Question

Which of the following ions disproportionates in aqueous solution? Write a balanced net ionic equation for the reaction. (a) \(\mathrm{Cr}^{3+}\) (b) \(\mathrm{Fe}^{3+}\) (c) \(\mathrm{Cu}^{+}\) (d) \(\mathrm{Cu}^{2+}\)

Short answer

Cu⁺ disproportionates in aqueous solution, with Cu⁺ → Cu + Cu²⁺.

Step-by-step solution

Identify Disproportionation Reaction

A disproportionation reaction is one where a substance undergoes both oxidation and reduction, producing two different products. To solve the exercise, you need to identify which of the given ions can both donate and accept electrons simultaneously, resulting in two different oxidation states.

Analyze Each Ion

Assess each ion to determine whether it can change to higher and lower oxidation states simultaneously:- (a) \(\mathrm{Cr}^{3+}\): Chromium often undergoes reduction or oxidation separately but doesn't typically show disproportionation.- (b) \(\mathrm{Fe}^{3+}\): Iron is more commonly observed changing oxidation states in both reduction and oxidation reactions with another species, but not in disproportionation.- (c) \(\mathrm{Cu}^{+}\): Copper ions can undergo disproportionation, as Cu(I) can oxidize to Cu(II) or reduce to Cu metal.- (d) \(\mathrm{Cu}^{2+}\): Copper in the +2 state is more stable and doesn't typically disproportionate.

Choose the Correct Ion

Based on analysis, \(\mathrm{Cu}^{+}\) is likely to disproportionate because it can transform into both a lower oxidation state (Cu) and a higher one (Cu²⁺) in solution.

Write the Balanced Net Ionic Equation

The balanced net ionic equation for the disproportionation of \(\mathrm{Cu}^{+}\) is:\[ 2\, \mathrm{Cu}^{+} (aq) \rightarrow \mathrm{Cu} (s) + \mathrm{Cu}^{2+} (aq) \]In this equation, one \(\mathrm{Cu}^{+}\) reduces to Cu(s), and another \(\mathrm{Cu}^{+}\) oxidizes to \(\mathrm{Cu}^{2+}\).

Key concepts

Redox Reactions

Redox reactions are processes where reduction and oxidation occur simultaneously. In these reactions, one species donates electrons (oxidation) while another species gains electrons (reduction). This exchange of electrons is crucial and can affect chemical properties. Consider how Oxygen often plays a role in these reactions by accepting electrons, thus being reduced. In the context of disproportionation, a single substance acts as both the oxidizing and reducing agent, leading to two different products.

Understanding how electrons transfer between different species helps in identifying disproportionation, as both oxidation and reduction happen to the same element. This unique type of redox can tell us a lot about the chemical behavior of certain ions.

Copper Ions

Copper exists in multiple oxidation states, primarily as \(\mathrm{Cu}^+\) and \(\mathrm{Cu}^{2+}\). The behavior of copper ions in solution is key to understanding disproportionation. \(\mathrm{Cu}^+\) is particularly interesting because it can easily oxidize to \(\mathrm{Cu}^{2+}\) or reduce to copper metal (Cu).

This dual behavior allows it to undergo disproportionation. \(\mathrm{Cu}^{+}\) ions reacting in water can form both solid copper and \(\mathrm{Cu}^{2+}\) ions, showcasing its versatility. Copper's rich chemistry is why it appears in various industrial and biological processes, and understanding its ions is vital for chemists.

Oxidation States

The oxidation state indicates the degree of oxidation of an atom in a compound. It represents the number of electrons lost or gained. In redox chemistry, these states help identify how electrons are transferred.

For disproportionation, the same element must shift between different oxidation states. With \(\mathrm{Cu}^{+}\), you see a move to both \(\mathrm{Cu}^{2+}\) and Cu metal, which are different oxidation states. Recognizing these transitions is crucial. When you look at any ion, considering possible shifts in oxidation states can provide insight into its chemical behavior.

Net Ionic Equations

Net ionic equations simplify chemical reactions by focusing on the ions involved in the actual reaction, omitting spectator ions. These equations provide a clear view of the actual chemical change taking place.

In disproportionation, writing a balanced net ionic equation shows how electrons are transferred. For example, the equation for \(\mathrm{Cu}^{+}\) disproportionation is:\[ 2\, \mathrm{Cu}^{+} (aq) \rightarrow \mathrm{Cu} (s) + \mathrm{Cu}^{2+} (aq) \]This equation tells us that two \(\mathrm{Cu}^{+}\) ions change into copper metal and \(\mathrm{Cu}^{2+}\) ions. Net ionic equations not only clarify the reaction but help predict the products formed.