Problem 77 - Chemistry The Science in Context

Question

Chromium(III) hydroxide is amphiprotic. Write chemical equations showing how an aqueous suspension of this compound reacts to the addition of a strong acid and a strong base.

Step-by-Step Solution

Answer

Answer: When chromium(III) hydroxide reacts with a strong acid, such as hydrochloric acid (HCl), the balanced chemical equation is: Cr(OH)₃ + 3HCl → CrCl₃ + 3H₂O When chromium(III) hydroxide reacts with a strong base, such as sodium hydroxide (NaOH), the balanced chemical equation is: Cr(OH)₃ + 3NaOH → Na₃[Cr(OH)₆]

Step 1: 1. Identify chromium(III) hydroxide formula

Chromium(III) hydroxide is an amphiprotic compound composed of one chromium (Cr) atom and three hydroxide (OH) groups. The chemical formula is Cr(OH)₃.

Step 2: 2. Identify a strong acid and a strong base for the reactions

A common strong acid is hydrochloric acid (HCl), and a common strong base is sodium hydroxide (NaOH). We can use these two compounds for the reactions with chromium(III) hydroxide.

Step 3: 3. Write the chemical equation for the reaction of chromium(III) hydroxide with a strong acid

When chromium(III) hydroxide reacts with hydrochloric acid, Cr(OH)₃ acts as a base, and HCl acts as an acid. The balanced chemical equation for this reaction is: Cr(OH)₃ + 3HCl → CrCl₃ + 3H₂O In this reaction, the hydrogen ions (H⁺) from HCl and hydroxide ions (OH⁻) from Cr(OH)₃ combine to form water molecules, and the chromium and chloride ions form chromium(III) chloride (CrCl₃).

Step 4: 4. Write the chemical equation for the reaction of chromium(III) hydroxide with a strong base

When chromium(III) hydroxide reacts with sodium hydroxide, Cr(OH)₃ can act as an acid. The balanced chemical equation for this reaction is: Cr(OH)₃ + 3NaOH → Na₃[Cr(OH)₆] In this reaction, the chromium(III) hydroxide reacts with sodium hydroxide to form a complex anion with a chromium center and six hydroxide groups surrounding it. The sodium ions (Na⁺) bond with this complex anion to form sodium hexahydroxochromate(III), Na₃[Cr(OH)₆].

Key Concepts

Chromium(III) Hydroxide

Chromium(III) hydroxide, with the formula \(Cr(OH)_3\), is an interesting compound due to its amphiprotic nature. This means it can act both as an acid and as a base. This dual ability allows it to participate in a variety of chemical reactions depending on the environment it is in.
When suspended in water, chromium(III) hydroxide can interact with both acids and bases, displaying its versatility. The element chromium, found in this compound, belongs to the transition metals, which are known for their varied oxidation states and complex formation capabilities. Such properties make chromium compounds important in both academic studies and industrial applications.
Structurally, chromium(III) hydroxide consists of a central chromium ion coordinated by three hydroxide ions. Its amphiprotic nature stems from its ability to either donate hydroxide ions when acting as an acid or accept protons when acting as a base.

Chemical Reactions with Acids and Bases

Chemical reactions involving acids and bases are fundamental aspects of chemistry. Chromium(III) hydroxide's ability to react with both shows its amphiprotic nature. When chromium(III) hydroxide encounters a strong acid, like hydrochloric acid \((HCl)\), it behaves as a base. The acid donates protons \((H^+)\) which react with the hydroxide ions \((OH^−)\) from chromium hydroxide to form water \((H_2O)\). Additionally, the remaining components form chromium(III) chloride \((CrCl_3)\).

• Acid reaction: \(Cr(OH)_3 + 3HCl \rightarrow CrCl_3 + 3H_2O\)

Conversely, when chromium(III) hydroxide is introduced to a strong base like sodium hydroxide \((NaOH)\), it demonstrates acidic behavior. This involves the chromium(III) hydroxide reacting with additional hydroxide ions to form a complex compound called sodium hexahydroxochromate(III) \((Na_3[Cr(OH)_6])\).

• Base reaction: \(Cr(OH)_3 + 3NaOH \rightarrow Na_3[Cr(OH)_6]\)

These reactions illustrate the principle that an amphiprotic substance can adapt its role depending on the nature of the reagent it encounters.

Balanced Chemical Equations

Balanced chemical equations are key to understanding and describing chemical reactions accurately. Each equation must have the same number of each type of atom on both sides to satisfy the law of conservation of mass.
When balancing equations for reactions involving chromium(III) hydroxide, it’s essential to ensure all elements, including chromium, hydroxide, sodium, and chlorine, are accounted for. This step clarifies how matter is conserved and which compounds result from the reactions.

• The balanced equation for chromium(III) hydroxide with hydrochloric acid: \(Cr(OH)_3 + 3HCl \rightarrow CrCl_3 + 3H_2O\)
• The balanced equation for chromium(III) hydroxide with sodium hydroxide: \(Cr(OH)_3 + 3NaOH \rightarrow Na_3[Cr(OH)_6]\)

Writing these equations correctly means balancing the charge as well as the mass. This precision ensures that the chemical quantities represented are consistent with experimental outcomes. Understanding how to balance these equations is crucial for anyone studying chemistry, as it highlights both the underlying principles of chemical reactions and their practical implications.