Activation Energy Examples: 47 Problems with Answers

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

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 minimum kinetic energy that reactant particles must possess upon collision in order to break existing bonds and initiate a chemical reaction, represented as the energy barrier $E_a$ between reactants and products.

The 'hill' reactants must climb over before the reaction can proceed.

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: Activation Energy starts by naming reactants and products, then checks conservation with a balanced equation.

Common stuck point: Students often know a formula related to activation energy 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

Explain why a match must be struck before it ignites, even though combustion is exothermic.

Answer

E_{a} must be overcome to initiate the reaction

First step

Combustion is exothermic overall, but it requires an initial input of energy to start — this is the activation energy (

E_a

Full solution

2
Striking the match provides friction-generated heat that overcomes the activation energy barrier.
3
Once started, the exothermic reaction sustains itself by providing energy to activate neighboring molecules.

Activation energy is the minimum energy that reactant molecules must possess for a successful collision. Even highly exothermic reactions need this initial push.

Example 2

medium

On an energy diagram, Reaction A has

E_a = 50\,\text{kJ/mol}

and Reaction B has

E_a = 120\,\text{kJ/mol}

. Both occur at the same temperature. Which reaction is faster and why?

Example 3

medium

A catalyst drops

E_a

from

100\,\text{kJ/mol}

60\,\text{kJ/mol}

for a reaction with

\Delta H = -20\,\text{kJ/mol}

. State the new reverse

E_a

Example 4

medium

Sketch (describe) the energy diagram for an exothermic reaction with

E_a = 50

and

\Delta H = -30\,\text{kJ/mol}

. Where is the peak relative to products?

Example 5

hard

Using the two-point Arrhenius form, if

k

doubles when T rises from

300\,\text{K}

310\,\text{K}

, estimate

E_a

. Use

R = 8.314\,\text{J/(mol}\cdot \text{K)}

Example 6

challenge

A reaction has

A = 5.0\times 10^{12}\,\text{s}^{-1}

and

E_a = 90\,\text{kJ/mol}

. Estimate

k

400\,\text{K}

. Use

R = 8.314\,\text{J/(mol}\cdot \text{K)}

Practice Problems

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

Example 1

easy

Does a catalyst change the activation energy of a reaction? How?

Example 2

medium

Paper burns exothermically, but a sheet of paper can sit at room temperature for days without catching fire. Why?

Example 3

easy

What is activation energy?

Example 4

easy

On an energy diagram, what does activation energy correspond to?

Example 5

easy

Do exothermic reactions have an activation energy?

Example 6

easy

Does a catalyst raise or lower the activation energy?

Example 7

easy

Does increasing temperature help reactant particles overcome activation energy?

Example 8

easy

Is activation energy the same as the overall energy change

\Delta H

Example 9

easy

Why must reactant particles collide with at least the activation energy to react?

Example 10

easy

A spark ignites a fuel-air mixture. What is the spark providing?

Example 11

medium

A reaction has

E_a = 75

kJ and

\Delta H = -40

kJ. How high is the peak above the products?

Example 12

medium

The reverse reaction's activation energy: forward

E_a = 50

kJ and

\Delta H = -30

kJ. Find

E_{a,\text{reverse}}

Example 13

medium

Why does a catalyst lowering

E_a

from 80 kJ to 50 kJ speed up the reaction?

Example 14

medium

Does a catalyst change

\Delta H

when it lowers

E_a

? Explain.

Example 15

medium

For an endothermic reaction, why is the forward activation energy larger than

\Delta H

Example 16

medium

At a fixed temperature, which reaction is faster: one with

E_a = 40

kJ or one with

E_a = 90

kJ? Why?

Example 17

medium

A reaction will not proceed at room temperature despite being exothermic. Explain using activation energy.

Example 18

medium

How does activation energy explain why food spoils faster at room temperature than in a fridge?

Example 19

medium

A reaction has

E_a = 65

kJ and

\Delta H = +25

kJ (endothermic). Are the products above or below the reactants, and by how much?

Example 20

challenge

Forward

E_a = 60

kJ, reverse

E_a = 100

kJ. Find

\Delta H

and classify the forward reaction.

Example 21

challenge

A catalyst lowers

E_a

from 100 kJ to 60 kJ for a reaction with

\Delta H = +20

kJ. State the new forward

E_a

, the reverse

E_a

, and whether

\Delta H

changed.

Example 22

challenge

Explain why a catalyst speeds up both the forward and reverse reactions equally, using activation energy.

Example 23

easy

What symbol is commonly used for activation energy?

Example 24

easy

Name the high-energy intermediate at the top of an energy diagram peak.

Example 25

easy

True or false: every reaction, exothermic or endothermic, has an activation energy.

Example 26

easy

Does a catalyst change the activation energy or the enthalpy of a reaction?

Example 27

easy

What units are typically used to express activation energy?

Example 28

easy

Which Arrhenius equation parameter represents activation energy:

A

E_a

, or

R

Example 29

medium

Forward

E_a = 85\,\text{kJ/mol}

and

\Delta H = +30\,\text{kJ/mol}

. Find the reverse activation energy.

Example 30

medium

Reactants sit at

20\,\text{kJ/mol}

and the peak is at

95\,\text{kJ/mol}

. What is

E_a

Example 31

medium

Why does milk spoil more slowly when refrigerated?

Example 32

medium

Hydrogen and oxygen can coexist at room temperature without reacting. Why?

Example 33

medium

Two reactions occur at the same temperature. Reaction X has

E_a = 30\,\text{kJ/mol}

, reaction Y has

E_a = 90\,\text{kJ/mol}

. Which has a larger rate constant

k

Example 34

medium

Increasing T from

300\,\text{K}

310\,\text{K}

roughly doubles many reaction rates. Why?

Example 35

medium

An endothermic reaction has

\Delta H = +40\,\text{kJ/mol}

and reverse

E_a = 25\,\text{kJ/mol}

. Find the forward

E_a

Example 36

hard

Enzyme catalase lowers

E_a

for

\text{H}_2\text{O}_2

decomposition from

75

8\,\text{kJ/mol}

. By approximately what factor does

k

increase at

310\,\text{K}

? Use

RT \approx 2.58\,\text{kJ/mol}

Example 37

hard

Reaction A:

E_a = 50

\Delta H = -10

. Reaction B:

E_a = 50

\Delta H = -60

. Compare forward rates (assume similar

A

factors).

Example 38

hard

For a reaction with

E_a = 60\,\text{kJ/mol}

, by what factor does

k

change when T goes from

298\,\text{K}

323\,\text{K}

? Use

R = 8.314\,\text{J/(mol}\cdot \text{K)}

Example 39

hard

A reaction has the same forward and reverse activation energies. What is

\Delta H

Example 40

hard

A platinum catalyst lowers

E_a

for a reaction from

180

90\,\text{kJ/mol}

. Does the equilibrium constant change?

Example 41

challenge

A reaction is thermodynamically favorable (

\Delta G < 0

) but does not proceed at room temperature. Explain in one sentence using activation energy.

← Back to Activation Energy Practice Mode

Related Concepts

Chemical Reaction Catalyst Reaction Rate

Background Knowledge

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

chemical reaction