Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

When othophosphoric acid is heated to \(600^{\circ} \mathrm{C}\), the product formed is (a) \(\mathrm{HPO}_{3}\) (b) \(\mathrm{P}_{2} \mathrm{O}_{5}\) (c) \(\mathrm{PH}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{PO}_{3}\)

Step-by-Step Solution

Answer

The product formed is (a) \(\mathrm{HPO}_3\).

Step 1: Identify the Initial Reactant

The problem involves the heating of orthophosphoric acid. Orthophosphoric acid is chemically represented as \(\mathrm{H}_3\mathrm{PO}_4\).

Step 2: Analyze the Heating Process

When \(\mathrm{H}_3\mathrm{PO}_4\) is heated to a high temperature, particularly around \(600^{\circ} \mathrm{C}\), it undergoes a dehydration process. Dehydration involves the removal of water (\(\mathrm{H}_2\mathrm{O}\)) molecules.

Step 3: Determine the Dehydration Reaction

During heating, orthophosphoric acid loses water and forms metaphosphoric acid. This can be represented as: \[\mathrm{H}_3\mathrm{PO}_4 \rightarrow \mathrm{HPO}_3 + \mathrm{H}_2\mathrm{O}\].

Step 4: Identify the Product

According to the reaction given in Step 3, the main product formed when \(\mathrm{H}_3\mathrm{PO}_4\) is heated is \(\mathrm{HPO}_3\), which is metaphosphoric acid.

Step 5: Verify the Correct Option

From the given options, the compound \(\mathrm{HPO}_3\) corresponds to option (a). Therefore, option (a) \(\mathrm{HPO}_3\) is the correct answer.

Key Concepts

Dehydration Reaction

A dehydration reaction is a type of chemical reaction that involves the removal of water molecules from a compound. In the context of orthophosphoric acid, this process is crucial for transforming it into another compound, like metaphosphoric acid. Such a transformation happens when orthophosphoric acid is subjected to heat, leading to the loss of its water content.

During the dehydration reaction, the water molecule comes from the integrated chemical structure of the reactant. This loss of water is what drives the reaction forward in creating a new substance.

Some important points about dehydration reactions include:

• They often require high temperatures to initiate.
• They result in the breaking down of a larger molecule into smaller, sometimes more reactive products.
• The specific outcome of a dehydration reaction can depend on the initial compound's nature and the environmental conditions.

Metaphosphoric Acid

Metaphosphoric acid is a product formed from the dehydration of orthophosphoric acid. Chemically represented as \(\mathrm{HPO}_3\), it plays a role in various chemical and industrial applications. This compound stands out due to its unique formation process, stemming directly from the application of heat and the subsequent removal of water.

Some characteristics of metaphosphoric acid include:

• It appears as a colorless solid, showcasing its distinctive solid-state at room temperature.
• It is derived directly from \(\mathrm{H}_3\mathrm{PO}_4\) under high-heat conditions, such as the heating to \(600^{\circ} \mathrm{C}\).
• It can also act as a dehydrating agent, much like its formation involves the removal of water molecules from another compound.

This compound helps in understanding the conversion processes of different phosphoric acids under specific conditions.

Heating Process

The heating process is an essential step in driving chemical reactions like the dehydration of orthophosphoric acid. By subjecting substances to increased temperatures, the kinetic energy of molecules rises, enabling and accelerating reactions.

• Applying heat often provides the necessary energy to overcome reaction barriers, thus initiating transformations like dehydration.
• High temperature ensures more effective interactions among molecules, leading to new product formations.
• Specific temperature levels, such as the \(600^{\circ} \mathrm{C}\) used for orthophosphoric acid, are critical for achieving the desired chemical conversion to metaphosphoric acid.

An understanding of the heating process is vital for controlling the outcomes of chemical reactions and ensuring that the desired product is formed.