Exothermic Reaction Examples in Chemistry

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

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

Concept Recap

A chemical reaction that releases energy (usually as heat or light) to the surroundings, resulting in an increase in surrounding temperature and a negative enthalpy.

The reaction gives off heat—you can feel the surroundings get warmer as it proceeds.

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

Common stuck point: Students often know a formula related to exothermic reaction 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 burning methane (

\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}

) is an exothermic reaction.

Answer

\Delta H < 0 \quad\text{(exothermic)}

First step

In an exothermic reaction, the energy released when new bonds form in the products is greater than the energy required to break bonds in the reactants.

Full solution

2
The C–O and O–H bonds in $\text{CO}_2$ and $\text{H}_2\text{O}$ are very strong, releasing more energy than is needed to break the C–H and O=O bonds.
3
The excess energy is released as heat and light ( $\Delta H < 0$ ).

Exothermic reactions release energy to the surroundings. Combustion reactions are classic examples — the heat released is what makes fuels useful for energy production.

Example 2

medium

A reaction releases

890

kJ of heat per mole of methane burned. If

3.2

g of

\text{CH}_4

is burned (

M = 16\,\text{g/mol}

), how much heat is released?

Example 3

medium

The combustion of octane releases

5470

kJ/mol. How much heat is released when

11.4

g of octane (

M = 114\,\text{g/mol}

) burns?

Example 4

medium

For a reaction, bonds broken absorb

840

kJ and bonds formed release

1020

kJ. Find

\Delta H

and classify.

Bonds broken: 840 kJ; bonds formed: 1020 kJ. Find ΔH and classify.

Example 5

medium

50.0\,\text{mL}

acid-base neutralization in a coffee-cup calorimeter raises

100.0

g of solution from

21.0\,°\text{C}

27.5\,°\text{C}

(

c = 4.18\,\text{J/g·°C}

). Find

q

released.

Example 6

medium

Combustion of

4.40

g propane (

M = 44.0\,\text{g/mol}

) releases

222\,\text{kJ}

. Calculate

\Delta H

per mole of propane burned.

Example 7

hard

Burning

2.00

g of ethanol (

M=46.0

g/mol;

\Delta H_c = -1367\,\text{kJ/mol}

) heats

250

g of water. Assuming all heat transfers, find

\Delta T

of the water. (

c=4.18\,\text{J/g·°C}

)

Example 8

hard

Given

\Delta H_f^°

: CO

_2

(g)

= -393.5

, H

_2

O(l)

= -285.8

, C

_2

_2

(g)

= +226.7

kJ/mol. Find

\Delta H

for

2\text{C}_2\text{H}_2 + 5\text{O}_2 \rightarrow 4\text{CO}_2 + 2\text{H}_2\text{O}

and classify.

Example 9

challenge

A reaction has

\Delta H = -120\,\text{kJ}

and

E_a = 60\,\text{kJ}

. A catalyst lowers

E_a

30\,\text{kJ}

. State the new

E_a

for the reverse reaction and

\Delta H

for the reverse.

With catalyst, Eₐ drops from 60 kJ to 30 kJ. Find Eₐ for the reverse reaction and ΔH_reverse.

Practice Problems

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

Example 1

easy

A hand warmer packet heats up when activated. Is this an exothermic or endothermic process? Explain.

Example 2

easy

A reaction mixture causes the beaker and surrounding air to become warmer. Is the reaction exothermic or endothermic? Explain.

Example 3

easy

Does an exothermic reaction release or absorb heat?

Example 4

easy

What is the sign of

\Delta H

for an exothermic reaction?

What is the sign of ΔH for this reaction?

Example 5

easy

A reaction makes the beaker feel warm. Exothermic or endothermic?

Example 6

easy

Is combustion (burning fuel) exothermic or endothermic?

Example 7

easy

Do exothermic reactions still need activation energy to start?

Example 8

easy

On an energy diagram, are the products higher or lower than reactants for an exothermic reaction?

Example 9

easy

Are all exothermic reactions fast or explosive?

Example 10

easy

Acid-base neutralization warms the solution. What does this tell you about its energy change?

Example 11

medium

A reaction has

\Delta H = -200

kJ. Is it exothermic, and how much energy is released per the equation?

A reaction has ΔH = −200 kJ. Exothermic or endothermic?

Example 12

medium

Bond energy: breaking reactant bonds needs 500 kJ; forming product bonds releases 700 kJ. Find

\Delta H

and classify.

Bonds broken: 500 kJ absorbed; bonds formed: 700 kJ released. What is ΔH?

