Question

Predict the effect of decreasing the container volume on the amounts of each reactant and product in the following reactions: (a) \(\mathrm{H}_{2}(g)+\mathrm{Cl}_{2}(g) \rightleftharpoons 2 \mathrm{HCl}(g)\) (b) \(2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}(l)\)

Short answer

For reaction (a), \(\text{H}_{2}\) and \(\text{Cl}_{2}\) increase, \(\text{HCl}\) decreases. For reaction (b), \(\text{H}_{2}\) and \(\text{O}_{2}\) decrease, \(\text{H}_{2}\text{O}\) increases.

Step-by-step solution

Understand Le Chatelier's Principle

Le Chatelier's Principle states that if an external change is applied to a system at equilibrium, the system adjusts itself to minimize that change.

Reaction (a): Analyzing Changes

For the reaction \(\text{H}_{2}(g)+\text{Cl}_{2}(g) \rightleftharpoons 2 \text{HCl}(g)\), decreasing the container volume increases the pressure. Since there are more moles of gas on the product side (2 moles) compared to the reactant side (1 mole \text{H}_{2} + 1 mole \text{Cl}_{2}), the system will shift towards the reactants to counteract the pressure increase.

Prediction for Reaction (a)

The amount of \(\text{H}_{2}\) and \(\text{Cl}_{2}\) will increase, while the amount of \(\text{HCl}\) will decrease when the container volume is decreased.

Reaction (b): Analyzing Changes

For the reaction \(\text{2 H}_{2}(g)+\text{O}_{2}(g) \rightleftharpoons \text{2 H}_{2}\text{O}(l)\), decreasing the container volume again increases the pressure. Since there are 3 moles of gas on the reactant side and none on the product side (liquid does not significantly affect pressure), the system will shift towards the products to reduce the pressure.

Prediction for Reaction (b)

The amount of \(\text{H}_{2}\) and \(\text{O}_{2}\) will decrease, while the amount of \(\text{H}_{2}\text{O}\) will increase when the container volume is decreased.

Key concepts

equilibrium shift

When we discuss an equilibrium shift, we're referring to how a chemical reaction adjusts to return to equilibrium after a disturbance. Le Chatelier's Principle guides this shift. It states that a system at equilibrium will adjust itself to minimize the impact of changes. In exercise (a), changing the container volume affects the system. When volume decreases, pressure increases. This information tells us that the system will shift towards the side with fewer gas molecules to counteract this increase. For reaction (a), \(\text{H}_{2}(g) + \text{Cl}_{2}(g) \rightleftharpoons 2 \text{HCl}(g)\), there are 1 mole each of \text{H}_{2} and \text{Cl}_{2} (total 2 moles) on the reactant side but 2 moles of \text{HCl} on the product side. Hence, decreasing volume causes the equilibrium to shift towards the reactants, increasing \text{H}_{2} and \text{Cl}_{2} and decreasing \text{HCl}. The shift maintains equilibrium and minimizes the pressure change.

pressure changes

Pressure changes are a significant factor in reactions involving gases. According to Le Chatelier's Principle, if a reaction at equilibrium experiences an increase in pressure, it will shift to reduce this pressure. Volume and pressure are inversely related: decreasing volume increases pressure and vice versa. In reaction (b), \(\text{2 H}_{2}(g) + \text{O}_{2}(g) \rightleftharpoons 2 \text{H}_{2}\text{O}(l)\), the reactant side has 3 moles of gas (2 \text{H}_{2} and 1 \text{O}_{2}), while the product side forms a liquid. Liquids hardly impact the pressure. In this scenario, when the container volume is reduced, pressure increases significantly since gases compress more than liquids. Therefore, the reaction shifts towards producing more liquid water (\text{H}_{2}\text{O}), decreasing the amounts of gaseous \text{H}_{2} and \text{O}_{2}. This adjustment helps to buffer the pressure change, stabilizing the system at a new equilibrium state.

reaction prediction

Predicting how a reaction will respond to changes in conditions requires understanding Le Chatelier's Principle. By knowing how equilibrium shifts and how changes in pressure impact the system, we can predict changes. Take the exercise's first reaction: \(\text{H}_{2}(g) + \text{Cl}_{2}(g) \rightleftharpoons 2 \text{HCl}(g)\). If the container's volume decreases, equilibrium shifts to reduce increased pressure. Therefore, reactants (\text{H}_{2} and \text{Cl}_{2}) will increase, and the product (\text{HCl}) will decrease. For reaction (b), \(\text{2 H}_{2}(g) + \text{O}_{2}(g) \rightleftharpoons 2 \text{H}_{2}\text{O}(l)\), reducing volume (which increases pressure) causes the reaction to shift towards the products to lower pressure. As a result, \text{H}_{2} and \text{O}_{2} (gases) will decrease, and \text{H}_{2}\text{O} (liquid) will increase. This prediction approach is invaluable in anticipating how chemical systems behave under different conditions.