Question

In a reaction \(\mathrm{A}_{2}(\mathrm{~g})+4 \mathrm{~B}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{AB}_{4}(\mathrm{~g}) ; \Delta \mathrm{H}<0\) The formation of \(\mathrm{AB}_{4}(\mathrm{~g})\) will be favoured by: (a) Low temperature and high pressure (b) High temperature and high pressure (c) Low temperature and low pressure (d) High temperature and low pressure

Short answer

Option (a): Low temperature and high pressure.

Step-by-step solution

Analyze Reaction and Reaction Type

The given reaction is \( \mathrm{A}_{2}(\mathrm{~g}) + 4 \mathrm{~B}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{AB}_{4}(\mathrm{~g}) \). The reaction is exothermic because \( \Delta \mathrm{H} < 0 \). In an exothermic reaction, heat is released, and lowering the temperature will favor the formation of products.

Use Le Chatelier's Principle for Pressure

According to Le Chatelier's Principle, if pressure is increased, the equilibrium will shift towards the side with fewer moles of gas to reduce pressure. In this reaction, we have 1 mole of \( \mathrm{A}_{2} \) and 4 moles of \( \mathrm{~B}_{2} \) on the reactant side (5 moles total), and 2 moles of \( \mathrm{AB}_{4} \) on the product side. The side with fewer moles is the product side, so an increase in pressure will favor the formation of \( \mathrm{AB}_{4} \).

Combine the Effects of Temperature and Pressure

Considering both temperature and pressure effects, low temperature will favor the formation of \( \mathrm{AB}_{4} \) due to the exothermic nature of the reaction, and high pressure will favor the formation of products due to the reduction in the number of gas moles. Hence, the formation of \( \mathrm{AB}_{4} \) will be most favored by low temperature and high pressure.

Key concepts

Exothermic Reactions

An exothermic reaction is a fascinating chemical process where heat is released as products form. In such reactions, \( \Delta H \)<0, indicating that the energy of products is lower than the energy of the reactants. This energy release is what makes an exothermic reaction different from an endothermic one, where heat is absorbed. Typically, lowering the temperature in an exothermic process favors the formation of products.

This is because in accordance with Le Chatelier's Principle, the system seeks to counteract the change. So, when the temperature decreases, the reaction shifts in the direction that produces heat, therefore favoring the exothermic pathway of forming more products.

• Exothermic reactions release energy.
• Lower temperatures promote the formation of products.
• In this particular case, the formation of \( \text{AB}_4 \) becomes favored.

Exothermic reactions are not only prevalent in chemical labs but also play a part in everyday life—think of the heat given off in combustion reactions like burning wood or coal.

Gaseous Equilibrium

Gaseous equilibrium plays a crucial role in many chemical reactions, especially those involving gases. When a reaction reaches equilibrium, the rates of the forward and reverse reactions are equal, and the concentrations of reactants and products remain constant.

One of the unique aspects of gaseous equilibrium is its sensitivity to changes in conditions such as pressure and temperature. According to Le Chatelier's Principle, any change in these external conditions causes the equilibrium to shift, counteracting the change. In gaseous reactions:

• An increase in pressure generally shifts the equilibrium towards the side with fewer gas molecules.
• A decrease in pressure allows the equilibrium to shift towards the side with more gas molecules.

For the reaction \( \text{A}_2(g) + 4 \text{B}_2(g) \rightleftharpoons 2 \text{AB}_4(g) \), the reactants comprise 5 moles total, while the product side consists of only 2 moles of gas.
Thus, increasing the pressure will favor the formation of \( \text{AB}_4 \), as the system attempts to minimize the number of gas molecules, effectively reducing pressure.

Effect of Pressure on Equilibrium

The effect of pressure on equilibrium in gaseous reactions is a fascinating concept rooted in Le Chatelier's Principle. It explains how a system at equilibrium responds to external pressure changes.

For gases, pressure changes can significantly affect equilibrium positions. This happens because gas molecules occupy space, and increasing pressure effectively pushes these molecules closer together, favoring the side with fewer moles.

• Increased pressure will shift equilibrium towards the fewer moles side.
• Decreased pressure shifts it towards the side with more moles.

In the example reaction \( \text{A}_2(g) + 4 \text{B}_2(g) \rightleftharpoons 2 \text{AB}_4(g) \), we have more moles on the reactant side compared to the product side.
Increasing the pressure thus shifts the equilibrium towards the product side where there are fewer moles of gas, thereby favoring the formation of \( \text{AB}_4 \).

Understanding the effects of pressure changes is vital for processes involved in industrial applications, where controlling yield and efficiency is crucial. It's a practical demonstration of how manipulating conditions can steer reactions towards desired products.