Problem 64 - Chemistry and Chemical Reactivity

Question

The photographs below show what occurs when a solution of potassium chromate is treated with a few drops of concentrated hydrochloric acid. Some of the bright yellow chromate ion is converted to the orange dichromate ion. \(2 \mathrm{CrO}_{4}^{2-}(\mathrm{aq})+2 \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq}) \rightleftharpoons \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\ell)\) (a) Explain this experimental observation in terms of Le Chatelier's principle. (b) What would you observe if you treated the orange solution with sodium hydroxide? Explain your observation.

Step-by-Step Solution

Answer

(a) Adding acid shifts the equilibrium to form orange dichromate. (b) Adding sodium hydroxide shifts it back to yellow chromate.

Step 1: Understanding the Reaction

The given reaction shows the conversion of yellow chromate ions \( \mathrm{CrO}_{4}^{2-} \) to orange dichromate ions \( \mathrm{Cr}_{2}\mathrm{O}_{7}^{2-} \) when treated with acid. This is a reversible reaction, and the equilibrium can shift according to Le Chatelier's principle.

Step 2: Applying Le Chatelier's Principle

According to Le Chatelier's principle, if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. Adding hydrochloric acid increases the concentration of \( \mathrm{H}_{3}\mathrm{O}^{+} \) ions, shifting the equilibrium to the right, converting chromate to dichromate which changes the solution color from yellow to orange.

Step 3: Effects of Sodium Hydroxide Addition

When sodium hydroxide is added, it neutralizes the \( \mathrm{H}_{3}\mathrm{O}^{+} \) ions by forming water. This reduces the concentration of \( \mathrm{H}_{3}\mathrm{O}^{+} \), causing the equilibrium to shift to the left as per Le Chatelier's principle, converting orange dichromate back to yellow chromate.

Step 4: Observing the Color Change

After adding sodium hydroxide, you would observe the solution's color changes from orange back to yellow, indicating the conversion of \( \mathrm{Cr}_{2}\mathrm{O}_{7}^{2-} \) to \( \mathrm{CrO}_{4}^{2-} \).

Key Concepts

Chemical Equilibrium

Chemical equilibrium is a state in which the forward and reverse reactions occur at the same rate, resulting in the concentrations of products and reactants remaining constant over time.
When a reaction reaches this state, it does not mean the reactions have stopped; they continue to occur.
Equilibrium simply implies that there is no net change in the concentrations of substances involved.

The concept of equilibrium is critical in understanding how changes in conditions can affect the outcome of chemical reactions.

• For equilibrium to be reached, the reaction must be reversible.
• The condition of equilibrium is dynamic, not static.

In chemical reactions such as the conversion of chromate to dichromate, equilibrium can be expressed by a balanced chemical equation.
In this case, the balance is influenced significantly by the addition of hydrogen ions, shifting the equilibrium position.

Chromate-Dichromate Equilibrium

The chromate-dichromate equilibrium involves an interconversion between chromate ions (\( \mathrm{CrO}_{4}^{2-} \)) and dichromate ions (\( \mathrm{Cr}_{2}\mathrm{O}_{7}^{2-} \)).
This equilibrium is sensitive to changes in the pH of the solution.
When acid is added to a chromate solution, the concentration of hydronium ions (\( \mathrm{H}_{3}\mathrm{O}^{+} \)) increases, causing the equilibrium to shift towards the formation of dichromate ions.

This shift corresponds to a noticeable change in color from yellow to orange due to the presence of dichromate ions.
Conversely, when a base such as sodium hydroxide is added, it decreases the concentration of hydronium ions, favoring the reverse reaction.

• Adding acid shifts the equilibrium towards dichromate formation.
• Adding base, like sodium hydroxide, shifts the equilibrium back towards chromate ions.

This principle helps explain the natural balance in various systems and shows how equilibrium is crucial for controlling reaction processes.

Acid-Base Reaction

Acid-base reactions play a pivotal role in shifting equilibrium states, such as in the chromate-dichromate system.
When an acid like hydrochloric acid is introduced, additional hydronium ions are produced in the solution.
This addition results in a forward shift as described by Le Chatelier's Principle.

An acid-base reaction in this context can be understood as the competition between acids and bases to donate or accept protons.

• Acids increase the hydronium ion concentration in a solution.
• Bases, like sodium hydroxide, remove hydronium ions by forming water.

When sodium hydroxide is added to the original orange dichromate solution, it acts as a base, reducing the hydronium ion concentration.
This lessens the forward reaction pressure, causing the equilibrium to reverse. This reaction exemplifies an interplay of acid and base influences on chemical equilibria.

Color Change in Reactions

Color change in chemical reactions is often an observable indicator of an equilibrium shift.
In the chromate-dichromate equilibrium, the visible switch from yellow to orange and vice versa demonstrates a chemical transformation.
This color change is primarily due to different light absorption properties of chromate and dichromate ions.

Each ion absorbs varying wavelengths of light, resulting in different perceived colors.

• Yellow is associated with a higher concentration of chromate ions.
• Orange indicates the presence of dichromate ions.

The ability to see such changes provides a practical way of monitoring shifts in equilibrium during reactions.
For a learner, understanding the cause behind these color changes can reinforce the dynamic nature of chemical equilibria and their sensitivity to environmental changes.