Question

Consider the following reactions. \(\mathrm{H}_{2}(g)+\mathrm{I}_{2}(g) \longrightarrow 2 \mathrm{HI}(g)\) and \(\mathrm{H}_{2}(g)+\mathrm{I}_{2}(s) \longrightarrow 2 \mathrm{HI}(g)\) List two property differences between these two reactions that relate to equilibrium.

Short answer

In summary, the property differences between these two reactions regarding equilibrium are: 1. The effect of pressure on the equilibrium position: - For the first reaction (with both reactants and products in the gas phase), pressure changes will affect the equilibrium position. For the second reaction (with solid iodine), pressure changes will have little to no impact. 2. Both reactions will be affected similarly by temperature changes, as both are governed by the same equilibrium constant expression and enthalpy change.

Step-by-step solution

Identify the differences between the reactions

In the first reaction, both reactants, hydrogen (H2) and iodine (I2), are in the gas phase. In the second reaction, hydrogen is in the gas phase, while iodine is in the solid phase. Apart from this difference, both reactions have the same product HI (g).

Consider the effect of pressure on equilibrium

For the first reaction, since all reactants and products are in the gas phase, changing the pressure of the system will influence the position of the equilibrium, according to Le Châtelier's principle. However, in the second reaction, since one of the reactants (iodine) is in solid form, changing the pressure of the system will have little to no impact on the solid and hence won't significantly influence the position of the equilibrium.

Consider the effect of temperature on equilibrium

For both reactions, changing the temperature will affect the position of the equilibrium. This is because both are governed by the same equilibrium constant expression, \(K_p = \frac{[HI]^2}{[H_2][I_2]}\), as well as the same enthalpy change for the reaction. So, if we summarize our findings: Property difference 1: The effect of pressure on the equilibrium position. - For the first reaction, pressure changes will affect the equilibrium position, while for the second reaction, pressure changes will have little to no impact. Property difference 2: Though not a difference, it's important to note that both reactions will be affected by temperature changes in a similar manner.

Key concepts

Le Châtelier's Principle

At the heart of predicting how a chemical reaction will respond to changes in conditions is Le Châtelier's principle. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, such as pressure, temperature, or concentration, the position of equilibrium will shift to counteract the change.

It’s a way of describing how a system adjusts to maintain balance. For instance, increasing the concentration of reactants will push the equilibrium towards producing more products, while increasing the concentration of products will shift the equilibrium towards the reactants. Understanding this principle is essential because it helps predict the direction of change in a chemical reaction under different conditions.

Phase of Reactants

In chemical reactions, the phase of reactants can significantly influence how the system achieves equilibrium. A reaction involving gases can behave quite differently from one involving solids or liquids. The phases can affect properties such as pressure and volume, which in turn can alter the equilibrium state based on Le Châtelier's principle.

For example, when all reactants are gases, as in our first reaction \(\mathrm{H}_{2}(g) + \mathrm{I}_{2}(g) \rightarrow 2 \mathrm{HI}(g)\), changes in pressure can influence the equilibrium position. In contrast, solid reactants, such as iodine in the second reaction \(\mathrm{H}_{2}(g) + \mathrm{I}_{2}(s) \rightarrow 2 \mathrm{HI}(g)\), are not affected by pressure changes. This is because solids and liquids have a much less compressible volume, meaning their concentrations don't change with pressure as gases do.

Equilibrium Constant Expression

The equilibrium constant expression for a reaction is a formula that helps us quantify the position of equilibrium. It is denoted by \(K\) and relates the concentrations of the products and reactants when the reaction is at equilibrium. For a general reaction \(aA(g) + bB(g) \leftrightarrow cC(g) + dD(g)\), the equilibrium constant expression is given as \(K = \frac{[C]^{c}[D]^{d}}{[A]^{a}[B]^{b}}\).

In our textbook example, the equilibrium constant expression for both reactions is \(K_p = \frac{[HI]^2}{[H_2][I_2]}\), regardless of the physical state of iodine. It's vital to understand that \(K\) reflects the balance point of the reaction and is dependent on temperature but not on pressure or the initial concentrations of reactants and products.

Effect of Pressure on Equilibrium

Pressure can be a crucial factor affecting equilibrium, especially for reactions involving gases. If the pressure on a system at equilibrium is increased, the system will adjust to lower the pressure. Normally, this means shifting towards the side with fewer moles of gas. Conversely, decreasing the pressure can shift the equilibrium in favor of producing more moles of gas.

In our first reaction, increasing the pressure would drive the system towards the formation of HI(g), since it reduces the total number of gas molecules from 2 to 1. However, for the second reaction, since I2 is a solid and doesn't contribute to the system's pressure, changes in pressure will have a negligible effect on the reaction's equilibrium position.

Temperature Impact on Equilibrium

Temperature changes are another major factor that can influence the position of equilibrium. According to Le Châtelier's principle, if a reaction is exothermic (releases heat), increasing the temperature will shift the equilibrium towards the reactants; conversely, if the reaction is endothermic (absorbs heat), an increase in temperature will shift the equilibrium towards the products.

This temperature dependency is rooted in the reaction's enthalpy change. Since both reactions mentioned in our exercise involve the same products and reactants, and by extension, the same enthalpy change, they will both be affected similarly by temperature changes. This means that the strategy for controlling the production of HI(g) via temperature adjustments would be identical for both reactions.