Problem 61 - Chemistry The Central Science In Si Units

Question

Consider the following exothermic equilibrium (Boudouard reaction) $$ 2 \mathrm{CO}(g) \rightleftharpoons \mathrm{C}(s)+\mathrm{CO}_{2}(g) $$ How will each of the following changes affect an equilibrium mixture of the three gases: \((\mathbf{a})\) a catalyst is added to the mixture; \((\mathbf{b}) \mathrm{CO}_{2}(g)\) is added to the system; (c) \(\mathrm{CO}(g)\) is added from the system; \((\mathbf{d})\) the reaction mixture is heated; (e) the volume of the reaction vessel is doubled; (f) the total pressure of the system is increased by adding a noble gas?

Step-by-Step Solution

Answer

(a) No effect; (b) shifts left; (c) shifts right; (d) shifts left; (e) shifts left; (f) no effect.

Step 1: Impact of Adding a Catalyst

A catalyst speeds up the rate of both the forward and reverse reactions equally without affecting the equilibrium position. Thus, adding a catalyst will not change the position of the equilibrium for this reaction.

Step 2: Effect of Adding CO₂ (Product)

According to Le Chatelier's Principle, if you add more of a product to the reaction mixture, the equilibrium will shift towards the left to counteract the change, favoring the formation of reactants. Therefore, adding \(\mathrm{CO}_2\) will shift the equilibrium towards the reactant side, increasing the concentration of \(\mathrm{CO}\).

Step 3: Adding More CO (Reactant)

Again, applying Le Chatelier's Principle, adding more of a reactant will shift the equilibrium towards the right, favoring the formation of products. Thus, adding \(\mathrm{CO}\) will shift the equilibrium towards the right, increasing the production of \(\mathrm{CO}_2\) and \(\mathrm{C}\).

Step 4: Effect of Heating (Temperature Increase)

The reaction is exothermic, meaning it releases heat. By increasing the temperature, the equilibrium will shift towards the endothermic direction, which is the left side (formation of \(\mathrm{CO}\)), to absorb the excess heat.

Step 5: Doubling the Volume of the Reaction Vessel

Doubling the volume of the vessel decreases the pressure. The system will respond by shifting towards the side with more gas molecules to increase the pressure. Since there are more moles of gas on the reactant side (2 moles of \(\mathrm{CO}\)) compared to the product side (0 moles \(\mathrm{C}\)), the equilibrium will shift to the left.

Step 6: Adding a Noble Gas (Increasing Total Pressure)

Adding a noble gas increases the total pressure but does not change the partial pressures of the reacting components, as noble gases do not react. Therefore, the equilibrium position remains unchanged.

Key Concepts

Understanding Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemistry used to predict how a chemical equilibrium will respond to changes in conditions. It states that if a dynamic equilibrium is disturbed by altering concentration, temperature, or pressure, the system will adjust to counteract this change and restore a new equilibrium.

Consider an exothermic reaction where heat is released. If the equilibrium is disturbed in this reaction, the system will act to re-establish equilibrium by either favoring the forward or reverse reaction. For example:

• If a reactant or product is added, the equilibrium will shift away from the added substance to minimize its effect.
• If heat is added (increasing temperature), in an exothermic reaction, the equilibrium will shift to absorb the heat by favoring the reverse reaction — which is endothermic.
• In a reaction involving gases, changing the volume affects pressure, prompting a shift toward the side with fewer or more moles, depending on the change.

Understanding this principle is critical for predicting how a reaction will behave under new conditions.

Exothermic Reactions and Temperature Changes

An exothermic reaction is one that releases energy in the form of heat. In these reactions, the enthalpy change (1 H) is negative, indicating an energy release. When it comes to equilibrium, exothermic reactions are sensitive to temperature changes.

According to Le Chatelier's Principle, if you increase the temperature of an exothermic reaction, the equilibrium will shift to absorb the excess heat. This means the equilibrium moves in the direction of the endothermic reaction, specifically the reactant side. Conversely, lowering the temperature will favor the exothermic pathway, encouraging the formation of more products.

In terms of practical application, understanding exothermic reactions helps us control industrial and natural processes. For instance, energy release in combustion engines or metabolic pathways depends on manipulating such reactions for desired outcomes.

The Role of Catalysts in Chemistry

Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. Importantly, they do not alter the equilibrium position of a reaction. Instead, catalysts work by providing an alternative pathway for the reaction with a lower activation energy.

In a chemical reaction, catalysts influence both the forward and reverse reactions equally. This means that while they help the system reach equilibrium faster, they do not change the concentration of reactants and products at equilibrium. Therefore, even if you add a catalyst to an exothermic equilibrium reaction, it will not affect the position of equilibrium; only the rate at which equilibrium is achieved is enhanced.

Catalysts are crucial in many areas, including industrial manufacturing and biological processes. They enable processes like the production of ammonia in the Haber-Bosch process and biological reactions in enzymes efficiently.