Activation Energy Examples in Chemistry

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.

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

Ea must be overcome to initiate the reactionE_a\text{ must be overcome to initiate the reaction}

First step

1
Combustion is exothermic overall, but it requires an initial input of energy to start — this is the activation energy (EaE_a).

Full solution

  1. 2
    Striking the match provides friction-generated heat that overcomes the activation energy barrier.
  2. 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 Ea=50 kJ/molE_a = 50\,\text{kJ/mol} and Reaction B has Ea=120 kJ/molE_a = 120\,\text{kJ/mol}. Both occur at the same temperature. Which reaction is faster and why?

Example 3

medium
A catalyst drops EaE_a from 100 kJ/mol100\,\text{kJ/mol} to 60 kJ/mol60\,\text{kJ/mol} for a reaction with ΔH=−20 kJ/mol\Delta H = -20\,\text{kJ/mol}. State the new reverse EaE_a.

Example 4

medium
Sketch (describe) the energy diagram for an exothermic reaction with Ea=50E_a = 50 and ΔH=−30 kJ/mol\Delta H = -30\,\text{kJ/mol}. Where is the peak relative to products?

Example 5

hard
Using the two-point Arrhenius form, if kk doubles when T rises from 300 K300\,\text{K} to 310 K310\,\text{K}, estimate EaE_a. Use R=8.314 J/(mol\cdotpK)R = 8.314\,\text{J/(mol·K)}.

Example 6

challenge
A reaction has A=5.0×1012 s−1A = 5.0\times 10^{12}\,\text{s}^{-1} and Ea=90 kJ/molE_a = 90\,\text{kJ/mol}. Estimate kk at 400 K400\,\text{K}. Use R=8.314 J/(mol\cdotpK)R = 8.314\,\text{J/(mol·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 ΔH\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 Ea=75E_a = 75 kJ and ΔH=−40\Delta H = -40 kJ. How high is the peak above the products?

Example 12

medium
The reverse reaction's activation energy: forward Ea=50E_a = 50 kJ and ΔH=−30\Delta H = -30 kJ. Find Ea,reverseE_{a,\text{reverse}}.

Example 13

medium
Why does a catalyst lowering EaE_a from 80 kJ to 50 kJ speed up the reaction?

Example 14

medium
Does a catalyst change ΔH\Delta H when it lowers EaE_a? Explain.

Example 15

medium
For an endothermic reaction, why is the forward activation energy larger than ΔH\Delta H?

Example 16

medium
At a fixed temperature, which reaction is faster: one with Ea=40E_a = 40 kJ or one with Ea=90E_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 Ea=65E_a = 65 kJ and ΔH=+25\Delta H = +25 kJ (endothermic). Are the products above or below the reactants, and by how much?

Example 20

challenge
Forward Ea=60E_a = 60 kJ, reverse Ea=100E_a = 100 kJ. Find ΔH\Delta H and classify the forward reaction.

Example 21

challenge
A catalyst lowers EaE_a from 100 kJ to 60 kJ for a reaction with ΔH=+20\Delta H = +20 kJ. State the new forward EaE_a, the reverse EaE_a, and whether ΔH\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: AA, EaE_a, or RR?

Example 29

medium
Forward Ea=85 kJ/molE_a = 85\,\text{kJ/mol} and ΔH=+30 kJ/mol\Delta H = +30\,\text{kJ/mol}. Find the reverse activation energy.

Example 30

medium
Reactants sit at 20 kJ/mol20\,\text{kJ/mol} and the peak is at 95 kJ/mol95\,\text{kJ/mol}. What is EaE_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 Ea=30 kJ/molE_a = 30\,\text{kJ/mol}, reaction Y has Ea=90 kJ/molE_a = 90\,\text{kJ/mol}. Which has a larger rate constant kk?

Example 34

medium
Increasing T from 300 K300\,\text{K} to 310 K310\,\text{K} roughly doubles many reaction rates. Why?

Example 35

medium
An endothermic reaction has ΔH=+40 kJ/mol\Delta H = +40\,\text{kJ/mol} and reverse Ea=25 kJ/molE_a = 25\,\text{kJ/mol}. Find the forward EaE_a.

Example 36

hard
Enzyme catalase lowers EaE_a for H2O2\text{H}_2\text{O}_2 decomposition from 7575 to 8 kJ/mol8\,\text{kJ/mol}. By approximately what factor does kk increase at 310 K310\,\text{K}? Use RT≈2.58 kJ/molRT \approx 2.58\,\text{kJ/mol}.

Example 37

hard
Reaction A: Ea=50E_a = 50, ΔH=−10\Delta H = -10. Reaction B: Ea=50E_a = 50, ΔH=−60\Delta H = -60. Compare forward rates (assume similar AA factors).

Example 38

hard
For a reaction with Ea=60 kJ/molE_a = 60\,\text{kJ/mol}, by what factor does kk change when T goes from 298 K298\,\text{K} to 323 K323\,\text{K}? Use R=8.314 J/(mol\cdotpK)R = 8.314\,\text{J/(mol·K)}.

Example 39

hard
A reaction has the same forward and reverse activation energies. What is ΔH\Delta H?

Example 40

hard
A platinum catalyst lowers EaE_a for a reaction from 180180 to 90 kJ/mol90\,\text{kJ/mol}. Does the equilibrium constant change?

Example 41

challenge
A reaction is thermodynamically favorable (ΔG<0\Delta G < 0) but does not proceed at room temperature. Explain in one sentence using activation energy.
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Background Knowledge

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

chemical reaction