Example 13

medium

Why is

\Delta H

negative for exothermic reactions even though energy is not 'lost'?

Example 14

medium

A hand warmer relies on iron oxidation releasing heat. Classify the reaction and explain the heat felt.

Example 15

medium

Compare activation energy and

\Delta H

for an exothermic reaction on an energy diagram.

Identify Eₐ and ΔH on this exothermic energy diagram.

Example 16

medium

Two reactions: A releases 150 kJ, B absorbs 150 kJ. Which is exothermic and what are their

\Delta H

signs?

Example 17

medium

Respiration:

C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O

releases energy. Classify and state what the energy is used for biologically.

Example 18

medium

If a reaction is exothermic, what happens to the temperature of an insulated reaction mixture?

Example 19

medium

A reaction releases 50 kJ. Write its

\Delta H

with sign and classify it.

A reaction releases 50 kJ. Write ΔH with its sign.

Example 20

challenge

A reaction has

E_a = 50

kJ and releases 120 kJ overall. Sketch the energy values: how high is the peak above reactants, and where are products relative to reactants?

Eₐ = 50 kJ; reaction releases 120 kJ. Where is the peak, and where are the products?

Example 21

challenge

Burning 1 mol methane releases 890 kJ. How much energy is released burning 0.5 mol, and what is

\Delta H

for 1 mol?

Example 22

challenge

A reaction is exothermic but does not start at room temperature. A catalyst is added and it proceeds. Explain in terms of

E_a

and

\Delta H

Example 23

easy

In an exothermic reaction, is the energy of the surroundings increasing or decreasing?

Example 24

easy

Burning propane in a barbecue is described by

\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}

. Is this exothermic or endothermic, and what evidence tells you so?

Example 25

easy

Sodium metal reacts vigorously with water and the beaker gets hot. Is the reaction exothermic or endothermic?

Example 26

medium

A reaction in a calorimeter raises

200

g of water from

22.0\,°\text{C}

30.0\,°\text{C}

. Using

c = 4.18\,\text{J/g·°C}

, find the heat released and classify the reaction.

Example 27

medium

The thermite reaction

2\text{Al} + \text{Fe}_2\text{O}_3 \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3

has

\Delta H = -852\,\text{kJ}

. How much energy is released when

0.50

mol Fe

_2

_3

reacts?

Example 28

medium

Why does an exothermic reaction inside a sealed insulated container show a rise in temperature even though

\Delta H

refers to constant-pressure heat?

Example 29

medium

Dissolving anhydrous CaCl

_2

in water releases heat. Classify the dissolution and explain in terms of lattice energy versus hydration energy.

Example 30

medium

For

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

\Delta H = -572\,\text{kJ}

per 2 mol H

_2

. How much heat is released by burning

0.25

mol H

_2

Example 31

medium

For a reversible reaction, if the forward reaction is exothermic, what is true about the reverse reaction's

\Delta H

Example 32

hard

In a bomb calorimeter (heat capacity

C = 10.5\,\text{kJ/°C}

), burning

1.50

g of a sample raises the temperature by

3.20\,°\text{C}

. Find the heat released per gram and classify.

Example 33

hard

Using Hess's law,

\Delta H_1 = -200\,\text{kJ}

for step 1 and

\Delta H_2 = -150\,\text{kJ}

for step 2. The overall reaction is the sum of step 1 and the reverse of step 2. Find overall

\Delta H

and classify.

Example 34

hard

A reaction has

\Delta H = -250\,\text{kJ/mol}

. If

40\%

of the released heat warms

500

g of water, how much does the water's temperature rise per mole reacted? (

c = 4.18\,\text{J/g·°C}

)

Example 35

hard

For a reaction with

\Delta H = -90\,\text{kJ}

and

\Delta S = -100\,\text{J/K}

, use

\Delta G = \Delta H - T\Delta S

T = 298\,\text{K}

to decide whether the (exothermic) reaction is spontaneous.

Example 36

hard

Two exothermic reactions release the same total energy: A is fast (seconds), B is slow (days). Does the difference in rate change

\Delta H

? Explain.

Example 37

challenge

A hand warmer uses iron oxidation:

4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3

\Delta H = -1648\,\text{kJ}

. If the warmer contains

5.60

g Fe (

M=55.85

) and

80\%

of released heat warms the user, how many kJ does the user receive?

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Related Concepts

Chemical Reaction Endothermic Reaction Enthalpy

Background Knowledge

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

chemical reaction