Question

For the following equations, tell in which direction, left or right, the equilibrium will shift when these changes are made: The temperature is increased, the pressure is increased by decreasing the volume of the reaction vessel, and a catalyst is added. (a) \(2 \mathrm{SO}_{3}(g)+197 \mathrm{~kJ} \rightleftharpoons 2 \mathrm{SO}_{2}(g)+\mathrm{O}_{2}(g)\) (b) \(4 \mathrm{NH}_{3}(g)+3 \mathrm{O}_{2}(g) \rightleftharpoons 2 \mathrm{~N}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(g)+1531 \mathrm{~kJ}\) (c) \(\mathrm{OF}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g) \rightleftharpoons \mathrm{O}_{2}(g)+2 \mathrm{HF}(g)+318 \mathrm{~kJ}\)

Short answer

Reaction (a): Temp increase -> right, Pressure increase -> left. Reaction (b): Temp increase -> left, Pressure increase -> left. Reaction (c): Temp increase -> left, Pressure increase -> no shift.

Step-by-step solution

- Analyze Reaction (a) for Temperature Increase

For the reaction \[2 \mathrm{SO}_{3}(g)+197 \mathrm{~kJ} \rightleftharpoons 2 \mathrm{SO}_{2}(g)+\mathrm{O}_{2}(g)\] the forward reaction is endothermic since it absorbs heat (positive ΔH). Increasing the temperature will shift the equilibrium to the right, towards the products, to absorb the added heat.

- Analyze Reaction (a) for Pressure Increase

For the reaction \[2 \mathrm{SO}_{3}(g)+197 \mathrm{~kJ} \rightleftharpoons 2 \mathrm{SO}_{2}(g)+\mathrm{O}_{2}(g)\], the forward reaction produces more gas molecules (3 moles of gas) compared to the reactants (2 moles of gas). Increasing the pressure by decreasing volume will shift the equilibrium to the left, towards the reactants, to reduce the pressure.

- Analyze Reaction (a) for Adding a Catalyst

Adding a catalyst does not shift the equilibrium position; it only speeds up the rate at which equilibrium is achieved.

- Analyze Reaction (b) for Temperature Increase

For the reaction \[4 \mathrm{NH}_{3}(g)+3 \mathrm{O}_{2}(g) \rightleftharpoons 2 \mathrm{~N}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(g)+1531 \mathrm{~kJ}\], the forward reaction is exothermic since it releases heat (negative ΔH). Increasing the temperature will shift the equilibrium to the left, towards the reactants, to counteract the added heat.

- Analyze Reaction (b) for Pressure Increase

For the reaction \[4 \mathrm{NH}_{3}(g)+3 \mathrm{O}_{2}(g) \rightleftharpoons 2 \mathrm{~N}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(g)+1531 \mathrm{~kJ}\], the forward reaction produces more moles of gas (8 moles of gas) compared to the reactants (7 moles of gas). Increasing the pressure by decreasing volume will shift the equilibrium to the left, towards the reactants, to reduce the pressure.

- Analyze Reaction (b) for Adding a Catalyst

Adding a catalyst does not shift the equilibrium position; it only speeds up the rate at which equilibrium is achieved.

- Analyze Reaction (c) for Temperature Increase

For the reaction \[\mathrm{OF}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g) \rightleftharpoons \mathrm{O}_{2}(g)+2 \mathrm{HF}(g)+318 \mathrm{~kJ}\], the forward reaction is exothermic since it releases heat (negative ΔH). Increasing the temperature will shift the equilibrium to the left, towards the reactants, to counteract the added heat.

- Analyze Reaction (c) for Pressure Increase

For the reaction \[\mathrm{OF}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g) \rightleftharpoons \mathrm{O}_{2}(g)+2 \mathrm{HF}(g)+318 \mathrm{~kJ}\], the forward reaction produces the same number of moles of gas (2 moles each side). Increasing the pressure by decreasing volume will not shift the equilibrium in either direction.

- Analyze Reaction (c) for Adding a Catalyst

Adding a catalyst does not shift the equilibrium position; it only speeds up the rate at which equilibrium is achieved.

Key concepts

Le Chatelier's principle

Le Chatelier's principle helps us predict how a change in conditions can affect chemical equilibrium. It states that if an external change is applied to a system at equilibrium, the system will adjust itself to counteract that change and restore a new equilibrium. For instance, if we add more reactants, the system will produce more products to balance. Similarly, if the temperature, pressure, or concentration changes, the equilibrium will shift to minimize that change. This principle is useful in understanding how different factors like temperature and pressure influence a reaction.

endothermic and exothermic reactions

Reactions can either release or absorb heat:

• **Endothermic reactions** absorb heat from their surroundings, which means they have a positive ΔH (enthalpy change). An example is the decomposition of sulfur trioxide to sulfur dioxide and oxygen. The forward direction of this reaction absorbs heat.
• **Exothermic reactions** release heat into their surroundings, indicating a negative ΔH. For example, the synthesis of water from ammonia and oxygen releases heat.

Increasing the temperature in an endothermic reaction will shift the equilibrium towards the products, because the system absorbs the excess heat. Conversely, in an exothermic reaction, increasing temperature will shift the equilibrium towards the reactants, as the system tries to release the added heat.

catalysts effect on equilibrium

Catalysts increase the rate at which equilibrium is achieved without being consumed in the reaction. They do this by lowering the activation energy needed for the reaction. However, it's important to note that catalysts do not affect the position of equilibrium. They only help the reactants and products reach equilibrium faster. For example, adding a catalyst to the reaction between nitrogen and hydrogen to form ammonia will speed up the time it takes to reach equilibrium but will not change the relative concentrations of the reactants and products at equilibrium.

effects of pressure changes on gases

Pressure changes can significantly affect reactions involving gases. According to Le Chatelier's principle, increasing the pressure by decreasing the volume will shift the equilibrium towards the side of the reaction with fewer gas molecules. Conversely, decreasing the pressure by increasing the volume will shift the equilibrium towards the side with more gas molecules. For example:

• In the reaction where sulfur trioxide decomposes into sulfur dioxide and oxygen, increasing the pressure shifts the equilibrium to the left, favoring the formation of sulfur trioxide, which has fewer moles of gas.
• In the reaction producing water from ammonia and oxygen, increasing the pressure shifts the equilibrium to the left, towards the reactants, because there are more moles of gas on the product side.

temperature effects on chemical equilibrium

Temperature changes can also impact the equilibrium of a reaction. For endothermic reactions, an increase in temperature shifts the equilibrium to the right, favoring the formation of products. For exothermic reactions, an increase in temperature shifts the equilibrium to the left, favoring the formation of reactants. Let's look at specific examples from the exercise:

• For the endothermic reaction where sulfur trioxide decomposes, increasing the temperature shifts the equilibrium to the right, producing more sulfur dioxide and oxygen.
• For the exothermic reaction between ammonia and oxygen forming nitrogen and water, increasing the temperature shifts the equilibrium to the left, resulting in more ammonia and oxygen.
• In the exothermic reaction producing oxygen and hydrogen fluoride from oxygen difluoride and water, increasing temperature shifts the equilibrium towards the reactants, producing less chlorine and hydrogen fluoride.

Therefore, understanding the temperature's role is crucial for manipulating and optimizing chemical reactions.