Endothermic Exothermic Reactions Difference - Definition, Examples, Important Details

Exothermic and Endothermic Reactions

Many chemical reactions involve the exchange of energy with their surroundings in the form of heat, light, or sound. Based on this energy change, reactions are classified as exothermic and endothermic.

Exothermic Reactions

An exothermic reaction is a chemical reaction in which energy is released to the surroundings. The release of energy often occurs as heat, and in some cases as light or sound. Such reactions may occur spontaneously and are often accompanied by an increase in entropy (ΔS > 0) of the surroundings.

Exothermic reactions are characterized by:

• Negative heat flow (heat flows out of the system)
• Decrease in enthalpy of the system (ΔH < 0)
• Rise in the temperature of the surroundings

In laboratory conditions, strongly exothermic reactions may produce significant heat or even explosions, such as combustion reactions.

Endothermic Reactions

An endothermic reaction is a chemical reaction that absorbs energy from the surroundings in order to proceed. These reactions do not occur spontaneously under normal conditions and require continuous energy input.

Endothermic reactions are characterized by:

• Positive heat flow (heat flows into the system)
• Increase in enthalpy of the system (ΔH > 0)
• Decrease in the temperature of the surroundings

During an endothermic reaction, the absorbed energy is used to break bonds in reactant molecules, making external energy supply essential.

Related Topics Link

• Endothermic Reaction
• Decomposition Reaction
• Endothermic Exothermic Reactions Difference
• Celsius to Kelvin
• Specific Heat of Water

Examples of Endothermic and Exothermic Reactions

Photosynthesis is a prime example of an endothermic chemical reaction. In this process, plants usually use energy from the sun to convert carbon dioxide and water into sugars and even oxygen. This reaction usually requires 15MJ of energy (sunlight) for every kilogram of given sugar produced.

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

Some examples of endothermic Reactions include:

1. Dissolve ammonium chloride in water

2. Crack of alkanes

3. Nucleosynthesis is heavier than nickel in stars

4. Evaporation of liquid water

5. Melting snow

An example of an exothermic reaction is a mixture of sodium and chlorine to produce table salt. This reaction produces 411 kJ of energy in each salt molecule produced:

$\mathrm{Na}(s)+\frac{1}{2} \mathrm{Cl}_2(g) \rightarrow \mathrm{NaCl}(s)$

Some examples of exothermic reaction include:

1. Thermite reaction

2. Neutral reactions (e.g., mixing acid and base to form salt and water)

3. Multiple reaction polymerization

4. Fuel burning

5. Breathing

6. Nuclear fusion

7. Iron corrosion (oxidation reaction)

8. Neutralization of acid in water or base

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

Endergonic and Exergonic Reactions

Chemical reactions can also be classified based on changes in Gibbs free energy (ΔG). On this basis, reactions are called exergonic or endergonic.

Exergonic Reactions

An exergonic reaction is a reaction that releases free energy and occurs spontaneously under given conditions.

Characteristics:

• ΔG < 0 (negative Gibbs free energy change)
• Energy is released to the surroundings
• Products have lower free energy than reactants
• Reaction is thermodynamically spontaneous

Examples:

• Cellular respiration
• Combustion reactions

Endergonic Reactions

An endergonic reaction is a reaction that absorbs free energy and does not occur spontaneously.

Characteristics:

• ΔG > 0 (positive Gibbs free energy change)
• Energy must be supplied from outside
• Products have higher free energy than reactants
• Often coupled with exergonic reactions

Examples:

• Photosynthesis
• Active transport in cells

Also read -

Some Solved Examples

Question 1. An exothermic reaction is characterized by

A. ΔH > 0
B. Absorption of heat
C. Release of heat
D. Decrease in entropy only

Solution:
Exothermic reactions release heat to the surroundings and have ΔH < 0.

Hence, the correct answer is option (C)

Question 2. Which of the following reactions is endothermic?

A. Combustion of methane
B. Neutralization of acid and base
C. Photosynthesis
D. Rusting of iron

Solution:
Photosynthesis absorbs energy from sunlight, hence it is endothermic.

Hence, the correct answer is option (C)

Question 3. For an endothermic reaction, which statement is correct?

A. ΔH < 0
B. Temperature of surroundings increases
C. Heat is absorbed by the system
D. Energy of products is less than reactants

Solution:
Endothermic reactions absorb heat, and products have higher enthalpy.

Hence, the correct answer is option (C)

Question 4. Which of the following has a negative enthalpy change?

A. Dissociation of N₂
B. Melting of ice
C. Burning of coal
D. Photosynthesis

Solution:
Combustion reactions are exothermic, so ΔH < 0.

Hence, the correct answer is option (C)

Question 5. During an exothermic reaction

A. Energy of products > reactants
B. Heat flows into the system
C. Surroundings get warmer
D. ΔH is positive

Solution:
Heat released raises the temperature of surroundings.

Hence, the correct answer is option (C)

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