Question

Coal can be used to generate hydrogen gas (a potential fuel) by the endo- thermic reaction: $$\mathrm{C}(s)+\mathrm{H}_{2} \mathrm{O}(g) \rightleftharpoons \operatorname{cog}(g)+\mathrm{H}_{2}(g)$$ If this reaction mixture is at equilibrium, predict whether each disturbance will result in the formation of additional hydrogen gas, the formation of less hydrogen gas, or have no effect on the quantity of hydrogen gas. \begin{equation} \begin{array}{l}{\text { a. adding } C \text { to the reaction mixture }} \\\ {\text { b. adding } \mathrm{H}_{2} \mathrm{O} \text { to the reaction mixture }} \\ {\text { c. raising the temperature of the reaction mixture }} \\\ {\text { d. increasing the volume of the reaction mixture }} \\ {\text { e. adding a catalyst to the reaction mixture }} \\ {\text { f. adding an inert gas to the reaction mixture }}\end{array} \end{equation}

Short answer

a. More H2, b. More H2, c. More H2, d. No effect, e. No effect, f. No effect

Step-by-step solution

Analyzing the effect of adding carbon (C)

Adding more reactant to a system at equilibrium (C in this case), according to Le Chatelier's principle, will shift the equilibrium to the right in order to consume the added reactant, thus producing more products, which includes hydrogen gas.

Analyzing the effect of adding water (H2O)

Introduction of more water vapor, which is a reactant, to the system will shift the equilibrium to the right, following Le Chatelier's principle. This shift will result in the production of more hydrogen gas.

Analyzing the effect of raising the temperature

Since the reaction is endothermic, increasing the temperature will add heat to the system, favoring the endothermic reaction to the right. Based on Le Chatelier's principle, raising the temperature will thus cause more hydrogen gas to be formed.

Analyzing the effect of increasing the volume

Increasing the volume of the reaction mixture lowers the pressure. For gas reactions, the equilibrium will shift toward the side with more moles of gas. In this reaction, both sides have the same number of moles of gas, so there should be no effect on the quantity of hydrogen gas produced.

Analyzing the effect of adding a catalyst

The addition of a catalyst will not impact the position of the equilibrium but rather the rate at which equilibrium is achieved. Consequently, a catalyst has no effect on the quantity of hydrogen gas.

Analyzing the effect of adding an inert gas

Adding an inert gas at constant volume increases the total pressure without changing the partial pressures of the reacting gases. Since the partial pressures are unchanged, the equilibrium position remains unaffected and there is no effect on the quantity of hydrogen gas produced.

Key concepts

Chemical Equilibrium

Understanding chemical equilibrium is essential when studying how reactions behave under various conditions. It refers to a state in a reversible reaction where the rate of the forward reaction equals the rate of the backward reaction, resulting in no net change in the concentration of reactants and products over time. It's important to note that this doesn't mean the reactants and products are in equal concentrations, but rather that they have reached a stable ratio.

When a reaction mixture is at equilibrium and a change is introduced, the system responds to counteract the change, a principle known as Le Chatelier's Principle. These changes can be in the form of concentration adjustments, temperature fluctuations, or volume and pressure variations. The system's response is to shift the equilibrium to the left or right in an effort to restore balance, which is a concept known as an 'equilibrium shift'.

For example, if additional carbon is added to our reaction mixture, the system will try to consume the excess carbon by shifting the equilibrium to the right, enhancing the production of hydrogen gas. This dynamic balance is crucial for the proper understanding of complex chemical processes.

Endothermic Reactions

Endothermic reactions are chemical reactions that absorb energy from their surroundings, typically in the form of heat. This means that they require energy input to proceed. In the context of our example, the production of hydrogen gas from coal and water vapor absorbs heat, indicating that the reaction is endothermic.

Increasing the temperature adds energy to the system, which favors the endothermic process. According to Le Chatelier's Principle, when the temperature rises, the equilibrium shifts in the direction that absorbs heat—in this case, to the right—resulting in the production of more hydrogen gas.

It's crucial for students to understand that temperature changes can significantly affect the yield of products in endothermic reactions. In practice, this knowledge can be applied to industrial chemical production, where temperature management can optimize the output of desired products.

Equilibrium Shift

An equilibrium shift occurs when a reaction at equilibrium experiences a change in conditions that upsets the balance of reactants and products. Le Chatelier's Principle provides a predictive framework to determine how the reaction adjusts in response to these changes.

As illustrated in the exercise, several changes can lead to an equilibrium shift:

• Adding more of a reactant or removing a product shifts the equilibrium toward the products, increasing their formation, as seen when adding carbon (C) or water (H₂O).
• Raising the temperature in an endothermic reaction also shifts the equilibrium toward products, as the system absorbs additional heat.
• If the reaction involves gases, changing the volume or pressure can shift the equilibrium. However, if the number of moles of gas is the same on both sides, as in our example, changes in volume will not affect the equilibrium.
• While introducing a catalyst or inert gas affects reaction rates or system pressure, it does not impact the equilibrium position of a reaction.

Understanding these shifts helps to control reaction conditions and achieve desired concentrations of products in various chemical processes.