higher concentration → more reactant particles per volume → more frequent collisions → faster rate.
3
Surface area: finer particles → more exposed surface → more collisions per unit time → faster rate.
4
Catalyst: lowers activation energy → greater fraction of successful collisions → faster rate.

All four factors relate to collision theory: reactions occur when particles collide with sufficient energy and proper orientation. Anything that increases effective collisions increases the rate.

Example 2

medium

In a reaction, the concentration of a reactant decreases from

0.80\,\text{M}

0.20\,\text{M}

over

120

seconds. Calculate the average rate of reaction.

Example 3

medium

For

2\text{NO}_2 \rightarrow 2\text{NO} + \text{O}_2

, NO

_2

disappears at

0.040\,\text{M/s}

. Find the rate of O

_2

formation.

Example 4

medium

For

\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3

, NH

_3

forms at

0.030\,\text{M/s}

. Find the rate of N

_2

disappearance.

Example 5

medium

Given initial-rate data: trial 1 [A]=0.10 M, rate=0.020; trial 2 [A]=0.30 M, rate=0.060 M/s. Find the order in A.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

Why does increasing temperature speed up a chemical reaction?

Example 2

medium

Two equal masses of calcium carbonate react with the same acid. One sample is powder and the other is large chips. Which reacts faster, and why?

Example 3

easy

A reactant concentration drops from

0.80\,\text{M}

0.50\,\text{M}

over

6\,\text{s}

. What is the average rate of disappearance?

Example 4

easy

Which has the faster reaction rate: iron rusting over years, or gunpowder exploding in milliseconds?

Example 5

easy

A product forms at

0.020\,\text{M}

4\,\text{s}

. What is the average rate of formation?

Example 6

easy

Name three factors that generally increase the rate of a reaction.

Example 7

easy

Does crushing a solid into powder speed up or slow down its reaction rate?

Example 8

easy

True or false: at constant conditions, the rate of a typical reaction is the same at the start and near the end.

Example 9

easy

2\text{A}\rightarrow\text{B}

, A disappears at

0.10\,\text{M/s}

. Using coefficients, at what rate does B form?

Example 10

easy

Increasing temperature by

10°\text{C}

often roughly doubles reaction rate. Does heating speed up or slow down most reactions?

Example 11

medium

A reactant falls from

1.00\,\text{M}

0.60\,\text{M}

in the first

10\,\text{s}

, then to

0.40\,\text{M}

in the next

10\,\text{s}

. Compare the two average rates.

Example 12

medium

For

\text{N}_2+3\text{H}_2\rightarrow2\text{NH}_3

\text{H}_2

is consumed at

0.090\,\text{M/s}

. Find the rate of formation of

\text{NH}_3

Example 13

medium

A gas reaction speeds up when the volume is halved at constant temperature. Explain in terms of rate factors.

Example 14

medium

Why does a catalyst increase reaction rate without being consumed?

Example 15

medium

A reaction's rate is measured as

0.012\,\text{M/s}

when

[\text{A}]=0.30\,\text{M}

. If rate

=k[\text{A}]

, find

k

Example 16

medium

Doubling

[\text{A}]

doubles the rate; doubling

[\text{B}]

quadruples the rate. Write the rate law form.

Example 17

medium

Explain why a reaction in solution slows as it proceeds even at constant temperature.

Example 18

medium

Over

5\,\text{s}

\text{O}_2

2\text{H}_2+\text{O}_2\rightarrow2\text{H}_2\text{O}

drops by

0.10\,\text{M}

. Find the rate of

\text{H}_2

disappearance.

Example 19

medium

If a reaction is second order in A (rate

=k[\text{A}]^2

) and

[\text{A}]

is doubled, by what factor does the rate change?

Example 20

challenge

Initial-rate data:

[\text{A}]_0=0.10,\,

rate

=0.002

;

[\text{A}]_0=0.20,\,

rate

=0.008\,\text{M/s}

. Find the order in A and

k

Example 21

challenge

A reaction is first order with

k=0.10\,\text{s}^{-1}

. If

[\text{A}]_0=0.80\,\text{M}

, what is the initial rate, and is the rate at

[\text{A}]=0.40\,\text{M}

half of it?

Example 22

challenge

Two reactions both consume A. Reaction 1 is zero order; reaction 2 is first order in A. As A is depleted, which reaction's rate stays constant, and why?

Example 23

easy

A reactant concentration drops from

1.00\,\text{M}

0.70\,\text{M}

over

5\,\text{s}

. What is the average rate of disappearance?

Example 24

easy

Does adding a catalyst increase, decrease, or not change the rate of a reaction?

Example 25

easy

For the reaction

\text{A} \rightarrow \text{B}

, A drops by

0.040\,\text{M}

2\,\text{s}

. Find the rate.

Example 26

easy

Which has a higher reaction rate at a given moment: a fresh acid–metal reaction or one near completion?

Example 27

medium

A first-order reaction has rate constant

k = 0.050\,\text{s}^{-1}

. What is the initial rate when

[\text{A}]_0 = 2.0\,\text{M}

Example 28

medium

For

\text{rate} = k[\text{A}]^2

, if

[\text{A}]

is tripled, by what factor does the rate change?

Example 29

medium

A reaction rate of

0.024\,\text{M/s}

is observed when

[\text{A}] = 0.40\,\text{M}

. If rate

= k[\text{A}]

, find

k

Example 30

medium

Why does compressing a gas mixture at constant temperature speed up the reaction?

Example 31

medium

For a zero-order reaction in A, what happens to the rate as

[\text{A}]

decreases?

Example 32

medium

For

2\text{A} + \text{B} \rightarrow \text{C}

, A disappears at

0.080\,\text{M/s}

. At what rate does B disappear?

Example 33

medium

Why is initial rate often used to determine rate laws?

Example 34

medium

A reaction's rate doubles when temperature rises from

25^\circ\text{C}

35^\circ\text{C}

. What approximate empirical rule does this match?

Example 35

hard

Initial-rate data: [A]=0.10, [B]=0.10, rate=

1.0\times10^{-3}

; [A]=0.20, [B]=0.10, rate=

4.0\times10^{-3}

; [A]=0.10, [B]=0.20, rate=

2.0\times10^{-3}

. Find the rate law.

Example 36

hard

For the rate law in the previous problem, find

k

using trial 1 data.

Example 37

hard

For a first-order reaction with

k = 0.020\,\text{s}^{-1}

, what fraction of A remains after

t = 1/k

seconds? (Use

e^{-1} \approx 0.37

Example 38

hard

Sketch in words: how does the rate of a typical reaction change over time at constant temperature?

Example 39

hard

\text{H}_2 + \text{I}_2 \rightarrow 2\text{HI}

, rate

=k[\text{H}_2][\text{I}_2]

. If both [H

_2

] and [I

_2

] are doubled, by what factor does rate change?

Example 40

hard

Two identical zinc samples react with HCl: one in

1\,\text{M}

HCl and one in

0.10\,\text{M}

HCl. Which produces H

_2

faster, and why?

Example 41

challenge

For a second-order reaction

2\text{A} \rightarrow \text{products}

with rate

=k[\text{A}]^2

k=0.50\,\text{M}^{-1}\text{s}^{-1}

, and

[\text{A}]_0=0.20\,\text{M}

, find the initial rate.

← Back to Reaction Rate Practice Mode

Related Concepts

Chemical Reaction Collision Theory Concentration

Background Knowledge

These ideas may be useful before you work through the harder examples.

chemical reaction