Endothermic Reaction - Definition, Examples, Differences, FAQs

Endothermic Reaction

An endothermic reaction is a chemical reaction in which the reactants absorb heat energy from the surroundings to form products. As a result, the temperature of the surroundings decreases, producing a cooling effect. Such reactions are characterized by a positive enthalpy change (ΔH > 0).

Concepts Behind Endothermic Reactions

To understand endothermic reactions, the following points must be kept in mind:

1. Bond breaking requires energy: Energy is needed to break the bonds present in reactant molecules.

2. Bond formation releases energy: Energy is released when new bonds are formed in product molecules.

In an endothermic reaction, the energy required to break bonds in reactants is greater than the energy released during bond formation in products. Hence, the excess energy is absorbed from the surroundings.

How to Identify Exothermic and Endothermic Reactions

Although bond breaking and bond formation cannot be directly observed, exothermic and endothermic reactions can be identified using the following methods:

1. Change in Temperature

The temperature change of the reaction mixture can be measured using a thermometer.

• If the temperature of the surroundings decreases, the reaction is endothermic, as energy is absorbed from the surroundings.
• If the temperature of the surroundings increases, the reaction is exothermic, as energy is released to the surroundings.

2. Enthalpy Change (ΔH)

Enthalpy (H) represents the total energy content of a system. The enthalpy change (ΔH) of a reaction depends on:

• Energy absorbed during bond breaking in reactants (always positive)
• Energy released during bond formation in products (always negative)

Based on ΔH:

• ΔH < 0 (Negative enthalpy change):
  More energy is released during product formation than is absorbed during bond breaking → Exothermic reaction

• ΔH > 0 (Positive enthalpy change):
  More energy is absorbed during bond breaking than is released during product formation → Endothermic reaction

Difference Between Exothermic Reaction and Endothermic Reaction

Read more :

• NCERT notes Class 11 Chemistry Chapter 6 Thermodynamics
• NCERT solutions for Class 11 Chemistry Chapter 6 Thermodynamics
• NCERT Exemplar Class 11 Chemistry Solutions Chapter 6 Thermodynamics

Endothermic Reaction Examples

1. Photosynthesis

Photosynthesis is essential for the existence of living organisms on Earth. It is an endothermic process because green plants absorb energy from sunlight to convert carbon dioxide and water into glucose and oxygen.

$6 \mathrm{CO}_2(\mathrm{~g})+6 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \xrightarrow{\text { sunlight }} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6(\mathrm{aq})+6 \mathrm{O}_2(\mathrm{~g})$

2. Melting of Ice

The melting of ice is an endothermic process. In ice, water molecules are tightly packed in a rigid structure. To change into liquid water, ice absorbs heat from the surroundings, allowing the molecules to move freely.

3. Baking bread and cooking eggs are also endothermic processes; energy is absorbed from the oven or pan to cook it.

4. Sublimation of dry ice: Sublimation occurs at a temperature and pressure below the critical point. The phase change requires energy to convert from a solid to a gas phase.

5. Instant ice pack to treat injuries.

SOME OTHER ENDOTHERMIC REACTION EQUATIONS:

1. Formation of Nitric Oxide

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

2. Thermal Decomposition of Magnesium Carbonate

$\mathrm{MgCO}_3(s)+\text { heat } \rightarrow \mathrm{MgO}(s)+\mathrm{CO}_2(g)$

3. Dissolution of Ammonium Nitrate

$\mathrm{NH}_4 \mathrm{NO}_3(s)+\text { heat } \rightarrow \mathrm{NH}_4^{+}(a q)+\mathrm{NO}_3^{-}(a q)$

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Some Solved Examples

Question 1: Which of the following is an endothermic process?

A. Burning of coal
B. Neutralization of acid and base
C. Photosynthesis
D. Rusting of iron

Solution:

Photosynthesis absorbs energy from sunlight; it is an endothermic process.

Hence, the correct answer is option (C)

Question 2: Melting of ice is endothermic because

A. Heat is released to surroundings
B. Bonds are formed
C. Heat is absorbed to overcome intermolecular forces
D. Entropy decreases

Solution:

Ice absorbs heat to overcome hydrogen bonding between water molecules.

Hence, the correct answer is option (C)

Question 3: Which reaction shows a positive enthalpy change $(\Delta \mathrm{H}>0)$ ?
A. Combustion of methane
B. Formation of NaCl
C. Thermal decomposition of $\mathrm{MgCO}_3$
D. Neutralization reaction

Solution:

Thermal decomposition reactions require heat and are endothermic.

Hence, the correct answer is option (C)

Question 4: For an endothermic reaction, which statement is correct?

A. ΔH < 0
B. Products have lower energy than reactants
C. Surroundings get warmer
D. Energy is absorbed from surroundings

Solution:

Endothermic reactions absorb heat, so ΔH > 0.

Hence, the correct answer is option (D)

Question 5: Which of the following equations represents an endothermic reaction?
A. $2 \mathrm{Na}+\mathrm{Cl}_2 \rightarrow 2 \mathrm{NaCl}$
B. $\mathrm{N}_2+\mathrm{O}_2+$ heat $\rightarrow 2 \mathrm{NO}$
C. $\mathrm{HCl}+\mathrm{NaOH} \rightarrow \mathrm{NaCl}+\mathrm{H}_2 \mathrm{O}$
D. $\mathrm{CH}_4+2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}$

Solution:

Formation of nitric oxide requires heat input.

Hence, the correct answer is option (B)