Endothermic Reaction Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Endothermic 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 absorbs energy (usually as heat) from the surroundings, resulting in a decrease in surrounding temperature and a positive enthalpy change ( $\Delta H > 0$ ).

The reaction needs heat to proceed — you can feel the surroundings get colder as it runs.

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

Common stuck point: Students often know a formula related to endothermic 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

Photosynthesis absorbs energy:

6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2

. Why is this endothermic?

Answer

\Delta H > 0 \quad\text{(endothermic — absorbs energy)}

First step

In an endothermic reaction, energy is absorbed from the surroundings because products have higher potential energy than reactants.

Full solution

2
For photosynthesis, the reactants are $\text{CO}_2$ and $\text{H}_2\text{O}$ ; the product is glucose, which stores more chemical potential energy in its bonds.
3
Since the system absorbs energy (sunlight) rather than releasing it, the enthalpy change is positive: $\Delta H > 0$ .

Endothermic reactions absorb energy from the surroundings. Plants use sunlight as the energy input for photosynthesis, storing that energy in the bonds of glucose.

Example 2

medium

An instant cold pack contains ammonium nitrate and water. When the inner bag is broken, the temperature drops from

25°\text{C}

5°\text{C}

. Classify the process and explain.

Example 3

medium

Photosynthesis absorbs 2803 kJ per mole of glucose. Is it endothermic or exothermic? What is the sign of

\Delta H

Example 4

medium

A cold pack absorbs

250\,\text{kJ}

to dissolve

50\,\text{g}

\text{NH}_4\text{NO}_3

(molar mass

80\,\text{g/mol}

). Estimate

\Delta H

per mole.

Example 5

medium

Decomposition:

2\text{KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_2

\Delta H = +89\,\text{kJ}

. How much energy is absorbed per mole of

\text{KClO}_3

Example 6

hard

\text{N}_2(g) + \text{O}_2(g) \rightarrow 2\text{NO}(g)

has

\Delta H = +180\,\text{kJ}

. How much heat is absorbed when

14\,\text{g}

\text{N}_2

reacts? (

\text{N}_2 = 28\,\text{g/mol}

)

Practice Problems

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

Example 1

easy

Cooking an egg requires continuous heat input. Is this an endothermic or exothermic process?

Example 2

easy

An instant cold pack becomes cold when activated. Is the process inside the pack exothermic or endothermic? Explain.

Example 3

easy

Does an endothermic reaction release or absorb heat?

Example 4

easy

What is the sign of

\Delta H

for an endothermic reaction?

Example 5

easy

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

Example 6

easy

Is photosynthesis endothermic or exothermic?

Example 7

easy

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

Example 8

easy

When an endothermic reaction runs, does the surroundings or the system get cold?

Example 9

easy

Thermal decomposition of

CaCO_3

requires constant heating. Is it endothermic?

Example 10

easy

Dissolving ammonium nitrate in water makes the solution cold. Classify the process.

Example 11

medium

A reaction has

\Delta H = +180

kJ. Is it endothermic, and how much energy is absorbed?

Example 12

medium

Bond energy: breaking reactant bonds needs 800 kJ; forming product bonds releases 600 kJ. Find

\Delta H

and classify.

Example 13

medium

Can an endothermic reaction occur spontaneously? Explain briefly.

Example 14

medium

Compare activation energy and

\Delta H

for an endothermic reaction on an energy diagram.

Example 15

medium

Why does an endothermic reaction mixture often feel cold to the touch?

Example 16

medium

A cold pack uses an endothermic dissolving process. If it absorbs 100 kJ, what is the sign and value of

\Delta H

Example 17

medium

Two reactions: X has

\Delta H = +90

kJ, Y has

\Delta H = -90

kJ. Which feels cold and which feels warm?

Example 18

medium

Photosynthesis stores 2870 kJ per mole of glucose formed. State

\Delta H

and classify.

Example 19

medium

A reaction absorbs 75 kJ. Write its

\Delta H

with sign and classify it.

Example 20

challenge

An endothermic reaction has

E_a = 120

kJ and

\Delta H = +60

kJ. How far below the peak are the products, measured from the peak?

Example 21

challenge

Forward reaction is endothermic with

\Delta H = +50

kJ. What is

\Delta H

for the reverse reaction, and is it exo- or endothermic?

Example 22

challenge

A student says an endothermic reaction with a catalyst becomes exothermic because it now starts easily. Critique this.

Example 23

easy

Melting an ice cube takes in heat. Is the process endothermic or exothermic?

Example 24

easy

A reaction has

\Delta H = +45\,\text{kJ}

. Classify it.

Example 25

easy

Evaporating water from skin cools the skin. What does this say about the phase change?

Example 26

easy

Baking bread requires an oven. Is the chemical process endothermic or exothermic?

Example 27

medium

Breaking the H-H bond in

\text{H}_2

requires

436\,\text{kJ/mol}

. Is bond-breaking endothermic?

Example 28

medium

A reaction has bonds broken totaling

1500\,\text{kJ}

and bonds formed totaling

1300\,\text{kJ}

. Find

\Delta H

and classify.

Example 29

medium

Why does dissolving table salt feel only slightly cool (vs ammonium nitrate, which feels strongly cool)?

Example 30

medium

An endothermic reaction is shifted toward products by Le Chatelier's principle when temperature is increased. Why?

Example 31

medium

If a reaction's energy diagram shows reactants at

50\,\text{kJ}

and products at

200\,\text{kJ}

, find

\Delta H

Example 32

medium

An endothermic reaction has

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

and

E_a = 200\,\text{kJ}

. What is

E_a

for the reverse reaction?

Example 33

medium

If a

200\,\text{g}

water bath loses

4.18\,\text{kJ}

as an endothermic reaction proceeds in it, by how much does the water cool? (specific heat

= 4.18\,\text{J/g}\cdot\text{K}

)

Example 34

medium

An ice pack absorbs heat at

\Delta H = +27\,\text{kJ/mol}

when dissolved. How much heat is absorbed if

0.50\,\text{mol}

dissolves?

Example 35

hard

An endothermic reaction has

\Delta H = +60\,\text{kJ/mol}

and

\Delta S = +250\,\text{J/K}\cdot\text{mol}

. At what temperature does it become spontaneous?

Example 36

hard

On an energy diagram, an endothermic reaction has activation energy

150\,\text{kJ}

and

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

. How far below the transition state are the products?

Example 37

hard

A reaction is at equilibrium. Adding heat shifts it to the right. Is the forward reaction endothermic or exothermic?

Example 38

hard

Critique: 'A reaction with

\Delta H > 0

cannot occur on its own.'

Example 39

challenge

A coffee-cup calorimeter holds

100\,\text{g}

of water at

25.0^\circ\text{C}

. Adding

5.00\,\text{g}

\text{NH}_4\text{NO}_3

(molar mass

80\,\text{g/mol}

) drops the temperature to

21.5^\circ\text{C}

. Compute

\Delta H_{\text{soln}}

per mole, using

c = 4.18\,\text{J/g}\cdot\text{K}

Example 40

challenge

\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)

has

\Delta H = +178\,\text{kJ/mol}

. How much heat is needed to decompose

1.00\,\text{kg}

\text{CaCO}_3

(molar mass

100\,\text{g/mol}

← Back to Endothermic Reaction Practice Mode

Related Concepts

Chemical Reaction Exothermic Reaction Activation Energy

Background Knowledge

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

chemical reaction