Problem 15 - Chemistry: Principles and Reactions

Question

At \(25^{\circ} \mathrm{C}, K=2.2 \times 10^{-3}\) for the reaction $$\mathrm{ICl}(g) \rightleftharpoons \frac{1}{2} \mathrm{I}_{2}(g)+\frac{1}{2} \mathrm{Cl}_{2}(g)$$ Calculate \(K\) at \(25^{\circ} \mathrm{C}\) for (a) the decomposition of ICl into one mole of iodine and chlorine. (b) the formation of two moles of \(\operatorname{ICl}(g)\).

Step-by-Step Solution

Answer

Question: Calculate the equilibrium constants at 25°C for (a) the decomposition of ICl into one mole of iodine and chlorine, and (b) the formation of two moles of ICl, given that the original reaction has an equilibrium constant of \(K = 2.2 \times 10^{-3}\). Answer: The equilibrium constants at 25°C, for (a) the decomposition of ICl into one mole of iodine and chlorine is \(2.2 \times 10^{-3}\), and for (b) the formation of two moles of ICl(g) is \(4.84 \times 10^{-6}\).

Step 1: Write the balanced equations for the given reactions

We need to write the balanced equations for the two reactions described in the problem. (a) Decomposition of ICl into one mole of iodine and chlorine: $$1\,\text{ICl} \rightleftharpoons \frac{1}{2}\,\text{I}_{2} + \frac{1}{2}\,\text{Cl}_{2}$$ (b) Formation of two moles of ICl(g): $$2\,\text{ICl} \rightleftharpoons \text{I}_{2} + \text{Cl}_{2}$$

Step 2: Determine the relationship between the equilibrium constants of the given reaction and the new reactions

If we multiply the stoichiometric coefficients of the original reaction by a factor 'n', the equilibrium constant of the new reaction (\(K_{new}\)) would be raised to the power of 'n': \(K_{new}=K^{n}\) (a) In the first reaction, we simply have the same stoichiometric coefficients as those in the original reaction, so we don't need to modify the equilibrium constant: \(K_{a}=K\) (b) The stoichiometric coefficients of the second reaction are twice those in the original reaction, so the new equilibrium constant would be the square of the original equilibrium constant: \(K_{b}=K^{2}\)

Step 3: Calculate the equilibrium constants for the new reactions

Now, we can calculate the new equilibrium constants using the relationship between the equilibrium constants and the stoichiometric coefficients of the reactions. (a) Decomposition of ICl into one mole of iodine and chlorine: $$K_{a} = K = 2.2 \times 10^{-3}$$ (b) Formation of two moles of ICl(g): $$K_{b} = K^2 = (2.2 \times 10^{-3})^2 = 4.84 \times 10^{-6}$$ So, the equilibrium constant, K, at 25°C for (a) the decomposition of ICl into one mole of iodine and chlorine is \(2.2 \times 10^{-3}\), and for (b) the formation of two moles of ICl(g) is \(4.84 \times 10^{-6}\).

Key Concepts

Chemical Equilibrium

At its core, chemical equilibrium is the state in which the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products. It's a dynamic balance, not static; molecules are constantly reacting, but in such a way that the overall amounts of reactants and products stay constant.

Take, for example, the reaction of iodine chloride (ICl) decomposing into iodine (I₂) and chlorine (Cl₂). At equilibrium, the rate at which ICl breaks down into I₂ and Cl₂ equals the rate at which I₂ and Cl₂ combine to form ICl. This doesn't mean all chemical activities cease; rather, there's a constant shuffle as molecules transition between states, but the observed concentrations remain unchanged.

Understanding this concept is foundational when studying chemical reactions, as many systems in biology, environmental science, and chemistry itself are governed by equilibrium dynamics.

Reaction Quotient

Moving to the reaction quotient (Q), this is a number that changes as a reaction progresses until it reaches chemical equilibrium, at which point Q becomes equal to the Equilibrium constant (K). The reaction quotient is calculated using the same formula as K but with the initial concentrations of the reactants and products, rather than their equilibrium concentrations.

For the given reaction \(\text{ICl}(g) \rightleftharpoons \frac{1}{2} \text{I}_{2}(g) + \frac{1}{2}\text{Cl}_{2}(g)\), if we measure the concentrations of ICl, I₂, and Cl₂ before equilibrium is established, we can calculate the reaction quotient (Q). In practice, comparing Q to K can predict the direction in which the reaction will proceed to reach equilibrium. If \(\text{Q} > \text{K}\), the reaction will proceed in the reverse direction, reducing the concentrations of products. Conversely, if \(\text{Q} < \text{K}\), the reaction will proceed forward, increasing the concentrations of products.

The reaction quotient plays a pivotal role in determining the shifts in reaction conditions—such as changes in concentration, pressure, or temperature—and predicting how the reaction will adapt to restore equilibrium.

Equilibrium Constant Calculation

The equilibrium constant calculation is crucial in quantitatively describing a chemical equilibrium. It allows us to understand the extent to which a reaction proceeds and provides insights into the stability of the products and reactants at equilibrium.

To calculate the equilibrium constant (\(\text{K}\)), we use the concentrations of the products raised to the power of their stoichiometric coefficients, divided by the concentrations of the reactants raised to the power of their stoichiometric coefficients. The provided exercise showcases how changing the coefficients—like when forming two moles of ICl from I₂ and Cl₂—affects the equilibrium constant. For such a case, if the stoichiometry is doubled, the equilibrium constant for the new reaction (\(\text{K}_\text{b}\)) is the original constant squared (\(\text{K}^2\)).

Remember that while K itself is a pure number and doesn't have units, the process to find it involves concentration units. Therefore, precision in measuring concentrations and respecting stoichiometry in calculations is imperative. The mathematical relationship between K and the coefficients highlights the sensitivity of equilibrium to changes in reaction conditions, a cornerstone concept for students' understanding of reaction dynamics.