Problem 77 - Chemistry: An Atoms-Focused Approach

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 the addition of a strong base.

Step-by-Step Solution

Answer

Answer: Chromium(III) hydroxide, Cr(OH)_3, reacts with a strong acid, such as HCl, by accepting protons (H^+) to form Cr^3+(aq) and Cl^-(aq) ions while releasing water. The balanced chemical equation is: Cr(OH)_3(s) + 3HCl(aq) -> Cr^3+(aq) + 3Cl^-(aq) + 3H_2O(l). When it reacts with a strong base, such as NaOH, it acts as an acid and donates protons (H^+) to form a chromate ion, CrO_4^2-, and sodium ions, Na^+, while releasing water. The balanced chemical equation is: 2Cr(OH)_3(s) + 6NaOH(aq) -> 2CrO_4^2-(aq) + 6Na^+(aq) + 6H_2O(l).

Step 1: Identify the Formula of Chromium(III) Hydroxide

The formula of Chromium(III) hydroxide can be determined by considering the charges of the ions involved. Chromium, in the +3 oxidation state (Cr^3+), carries a charge of +3, while the hydroxide ion (OH^-) carries a charge of -1. Therefore, the formula of Chromium(III) hydroxide is Cr(OH)_3.

Step 2: Write the Reaction between Chromium(III) Hydroxide and a Strong Acid (e.g., HCl)

To show the reaction between Chromium(III) hydroxide and a strong acid, we can choose hydrochloric acid (HCl) as an example. In this reaction, the chromium(III) hydroxide will act as a base, accepting protons (H^+) from the acid to form Cr^3+(aq) and Cl^-(aq) ions while releasing water: Cr(OH)_3(s) + 3HCl(aq) -> Cr^3+(aq) + 3Cl^-(aq) + 3H_2O(l) Remember, the reaction is balanced since the number of atoms of each element and the overall charge on each side are equal.

Step 3: Write the Reaction between Chromium(III) Hydroxide and a Strong Base (e.g., NaOH)

To show the reaction between Chromium(III) hydroxide and a strong base, we can choose sodium hydroxide (NaOH) as an example. In this reaction, the chromium(III) hydroxide will act as an acid, donating protons (H^+) to the base to form a chromate ion, CrO_4^2-, and sodium ions, Na^+, while releasing water: 2Cr(OH)_3(s) + 6NaOH(aq) -> 2CrO_4^2-(aq) + 6Na^+(aq) + 6H_2O(l) Once again, ensure that the reaction is balanced with an equal number of atoms and overall charge on each side.

Key Concepts

Chromium(III) Hydroxide

Chromium(III) hydroxide, represented as \( \text{Cr(OH)}_3 \), is a fascinating compound because it behaves as an amphiprotic substance. This means it can react both as an acid and a base. It is composed of chromium ions in a +3 oxidation state, coupled with hydroxide ions, \( \text{OH}^- \), each carrying a -1 charge. The compound's balanced charge is crucial to forming its chemical formula, which combines one chromium ion with three hydroxide ions.

• When interacting with acids, it offers its basic properties by accepting protons (\( \text{H}^+ \)).
• When mixed with bases, it reveals its acidic nature by donating protons.

Understanding these interactions further illuminates the amphiprotic nature and diverse reactivity of Chromium(III) hydroxide in different chemical environments.

Acid-Base Reactions

Acid-base reactions are fundamental chemical processes where an acid and a base interact. Chromium(III) hydroxide is remarkable in these reactions due to its dual role. For instance:

• With a strong acid like hydrochloric acid (\( \text{HCl} \)), Chromium(III) hydroxide behaves as a base. It accepts \( \text{H}^+ \) ions and dissociates to form soluble chromium ions \( \text{Cr}^{3+} \) and chloride ions \( \text{Cl}^- \), while water is produced as a byproduct. This is indicated through the chemical equation: \( \text{Cr(OH)}_3(s) + 3\text{HCl}(aq) \rightarrow \text{Cr}^{3+}(aq) + 3\text{Cl}^-(aq) + 3\text{H}_2\text{O}(l) \).
• When paired with a strong base like sodium hydroxide (\( \text{NaOH} \)), it displays acidic character by donating protons. The reaction results in the formation of chromate ions \( \text{CrO}_4^{2-} \) and sodium ions \( \text{Na}^+ \), again with water as a side product, demonstrated by the equation: \( 2\text{Cr(OH)}_3(s) + 6\text{NaOH}(aq) \rightarrow 2\text{CrO}_4^{2-}(aq) + 6\text{Na}^+(aq) + 6\text{H}_2\text{O}(l) \).

This dual amphiprotic action illustrates the versatility of acid-base reactions, showcasing how a single compound can adapt to either role based on its chemical partner.

Chemical Equations

Chemical equations are vital tools in chemistry that represent the transformation occurring in a chemical reaction. They indicate the reactants, products, and their stoichiometric relationships. For Chromium(III) hydroxide reactions:
* The equation shows all substances involved in their respective states: solid (s), liquid (l), aqueous (aq).* A balanced equation reflects an equal number of each type of atom on both sides. This also ensures that the overall charge is conserved.
For example, in the reaction of Chromium(III) hydroxide with hydrochloric acid: \[ \text{Cr(OH)}_3(s) + 3\text{HCl}(aq) \rightarrow \text{Cr}^{3+}(aq) + 3\text{Cl}^-(aq) + 3\text{H}_2\text{O}(l) \]Every chloride ion and water molecule is accounted for, demonstrating balance in both mass and charge.
Such precision is what allows chemists to predict how compounds like Chromium(III) hydroxide will interact with acids and bases, providing insights into their amphiprotic nature.