Question

Consider the following system at equilibrium: $$ 4 \mathrm{HCl}(g)+\mathrm{O}_{2}(g) \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}(g)+2 \mathrm{Cl}_{2}(g)+114.4 \mathrm{~kJ} $$ (a) Is the reaction endothermic or exothermic? (b) If \(\mathrm{O}_{2}\) is added, in which direction will the reaction shift in order to reestablish equilibrium? After the new equilibrium has been established, will the final molar concentrations of \(\mathrm{HCl}, \mathrm{O}_{2}, \mathrm{H}_{2} \mathrm{O}\), and \(\mathrm{Cl}_{2}\) increase or decrease? (c) If heat is added, in which direction will the equilibrium shift?

Short answer

(a) Exothermic. (b) Shift right, \(HCl\) and \(O_2\) decrease, \(H_2O\) and \(Cl_2\) increase. (c) Shift left.

Step-by-step solution

Understanding Exothermic and Endothermic Reactions

Determine if the reaction is endothermic or exothermic by checking the energy term. Since 114.4 kJ is produced on the product side, it means the reaction releases energy. Therefore, the reaction is exothermic.

Effect of Adding \(O_2\)

According to Le Chatelier's Principle, if \(O_2\) is added, the system will shift to counteract the change. Adding \(O_2\) will shift the reaction to the right to consume excess \(O_2\). As a result, the production of \(H_2O\) and \(Cl_2\) will increase, and the consumption of \(HCl\) and \(O_2\) will also increase.

Final Concentrations after Adding \(O_2\)

After reestablishing equilibrium: \(HCl\) will decrease because it is consumed to produce more \(H_2O\) and \(Cl_2\). \(O_2\) will decrease slightly as it is consumed but may remain higher than the initial amount before addition. \(H_2O\) and \(Cl_2\) concentrations will increase because the reaction shifts to the right.

Effect of Adding Heat

For an exothermic reaction, adding heat is like adding a product. According to Le Chatelier's Principle, the system will shift to the left to consume excess heat. Therefore, the concentrations of \(HCl\) and \(O_2\) will increase, and the concentrations of \(H_2O\) and \(Cl_2\) will decrease.

Key concepts

Equilibrium Shifts

In chemical reactions, equilibrium is achieved when the rates of the forward and reverse reactions are equal, leading to constant concentrations of the reactants and products. However, when an external change is applied to a system at equilibrium, Le Chatelier's Principle dictates that the system will adjust to counteract this change and restore a new equilibrium.

For example, if the concentration of a reactant or product is changed, the equilibrium will shift to either the left or the right to balance the disturbance. In the given reaction:
\(4 \mathrm{HCl}(g)+ \mathrm{O}_{2}(g) \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}(g)+2 \mathrm{Cl}_{2}(g)+114.4 \mathrm{~kJ} \),
adding more \(\mathrm{O_2}\) will shift the reaction to the right to use up the extra \(\mathrm{O_2}\). This means more \(\mathrm{H_2O}\) and \(\mathrm{Cl_2}\) will be produced, increasing their concentrations, while the concentrations of \(\mathrm{HCl}\) and \(\mathrm{O_2}\) will decrease as \(\mathrm{O_2}\) is consumed.

Conversely, if heat is added to the system, the reaction will shift in the direction that absorbs heat. For an exothermic reaction like this one, adding heat shifts the equilibrium to the left, thus increasing the concentrations of \(\mathrm{HCl}\) and \(\mathrm{O_2}\) while decreasing \(\mathrm{H_2O}\) and \(\mathrm{Cl_2}\).

Endothermic and Exothermic Reactions

Endothermic and exothermic reactions involve the absorption and release of energy, respectively. An endothermic reaction absorbs energy from its surroundings, indicated by a positive energy change. In contrast, an exothermic reaction releases energy, shown by a negative energy change or energy released on the product side.

The given reaction is exothermic, as evidenced by the release of 114.4 kJ on the product side. This means the system gives off heat when the reaction proceeds forward, contributing to an increase in the temperature of the surroundings.

Understanding whether a reaction is endothermic or exothermic is crucial for predicting how temperature changes will affect equilibrium. For instance, adding heat to an exothermic reaction at equilibrium will push the reaction to favor the reactants, as the system attempts to absorb the extra heat by shifting left. In our exercise, adding heat led to the production of more \(\mathrm{HCl}\) and \(\mathrm{O_2}\).

Molar Concentrations

Molar concentration refers to the amount of a substance (in moles) present in a given volume of solution (liters). It is denoted in units of mol/L and is an important factor in chemical equilibria and reactions.

For the given reaction, changes in molar concentrations directly affect the equilibrium position. When extra \(\mathrm{O_2}\) is added, it initially increases the molar concentration of \(\mathrm{O_2}\). According to Le Chatelier's Principle, the system counteracts by shifting the equilibrium to the right, reducing the \(\mathrm{O_2}\) concentration and consequently increasing the molar concentrations of \(\mathrm{H_2O}\) and \(\mathrm{Cl_2}\). The \(\mathrm{HCl}\) concentration decreases in this process as it is consumed.

Molar concentrations are essential for quantifying how components react and change within a chemical system. After equilibrium is reestablished in our exercise, the final molar concentrations will be different from the initial values, reflecting the system's response to the disturbance.