Problem 193 - A Complete Resource Book in Chemistry for JEE Main

Question

What is expected to happen when phosphine gas is mixed with chlorine gas? (a) the mixture only cools down (b) \(\mathrm{PCl}_{3}\) and \(\mathrm{HCl}\) are formed and the mixture warms up (c) \(\mathrm{PCl}_{5}\) and \(\mathrm{HCl}\) are formed and the mixture cools down (d) \(\mathrm{PH}_{3} \mathrm{Cl}_{2}\) is formed with warming up

Step-by-Step Solution

Answer

The correct answer is (b): \( \mathrm{PCl}_3 \) and \( \mathrm{HCl} \) are formed and the mixture warms up.

Step 1: Understand Reactants

Phosphine gas is represented by \( \mathrm{PH}_3 \) and chlorine gas by \( \mathrm{Cl}_2 \). The reaction to consider is between these two reactants.

Step 2: Determine Reaction Products

When phosphine gas \( \mathrm{PH}_3 \) reacts with chlorine gas \( \mathrm{Cl}_2 \), chemical reactions can lead to the formation of phosphorus trichloride \( \mathrm{PCl}_3 \) and hydrochloric acid \( \mathrm{HCl} \), which means option (b) is possible.

Step 3: Understand Reaction Energetics

The reaction between \( \mathrm{PH}_3 \) and \( \mathrm{Cl}_2 \) is exothermic. This means that heat is released, warming up the mixture.

Step 4: Analyze Options

Given the reaction forms \( \mathrm{PCl}_3 \) and \( \mathrm{HCl} \), and the reaction releases heat (exothermic), option (b) 'the mixture warms up' is validated, while other options do not match this outcome.

Step 5: Conclusion

Based on the chemical reaction and the energy change involved, the correct option is that \( \mathrm{PCl}_3 \) and \( \mathrm{HCl} \) are formed and the mixture warms up, which aligns with option (b).

Key Concepts

Phosphorus Trichloride Formation

When phosphine (\( \mathrm{PH}_3 \)) comes into contact with chlorine gas (\( \mathrm{Cl}_2 \)), a series of chemical interactions occur. The resulting product from this reaction is phosphorus trichloride (\( \mathrm{PCl}_3 \)). To understand how phosphorus trichloride forms, it's important to look into how the atoms in phosphine and chlorine rearrange themselves.

• In phosphine, phosphorus is bonded to three hydrogens.
• Chlorine is diatomic, meaning it exists as \(\mathrm{Cl}_2\) pairs.

During the reaction, chlorine atoms break their bond to pair with phosphorus, forming \( \mathrm{PCl}_3 \).
Phosphorus trichloride is a vital compound in various chemical processes and is broadly used in the manufacture of pesticides, plasticizers, and flame retardants.
Its simple formation through the reaction of phosphine and chlorine highlights the beauty of chemical reactivity, showcasing how even simple molecular rearrangements can result in completely new substances.

Exothermic Reactions

Not all chemical reactions are created equally when it comes to the energy exchange involved. In the case of phosphine reacting with chlorine, the reaction is exothermic. But what exactly does that mean?
An exothermic reaction is one that releases energy in the form of heat.

• Heat is given off to the surrounding environment.
• The result is an increase in temperature of the reaction mixture.

When phosphine and chlorine react, the newly formed bonds in \( \mathrm{PCl}_3 \) and \( \mathrm{HCl} \) release more energy than is required to break the original bonds in phosphine and chlorine.
This surplus energy is released as heat, and that’s why the reaction mixture warms up during the process.
Understanding exothermic reactions helps in gauging the energy changes associated with chemical processes. It's a crucial concept for chemistry students, as these reactions play significant roles in everyday life, from combustion in engines to the rusting of metal.

Hydrochloric Acid Production

The interaction between phosphine and chlorine doesn’t just stop at forming phosphorus trichloride. Another significant product is hydrochloric acid (\( \mathrm{HCl} \)). When we say 'acid,' we think of a substance with a sour taste and corrosive properties. Hydrochloric acid is common in both industrial applications and our own stomachs!

• In this reaction, hydrogen from phosphine pairs with chlorine atoms.
• The resulting product, \( \mathrm{HCl} \), is a strong acid.

Hydrochloric acid is widely used in laboratories and industries due to its reactivity.
Besides its industrial use, in the stomach, it helps digest food. By understanding how \( \mathrm{HCl} \) forms during reactions, like this one, students gain insight into fundamental chemistry concepts that relate directly to the environment and biological systems.