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. MissED THis? Read Section 16.9 ; Watch KCV 16.9 a. \(\mathrm{I}_{2}(g) \rightleftharpoons 2 \mathrm{l}(\mathrm{s})\) (volume is increased) b. \(2 \mathrm{H}_{2} \mathrm{~S}(\mathrm{~g})=2 \mathrm{H}_{2}(\mathrm{~s})+\mathrm{S}_{2}(\mathrm{~g})\) (volume is decreased) c. \(\mathrm{I}_{2}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{ICl}(\mathrm{g})\) (volume is decreased)

Short answer

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

Step-by-step solution

Le Chatelier's Principle

Understand that according to Le Chatelier's Principle, when a change is made to a system at equilibrium, the equilibrium will shift to oppose that change.

Analyzing the Effect of Volume Increase on Reaction A

For the reaction \(\mathrm{I}_{2}(g) \rightleftharpoons 2 \mathrm{I}(\mathrm{s})\), increasing the volume decreases the pressure. The equilibrium will shift towards the side with more moles of gas. However, since the product side has no gas, the reaction does not shift.

Analyzing the Effect of Volume Decrease on Reaction B

For the reaction \(2 \mathrm{H}_{2} \mathrm{S}(\mathrm{g})=2\mathrm{H}_{2}(\mathrm{s})+\mathrm{S}_{2}(\mathrm{g})\), decreasing the volume increases the pressure. The equilibrium will shift towards the side with fewer moles of gas, which is the left side. Therefore, the reaction shifts to the left.

Analyzing the Effect of Volume Decrease on Reaction C

For the reaction \(\mathrm{I}_{2}(\mathrm{g})+\mathrm{Cl}_{2}(\mathrm{g}) \rightleftharpoons 2 \mathrm{ICl}(\mathrm{g})\), decreasing the volume increases the pressure. The equilibrium will shift towards the side with the fewer moles of gas to reduce the pressure. Both sides have an equal amount of moles of gases, so the shift is not affected by the change in volume.

Key concepts

Equilibrium Shift

Understanding how equilibrium shifts when changes are made to a system is a foundational concept in chemistry. Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium will move to counteract the change. This principle helps predict the response of a system to stress, such as changes in concentration, pressure, volume, or temperature.

For example, consider a reaction where gases are involved. If the volume of the container is increased, the pressure decreases. According to Le Chatelier's Principle, the system will shift its equilibrium position to increase pressure, often by favoring the side of the reaction that produces more gas molecules. Conversely, if the volume is decreased, the pressure increases, and the system will shift to decrease pressure, favoring the side with fewer gas molecules.

Volume and Pressure Relationship

The relationship between volume and pressure in a gas system is described by Boyle's Law, which states that the pressure of a given mass of gas is inversely proportional to its volume when the temperature remains constant. In the context of chemical reactions, this relationship becomes particularly important.

When the volume of a gas is reduced, the pressure inside the container increases because the same number of gas particles now occupies a smaller space. This increase in pressure can affect the position of equilibrium in a reversible reaction, as the system seeks to minimize this pressure increase according to Le Chatelier's Principle.

Effects of Volume Change on Equilibrium

Changing the volume of the container in which a reversible reaction is taking place has direct consequences on the system's equilibrium. If the system contains gases, changing the volume can effectively change the concentration of the gaseous reactants and products, which in turn alters the pressure.

For reactions involving solids and liquids, volume changes do not affect the equilibrium position as much because their densities are not significantly altered by changes in pressure. However, when gases are part of the reaction, a decrease in volume (and subsequent increase in pressure) will typically shift the equilibrium towards the side with fewer moles of gas, as demonstrated in the provided exercise with the reaction involving hydrogen sulfide gas.

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions in a closed system become equal, resulting in no net change in the concentration of reactants and products over time. However, this does not mean that the reactants and products are present in equal amounts, but rather that they are at a specific ratio that remains constant at equilibrium.

It is important to recognize that equilibrium is dynamic; reactions continue to occur in both directions, but because their rates are equal, the macroscopic properties of the system, such as concentration, remain stable. Understanding chemical equilibrium is essential in predicting how a system will respond to external changes and in designing processes and reactions in the field of chemistry.