Endothermic Reaction

Nov 02, 2022, 16:45 IST

Endothermic reactions is a chemical reactions in which the reactants absorb heat energy from surroundings to form a products. These reactions lower the temperature of their surroundings, creating a cooling effect. Physical processes can also be endothermic – ice cubes absorb thermal energy from their surroundings and melt to form liquid water (they do not break or form any chemical bonds).

The release of energy usually accompanies the breaking of a chemical bond. Similarly, the formation of chemical bonds requires an input of energy. The energy supplied/released can take many forms (such as heat, light, and electricity). Endothermic reactions generally involve the formation of chemical bonds through the absorption of heat from the surroundings. On the other hand, exothermic reactions involve the release of thermal energy generated by bond breaking.

Difference Between Endothermic and Exothermic Reaction

The terms ‘Endo’ and ‘Exo’ have Greek roots, meaning ‘within’ and ‘out,’ respectively. The primary difference between endothermic and exothermic reactions is that the former absorbs heat from the surroundings, whereas the latter involves a release of heat.

Some other differences between these types of chemical reactions are tabulated below-

Endothermic Process v/s Endothermic Reaction

The human body uses the endothermic nature of evaporation to cool by itself. This process is done through sweating. The sweat (produced on the skin's surface) absorbs the heat from the skin to evaporate, creating a cooling effect.

However, sweating is not an exothermic reaction. Chemical reactions can involve breaking existing chemical bonds, forming new bonds, or both. Evaporation of sweat does not include chemical changes but involves a physical phase change (from liquid to vapor). Thus, evaporation is said to be physical endothermic process rather than endothermic reactions.

All the process that absorbs heat from their surroundings is endothermic. Thus, all the endothermic reactions are endothermic processes. However, the opposite is not true. Many endothermic processes involve physical changes rather than the chemical changes.

Endothermic Reaction Examples

When ammonium chloride (NH4Cl) dissolves in water, an endothermic reaction occurs. The salt dissociates into ammonium (NH4+) and the chloride (Cl–) ions. The chemical equation can be written as follows:

NH₄Cl (s) + H₂O (l) ⟶ NH₄Cl (aq) – Heat

Ammonium nitrate (NH4NO3), an important ingredient in instant cold packs, dissociates into ammonium cation (NH4+) and nitrate anion (NO3–) when dissolved in water. These ions further form ammonium hydroxide (NH4OH) and nitric acid (HNO3) (by reaction with OH– and H+ ions in water). This reaction is endothermic because it cools the surroundings by absorbing heat from them.

The formation of an nitric oxide from the reaction between the nitrogen and oxygen is endothermic because it absorbs approximately 180.5 kilojoules of heat for each mole of N2 and O2.

Other Endothermic Processes

• The melting of ice to form water
• Evaporation of liquid water, forming water vapour
• Sublimation of solid CO₂
• The baking of bread

Energy Level Diagram

A simple energy level diagram of endothermic and exothermic reactions is shown below. An activation energy is an energy that must be provided to the reactants for them to overcome the energy barrier and react.

In exothermic reactions, the potential energy of a product is generally lower than the energy of the reactant. In comparison, the potential energy of a product in the endothermic reaction is higher than that of the reactants. This gap in the potential energy accounts for the energy absorbed by the system during the chemical reaction.

Frequently Asked Question (FAQs)

Q1. How can you say if a reaction is endothermic?

Ans. If given ΔH is positive (+), the chemical reaction is endothermic because the less energy is being released when the products is formed than the energy used to break the reactants.

Q2. Define Endothermic reaction

Ans. The chemical reactions in which the reactants absorb a heat energy from the surroundings to form a products. These reactions lowers the temperature of their surrounding, to form a cooling effect.

Q3. Why are endothermic reactions positive?

Ans. If the reaction absorbs or uses more energy than it releases, than the reaction is said to be endothermic, also the enthalpy will be positive.

Q4. Which element has the most endothermic reaction?

Ans. F−F is the most endothermic.

Q5. Do endothermic reactions release heat?

Ans. An exothermic process releases heat, it cause the temperature of the immediate surroundings to rise. Whereas endothermic process absorbs heat and cools the surroundings.”

Q6. Can endothermic reaction freeze water?

Ans. The endothermicity of the reaction takes thermal energy away from its surroundings, so much so that the water around the beaker will freeze as a result.