The reactions in which energy is absorbed are known as:

Endothermic reaction

The correct answer is option 2. Endothermic reaction.

Let us delve deeper into endothermic reactions:

In chemistry, reactions involve the breaking and forming of chemical bonds. Depending on the energy changes associated with these bond-breaking and bond-forming processes, reactions can be classified as either exothermic or endothermic.

Endothermic reactions are those chemical reactions that absorb energy from the surroundings in the form of heat. This absorption of energy results in a decrease in the temperature of the surroundings. The energy absorbed is used to break bonds in the reactants, which requires input energy.

Key characteristics of endothermic reactions include:

1. Energy Absorption: Endothermic reactions absorb heat from the surroundings during the course of the reaction. This energy is often used to overcome the activation energy barrier required for the reaction to proceed.

2. Temperature Decrease: Since energy is absorbed from the surroundings, the temperature of the surroundings decreases. This cooling effect is one way to identify an endothermic reaction.

3. Examples: Some common examples of endothermic reactions include dissolving ammonium nitrate in water, the reaction between barium hydroxide and ammonium chloride, and the reaction between citric acid and baking soda in cold packs.

4. Equilibrium Shift: In some cases, endothermic reactions may be used to shift the equilibrium of a reversible reaction towards the products. By absorbing energy, the reaction can overcome the equilibrium position and favor the formation of products.

Overall, endothermic reactions play crucial roles in various chemical processes, including biological reactions, industrial processes, and environmental phenomena. They are characterized by the absorption of energy, resulting in a decrease in the temperature of the surroundings.