Question

For each of the questions, four choices have been provided. Select the correct alternative. In which among the following reactions, the formation of product is favoured by decreasing the temperature or volume? (a) \(2 \mathrm{SO}_{3(\mathrm{~g})} \rightleftharpoons 2 \mathrm{SO}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})}-\mathrm{q}\) (b) \(\mathrm{N}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})} \rightleftharpoons 2 \mathrm{NO}_{(\mathrm{g})}-\mathrm{q}\) (c) \(4 \mathrm{NH}_{3(\mathrm{~g})}+5 \mathrm{O}_{2(\mathrm{~g})} \rightleftharpoons 4 \mathrm{NO}_{(\mathrm{g})}+6 \mathrm{H}_{2} \mathrm{O}_{(\mathrm{g})}, \Delta \mathrm{H}\) \(=\) -ve (d) \(2 \mathrm{NO}_{(\mathrm{g})}+\mathrm{O}_{2(\mathrm{~g})} \rightleftharpoons 2 \mathrm{NO}_{2(\mathrm{~g})}, \Delta \mathrm{H}=-\mathrm{ve}\)

Short answer

Answer: (c) \(4 \mathrm{NH}_{3(\mathrm{~g})}+5 \mathrm{O}_{2(\mathrm{~g})} \rightleftharpoons 4 \mathrm{NO}_{(\mathrm{g})}+6 \mathrm{H}_{2} \mathrm{O}_{(\mathrm{g})}, \Delta \mathrm{H}=\) -ve

Step-by-step solution

Apply Le Chatelier's Principle for Temperature

Le Chatelier's Principle states that when a system at equilibrium is subjected to a change in temperature, pressure, or concentration of reactants or products, the system will adjust itself to re-establish equilibrium. For temperature changes, if the reaction is exothermic (ΔH < 0), decreasing the temperature will shift the equilibrium towards the formation of products. If the reaction is endothermic (ΔH > 0), decreasing the temperature will shift the equilibrium towards the formation of reactants.

Apply Le Chatelier's Principle for Volume

For volume changes, if the volume of the system is decreased, the equilibrium will shift towards the side with the fewer moles of gas. If the volume is increased, the equilibrium will shift towards the side with more moles of gas.

Analyze each reaction

(a) The given reaction is exothermic (ΔH < 0) and has more moles of gas on the left side. Therefore, decreasing the temperature will shift the equilibrium towards the formation of products, while decreasing the volume will shift the equilibrium towards the formation of reactants. This option does not satisfy both conditions. (b) The given reaction is endothermic (ΔH > 0) and has an equal number of moles of gas on both sides. Decreasing the temperature will shift the equilibrium towards the formation of reactants, while changing the volume will not affect the equilibrium. This option does not satisfy both conditions. (c) The given reaction is exothermic (ΔH < 0) and has more moles of gas on the left side. Therefore, decreasing the temperature will shift the equilibrium towards the formation of products, while decreasing the volume will shift the equilibrium towards the formation of products as well. This option satisfies both conditions. (d) The given reaction is exothermic (ΔH < 0) and has the same number of moles of gas on both sides. Decreasing the temperature will shift the equilibrium towards the formation of products, while changing the volume will not affect the equilibrium. This option does not satisfy both conditions.

Select the correct alternative

Based on the analysis in Step 3, the correct alternative is (c) \(4 \mathrm{NH}_{3(\mathrm{~g})}+5 \mathrm{O}_{2(\mathrm{~g})} \rightleftharpoons 4 \mathrm{NO}_{(\mathrm{g})}+6 \mathrm{H}_{2} \mathrm{O}_{(\mathrm{g})}, \Delta \mathrm{H}=\) -ve as both decreasing the temperature and volume favor the formation of products in this reaction.

Key concepts

Chemical Equilibrium

Chemical equilibrium is a state in a chemical reaction where the concentrations of reactants and products remain constant over time. This occurs when the forward and reverse reactions happen at equal rates. It's important to note that "constant concentrations" do not mean they are equal; rather, they stabilize at specific values. Achieving equilibrium does not imply that the reaction ceases, but that it continues with no net change in concentrations. This concept is essential to understanding how different conditions affect reaction dynamics.

• Equilibrium is dynamic; reactions still proceed in both directions.
• The equilibrium position represents a balance of the forward and reverse reactions.

Knowing this helps in predicting how factors like temperature, pressure, and volume changes influence the equilibrium position.

Exothermic Reactions

Exothermic reactions are chemical reactions that release energy, typically in the form of heat, as they occur. An exothermic process leads to products that are lower in energy than the reactants. The energy released can often be felt as warmth in the surrounding environment. These reactions are characterized by negative enthalpy change (\(\Delta H < 0\)).

• Common examples include combustion and many oxidation reactions.
• Exothermic reactions often make reactions favorable at lower temperatures.

This property becomes relevant when applying Le Chatelier's Principle because exothermic reactions will favor product formation when temperature decreases.

Effect of Temperature on Equilibrium

The effect of temperature changes on a system at equilibrium is explained by Le Chatelier's Principle. If the temperature of a system involving an exothermic reaction is decreased, the equilibrium shifts to the right, favoring the formation of products. This happens because the system seeks to counteract the change by generating more heat, a result of favoring the exothermic direction. In contrast, increasing the temperature promotes the reverse reaction for exothermic processes.

• Decreasing temperature favors product formation in exothermic reactions.
• Increasing temperature favors product formation in endothermic reactions.

Thus, the interplay between reaction heat dynamics and equilibrium shifts is crucial in chemical reactions and industrial applications.

Effect of Volume on Equilibrium

Volume changes affect gaseous equilibrium systems differently based on the mole count of gases on either side of the reaction. According to Le Chatelier's Principle, a decrease in volume (or increase in pressure) shifts the equilibrium toward the side with fewer gas molecules. Conversely, increasing volume favors the side with more gas molecules. These shifts occur because the system attempts to minimize the change in pressure caused by volume alterations.

• Decrease in volume = shift toward fewer gas moles.
• Increase in volume = shift toward more gas moles.

This principle helps predict how reactions will respond to physical changes, crucial in laboratory and industrial settings.