Problem 30 - General Chemistry: Principles and Modern Applications

Question

When yellow \(\mathrm{BaCrO}_{4}\) is dissolved in \(\mathrm{HCl}(\mathrm{aq}),\) a green solution is obtained. Write a chemical equation to account for the color change.

Step-by-Step Solution

Answer

The balanced chemical equation that accounts for the color change when BaCrO4 is dissolved in HCl is 2HCl(aq) + BaCrO4(aq) -> BaCl2(aq) + CrCl3(aq) + H2O

Step 1: Identify initial chemicals

Initially, there are two chemicals present, BaCrO4 (which is yellow) and HCl. The arrow in the chemical equation will indicate that a reaction is taking place.

Step 2: Determine the product(s)

The chromium in BaCrO4 gets reduced by HCl, changing its oxidation state and, as a result, its color. The resulting compound is CrCl3, which is green. The remaining compound is BaCl2.

Step 3: Balance the chemical equation

For the chemical equation to be consistent with the law of conservation of mass, we need to ensure that there are equal numbers of each type of atom on both the left and right sides of the equation. The balanced equation is:2HCl(aq) + BaCrO4(aq) -> BaCl2(aq) + CrCl3(aq) + H2O

Key Concepts

Oxidation States

Understanding oxidation states is crucial in following chemical reactions, especially those involving color changes due to electron transfer. Oxidation states, also known as oxidation numbers, are a way to keep track of electron distribution in molecules. They help to determine which atoms are oxidized and which are reduced.

In the context of the reaction between yellow \(\mathrm{BaCrO}_{4}\) and \(\mathrm{HCl}\), the oxidation state of chromium changes. Initially, chromium in \(\mathrm{BaCrO}_{4}\) is in a high oxidation state of +6. During the reaction, the chromium gets reduced, meaning it gains electrons, and its oxidation state decreases to +3 as it becomes part of \(\mathrm{CrCl}_{3}\).

Key points to remember about oxidation states in this context include:

• An increase in oxidation state indicates oxidation.
• A decrease in oxidation state indicates reduction.
• The compound's color change from yellow to green is a result of these oxidation-reduction processes.

Balancing Chemical Equations

Balancing chemical equations is essential for illustrating the conservation of mass in reactions. It ensures that the same number of each type of atom appears on both sides of a chemical equation. This balanced representation is achieved by adjusting coefficients before reactants and products.

For the reaction of \(\mathrm{BaCrO}_{4}\) and \(\mathrm{HCl}\) forming green \(\mathrm{CrCl}_{3}\), the equation is balanced as follows: 2 \(\mathrm{HCl(aq)}\) + \(\mathrm{BaCrO}_{4}(aq)\) \rightarrow \(\mathrm{BaCl}_{2}(aq)\) + \(\mathrm{CrCl}_{3}(aq)\) + \(\mathrm{H_{2}O}\).

This balanced equation shows:

• Two hydrogen atoms and two chlorine atoms react from \(\mathrm{HCl}\).
• One barium atom each in reactants and products to form \(\mathrm{BaCl}_{2}\).
• Keeping chromium and oxygen balanced through \(\mathrm{BaCrO}_{4}\) and resulting compounds.

Balancing helps visualize the full story of the reaction, ensuring that nothing is lost or created in isolation but only transformed.

Chromium Compounds

Chromium compounds are known for their vivid colors, which change depending on their oxidation states. This characteristic makes them useful as indicators in various chemical reactions and technological applications.

In the example of \(\mathrm{BaCrO}_{4}\) reacting with \(\mathrm{HCl}\), chromium is initially in the form of \(\mathrm{BaCrO}_{4}\), exhibiting a yellow color. After reduction, it turns into \(\mathrm{CrCl}_{3}\), displaying a green hue.

• Chromium's colors help in visually identifying its oxidation states and forms.
• The visible color change from yellow to green symbolizes the redox reaction occurring in the solution.
• Crucially, these shifts are not just physical but reflect underlying chemical transformations.

Chromium's behavior illustrates how chemical and visual observations go hand in hand in elucidating reaction pathways.