Question

Each reaction is allowed to come to equilibrium, and then the volume is changed as indicated. Predict the effect (shift right, shift left, or no effect) of the indicated volume change. \begin{equation}\begin{array}{l}{\text { a. } \mathrm{CO}(g)+\mathrm{H}_{2} \mathrm{O}(g) \rightleftharpoons \mathrm{CO}_{2}(g)+\mathrm{H}_{2}(g) \text { (volume is decreased) }} \\ {\text { b. } \mathrm{PCl}_{3}(g)+\mathrm{Cl}_{2}(g) \rightleftharpoons \mathrm{PCl}_{5}(g)(\text { volume is increased) }} \\ {\text { c. } \mathrm{CaCO}_{3}(s) \rightleftharpoons \mathrm{CaO}(s)+\mathrm{CO}_{2}(g)(\text { volume is increased) }}\end{array}\end{equation}

Short answer

a) No effect, b) Shift right, c) Shift left

Step-by-step solution

Consider Le Châtelier’s Principle

Le Châtelier’s Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. Increasing the pressure by decreasing the volume will shift the equilibrium to the side with fewer gas molecules. Conversely, decreasing the pressure by increasing the volume will shift the equilibrium to the side with more gas molecules.

Analyze Reaction (a)

For the reaction CO(g) + H\(_2\)O(g) \(\rightleftharpoons\) CO\(_2\)(g) + H\(_2\)(g), there are the same number of gas molecules on both sides of the equation (2 molecules). Therefore, decreasing the volume will have no effect on the position of the equilibrium of this reaction.

Analyze Reaction (b)

For the reaction PCl\(_3\)(g) + Cl\(_2\)(g) \(\rightleftharpoons\) PCl\(_5\)(g), there are more gas molecules on the left side (2 molecules) than on the right side (1 molecule). Increasing the volume would shift the equilibrium to the left side to increase the number of gas molecules, according to Le Châtelier’s Principle.

Analyze Reaction (c)

For the reaction CaCO\(_3\)(s) \(\rightleftharpoons\) CaO(s) + CO\(_2\)(g), solids do not affect the equilibrium position as their concentration does not change with volume. Therefore, increasing the volume would shift the equilibrium to the right to increase the number of gas molecules, which in this case is CO\(_2\)(g).

Key concepts

Chemical Equilibrium

Chemical equilibrium is a state in a chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain stable over time, although the actual exchange of substances continues. This concept is crucial in understanding how reactions behave when they reach a balance between the two opposing processes.

Imagine a see-saw perfectly balanced with equal weights on both sides—this represents the equilibrium state of a chemical reaction. Now, if you add or remove weight from one side, the see-saw will tilt to one side until a new balance is achieved. This addition or removal of weight is analogous to changes in reaction conditions, such as concentration, temperature, or volume, which can disrupt the established equilibrium of a chemical system.

Reaction Volume Change

Reaction volume change refers to the alteration of the space available for a reaction to occur. It can have a significant impact on the gas phase reactions especially, due to the compressible nature of gases. Reducing the reaction volume increases the pressure, thereby essentially pushing the gas molecules closer together.

When the volume is increased in a reaction involving gases, the pressure decreases and the gas molecules spread further apart. It is analogous to blowing up a balloon (increasing volume) and then letting the air out (decreasing volume). These volume changes can be used strategically to steer a reaction towards the desired products using Le Châtelier's principle.

Equilibrium Shift

An equilibrium shift occurs when a dynamic equilibrium is disturbed, causing the system to favor either the forward or reverse reaction in order to re-establish equilibrium. Depending on the disturbance, the equilibrium can shift to the right (towards more products) or to the left (towards more reactants).

The position of equilibrium is sensitive to changes in pressure, concentration, and temperature, each of which can alter the balance. For example, an increase in the concentration of a reactant typically shifts the equilibrium to the right, producing more products, while an increase in temperature can shift equilibrium depending on whether the reaction is exothermic or endothermic.

• If a reaction releases heat (exothermic), increasing the temperature tends to shift the equilibrium to the left, favoring the reactants.
• For a reaction that absorbs heat (endothermic), raising the temperature shifts the equilibrium to the right, favoring the products.

Understanding these dynamic changes helps chemists control the outcome of reactions to maximize product yield or to optimize reaction conditions.

Gas Molecule Behavior

Gas molecule behavior is influenced by the Kinetic Molecular Theory, which describes how gas particles move and interact. Gas molecules are in constant, rapid motion and collide with each other and the walls of their container. These collisions are what account for the pressure exerted by gases.

When the volume of a gas's container is decreased, these molecules have less space to move around, so collisions become more frequent, resulting in an increase in pressure. Conversely, increasing the volume gives molecules more space, collisions are less frequent, and the pressure drops. Gas behaviors like these are absolutely pivotal when considering reactions under various conditions and can drastically affect chemical equilibria in a closed system.