Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

The temperature rise in the tank was attributed to (a) Dissolution of \(\mathrm{CH}_{3}-\mathrm{N}=\mathrm{C}=\mathrm{O}\) in water (b) Dissolution of \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) in water. (c) Dissolution of \(\mathrm{NH}_{3}\) in water. (d) Dissolution of \(\mathrm{CO}_{2}\) in water.

Step-by-Step Solution

Answer

(c) Dissolution of \( \mathrm{NH}_{3} \) in water causes the temperature rise.

Step 1: Understand the Exercise

The problem is asking for the reason why the temperature rise in the tank occurred. You need to evaluate which dissolution process is exothermic, meaning it releases heat when the specific compound dissolves in water.

Step 2: Analyze the Options

Consider each option and think about the exothermic or endothermic nature of each dissolution reaction. Recall that when gases dissolve in water, associations between molecules can release or absorb heat. We will consider general reactions of dissolution in water for each compound.

Step 3: Exothermic Dissolution Identification

Recall that ammonia (NH3) dissolving in water is a highly exothermic reaction. This means when NH3 dissolves in water, it releases a significant amount of heat, causing the water's temperature to rise. This is because NH3 forms ammonium ions in water, releasing energy.

Step 4: Conclude the Solution

Among the given options, the correct answer would be the one where an exothermic reaction causes the temperature to increase. Based on our analysis, an exothermic reaction is most associated with the dissolution of NH3 in water.

Key Concepts

Dissolution of Ammonia

When we talk about the dissolution of ammonia (\(\text{NH}_3\)) in water, we're looking at a fascinating interaction between molecules. Ammonia, a gas at room temperature, can dissolve in water to form ammonium hydroxide (NH4OH). But what actually happens during this process? When \(\text{NH}_3\) dissolves, it interacts with water molecules and forms ammonium ions (\(\text{NH}_4^+\)) and hydroxide ions (\(\text{OH}^-\)).

This chemical interaction is an exothermic process, meaning it releases energy as heat. The release of energy is primarily due to the formation of these new bonds, particularly between water molecules and ammonia. This is why when ammonia dissolves in water, the temperature of the water rises. This exothermic reaction is crucial when evaluating energy changes in chemical systems.

Temperature Changes

In the context of chemistry, temperature changes during reactions can tell us a lot about the nature of that reaction. But why do we see some reactions heat up while others cool down? The key lies in understanding endothermic and exothermic reactions.

### Exothermic Reactions
When a reaction releases heat, it's known as an exothermic reaction. Dissolution of ammonia is a classic example. Here, the energy released from forming ammonium ions and hydroxide ions is greater than the energy required to initially dissolve the ammonia gas. This excess energy is released as heat, raising the temperature.

### Endothermic Reactions
Conversely, not all chemical reactions increase the temperature. Endothermic reactions absorb energy from their surroundings, leading to a drop in temperature. Understanding whether a reaction is exothermic or endothermic helps in predicting whether the reaction mixture will warm up or cool down, as seen with the dissolution of ammonia.

Chemical Reactions in Aqueous Solutions

Chemical reactions that occur in water, or aqueous solutions, are fundamental to a lot of everyday processes. These reactions often involve the formation or breaking of bonds, releasing or absorbing energy.

### The Role of Water
Water acts as a great solvent because it's polar, meaning it has a positive and a negative end. This polarity helps \(\text{NH}_3\) dissolve by allowing water molecules to surround and interact with the \(\text{NH}_3\) molecules effectively. The dissolution involves the water molecules breaking apart the ammonia molecules and reforming into new compounds like ammonium ions, leading to energy release.

### Energy Changes
Aqueous reactions can vary widely in their energy changes. It is the ability of water molecules to facilitate these changes that makes it a dominant medium for many chemical reactions. In cases like the dissolution of ammonia, the polar nature of water molecules allows new bonds to form, leading to an exothermic reaction where heat is liberated, which is indicative of the broader theme of energy dynamics in aqueous solutions.