Question

\(\mathrm{CrO}_{3}\) dissolves in aqueous \(\mathrm{NaOH}\) to give (a) \(\mathrm{Cr}(\mathrm{OH})_{3}\) (b) \(\mathrm{Cr}(\mathrm{OH})_{2}\) (c) \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) (d) \(\mathrm{CrO}_{4}^{2-}\)

Short answer

The main product is \(\mathrm{CrO}_{4}^{2-}\) (option d).

Step-by-step solution

Understand the Reactants

The problem involves chromium trioxide, \(\mathrm{CrO}_{3}\), and aqueous sodium hydroxide, \(\mathrm{NaOH}\). \(\mathrm{CrO}_{3}\) is a chromium(VI) oxide, and \(\mathrm{NaOH}\) is a strong base. We need to understand how these reactants interact in water.

Write the Reaction

When \(\mathrm{CrO}_{3}\) is dissolved in aqueous \(\mathrm{NaOH}\), a reaction occurs to form a chromate ion. The overall balanced reaction is:\[\mathrm{CrO}_{3} + 2 \mathrm{OH}^{-} \rightarrow \mathrm{CrO}_{4}^{2-} + \mathrm{H}_{2}\mathrm{O}\]In this reaction, \(\mathrm{CrO}_{3}\) reacts with hydroxide ions \(\mathrm{OH}^{-}\) from \(\mathrm{NaOH}\) to form \(\mathrm{CrO}_{4}^{2-}\), the chromate ion, and water.

Determine the Product

According to the reaction from Step 2, the main product when \(\mathrm{CrO}_{3}\) dissolves in aqueous \(\mathrm{NaOH}\) is \(\mathrm{CrO}_{4}^{2-}\), which is the chromate ion. This matches option (d) given in the exercise.

Key concepts

Chromium trioxide

Chromium trioxide, denoted as \(\mathrm{CrO}_{3}\), is a powerful oxidizing agent. It appears as a bright red crystalline solid. It's often utilized in the preparation of other chromium compounds and in chromium plating. In organic chemistry, it's sometimes used as a reagent to oxidize primary alcohols to aldehydes or carboxylic acids. Being an oxide of chromium, \(\mathrm{CrO}_{3}\) features chromium in a +6 oxidation state, which is particularly toxic. Hence, it requires careful handling. When dissolved in water or a basic solution like sodium hydroxide, \(\mathrm{CrO}_{3}\) undergoes significant transformation to produce chromate ions. This chemical behavior is essential in understanding its broader applications and its role in various chemical reactions.

Sodium hydroxide

Sodium hydroxide (\(\mathrm{NaOH}\)), commonly known as lye or caustic soda, is a strong base used in many industrial and chemical processes. It appears as white, solid pellets or a concentrated aqueous solution and is highly soluble in water. As a strong base, \(\mathrm{NaOH}\) dissociates completely in water to give sodium (\(\mathrm{Na^+}\)) and hydroxide ions (\(\mathrm{OH}^-\)). This disassociation is crucial to the reaction involving chromium trioxide. The hydroxide ions serve as key reactants, facilitating various chemical transformations, including the formation of chromate ions from chromium trioxide.When working with \(\mathrm{NaOH}\), it's important to note that it is highly corrosive. It can cause severe burns upon contact with skin or damage to materials. Therefore, proper safety measures, such as wearing gloves and goggles, are essential when handling this chemical.

Chemical reactions

Chemical reactions involve the transformation of reactants into products. In this context, we look at how \(\mathrm{CrO}_{3}\) reacts with \(\mathrm{NaOH}\) to yield chromate ions. The fundamental reaction is:

• \(\mathrm{CrO}_{3} + 2 \mathrm{OH}^- \rightarrow \mathrm{CrO}_{4}^{2-} + \mathrm{H}_{2}\mathrm{O}\)

Here, chromium trioxide reacts with hydroxide ions from sodium hydroxide. This leads to the formation of chromate ions and water.Understanding the stoichiometry is essential. Each molecule of \(\mathrm{CrO}_{3}\) reacts with two hydroxide ions to yield a single chromate ion. This balanced equation helps predict the outcome of mixing these substances under controlled conditions. Such reactions underpin numerous industrial and laboratory processes, including those involved in metal treatment and synthesis of compounds.

Chromate ion

The chromate ion, \(\mathrm{CrO}_{4}^{2-}\), is formed when chromium trioxide is dissolved in a basic solution like sodium hydroxide. It is characterized by a tetrahedral geometry around the chromium atom, where four oxygen atoms are symmetrically positioned. Chromate ions are often used in industrial settings for pigment production, corrosion resistance, and wood preservation. They can also serve as a precursor to other chromium compounds.One of the interesting aspects of chromate ions is their ability to exist in equilibrium with dichromate ions (\(\mathrm{Cr}_2\mathrm{O}_7^{2-}\)), depending on the pH of the solution. At higher pH levels, chromate ions are favored, while at lower pH, dichromate ions are more stable. This property makes the chromate-dichromate system a subject of study in acid-base equilibria.