Understanding Quick Lime and Slaked Lime Reaction

The question asks about the type of chemical reaction that occurs when quick lime reacts with water to form slaked lime. Let's break down this specific reaction and understand the energy changes involved.

Chemical Equation for the Reaction

Quick lime is calcium oxide ($\text{CaO}$). When it reacts with water ($\text{H}_2\text{O}$), it forms slaked lime, which is calcium hydroxide ($\text{Ca(OH)}_2$). The chemical equation for this reaction is:

$\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)}$

This reaction is also commonly known as the slaking of lime.

Energy Changes in the Quick Lime Reaction

When quick lime is added to water, a vigorous reaction takes place. A significant amount of heat is released during this process. You can observe the water getting hot, sometimes even boiling, and steam being produced. The release of heat is a key indicator of the type of reaction occurring.

Types of Chemical Reactions Based on Energy

Chemical reactions can be classified based on whether they absorb or release energy (usually in the form of heat or light). The main types are:

• Endothermic reactions: These reactions absorb energy from the surroundings. This causes the temperature of the surroundings to decrease. The products have higher energy than the reactants.
• Exothermic reactions: These reactions release energy into the surroundings. This causes the temperature of the surroundings to increase. The products have lower energy than the reactants.

Let's also briefly look at the other reaction types mentioned in the options:

• Displacement reaction: A reaction where a more reactive element displaces a less reactive element from its compound. For example, $\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}$. This is not what happens when quick lime reacts with water.
• Decomposition reaction: A reaction where a single compound breaks down into two or more simpler substances. For example, $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$. This is the reverse process of forming quick lime, not its reaction with water.

Analyzing the Quick Lime Reaction

As observed, the reaction between quick lime ($\text{CaO}$) and water ($\text{H}_2\text{O}$) produces a large amount of heat. This means energy is being released into the surroundings.

Comparing this characteristic with the definitions:

• Is it a displacement reaction? No, no element is displacing another.
• Is it an endothermic reaction? No, because heat is released, not absorbed.
• Is it a decomposition reaction? No, two reactants combine, they don't break down from a single compound.
• Is it an exothermic reaction? Yes, because heat is released during the reaction.

Therefore, the reaction of quick lime ($\text{CaO}$) with water ($\text{H}_2\text{O}$) to produce slaked lime ($\text{Ca(OH)}_2$) is a clear example of an exothermic reaction.

Revision Table: Comparing Reaction Types

Additional Information: Applications of Quick Lime Reaction

The exothermic reaction of quick lime with water has practical applications. For instance, it is used in the production of cement and mortar. The heat generated during the slaking process can also be used in certain industrial applications. This reaction is also important in the calcium cycle in nature and industry.