Problem 23 - Chemistry in 30 days

Question

\(\mathrm{BiCl}_{3}\) (aq) \(+\mathrm{H}_{2} \mathrm{O}\) (1) \(\rightarrow \mathrm{BiOCl}(\mathrm{s})+2 \mathrm{HCl}\) (aq). If a little conc. \(\mathrm{HCl}\) is added (a) the solution becomes cloudier (b) the solution becomes clearer (c) nothing happens (d) data insufficient to conclude

Step-by-Step Solution

Answer

(b) the solution becomes clearer

Step 1: Understand the Reaction

The given chemical reaction involves the conversion of bismuth chloride (\(\mathrm{BiCl}_{3}\)) and water into bismuth oxychloride (\(\mathrm{BiOCl}\)) and hydrochloric acid (\(\mathrm{HCl}\)). Initially, the reaction leads to the formation of a solid (\(\mathrm{BiOCl}\)), causing the solution to become cloudy.

Step 2: Le Chatelier’s Principle

Le Chatelier’s Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. Here, adding \(\mathrm{HCl}\) to the reaction will increase the concentration of \(\mathrm{HCl}\) on the product side.

Step 3: Predict the Effect of Adding \(\mathrm{HCl}\)

When the concentration of \(\mathrm{HCl}\) is increased, the equilibrium will shift to the left to reduce the concentration of \(\mathrm{HCl}\). This shift will cause the formed solid \(\mathrm{BiOCl}\) to dissolve back into solution, making the solution clearer.

Key Concepts

Le Chatelier's Principle

In chemistry, Le Chatelier's Principle is an essential concept to understand how equilibria react to changes. This principle states that when a system at equilibrium is subjected to a change in concentration, pressure, or temperature, the system will adjust itself to partially counteract the effect of the change and restore a new equilibrium. Essentially, it tries to "balance out" shifts to maintain stability.

For instance, if you add more of a reactant or product to a system, the equilibrium will shift away from the added substance to reduce its effect. Conversely, removing a substance will cause the equilibrium to shift toward the side where that substance was removed. This is like trying to squash a balloon on one side – it will bulge out somewhere else. Understanding this principle can help predict the behavior of chemical reactions under stress.

Bismuth Chloride Reaction

The reaction involving bismuth chloride (\(\mathrm{BiCl}_3\)) with water is an intriguing chemical process. When \(\mathrm{BiCl}_3\) dissolves in water, it reacts to form bismuth oxychloride (\(\mathrm{BiOCl}\)) as a solid and hydrochloric acid (\(\mathrm{HCl}\)). This transformation can be represented as follows:
\[\mathrm{BiCl}_{3} (aq) + \mathrm{H}_2 \mathrm{O} (l) \rightarrow \mathrm{BiOCl}(s) + 2 \mathrm{HCl}(aq)\]
In this reaction, bismuth oxychloride precipitates out as a white solid, which is why the solution initially turns cloudy. This cloudiness indicates that a solid product has formed. The presence of \(\mathrm{HCl}\) as a byproduct will influence the equilibrium, and any changes in the concentration of \(\mathrm{HCl}\) will have consequences for the appearance of the solution.

Dynamic Equilibrium

Dynamic equilibrium is a state in a chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction. Although reactions are still occurring, there is no net change in the concentration of reactants and products. This is like a busy street with as many cars leaving as there are arriving – the traffic pattern remains constant over time.

In the context of the bismuth chloride reaction, when equilibrium is established, the amounts of \(\mathrm{BiCl}_3\), \(\mathrm{H}_2\mathrm{O}\), \(\mathrm{BiOCl}\), and \(\mathrm{HCl}\) remain constant. However, if the system is disturbed by adding substances or changing conditions, the dynamic equilibrium will shift according to Le Chatelier's Principle to maintain balance.

Effect of Concentration Changes on Equilibrium

Changing the concentration of substances in a system at equilibrium can have significant effects. According to Le Chatelier's Principle, such a change will cause the equilibrium to shift to counteract this change.

In the example of the bismuth chloride reaction, adding \(\mathrm{HCl}\) increases its concentration in the solution. As a response, the equilibrium will shift to the left, favoring the reverse reaction to consume some of the excess \(\mathrm{HCl}\). This causes the solid \(\mathrm{BiOCl}\) to dissolve, turning the solution clearer. Thus, understanding the impact of changing concentrations can help you predict how a chemical system will adapt, providing insights into controlling and optimizing reactions.