Problem 58 - General Chemistry

Question

Write the equation for the acid ionization of the \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\) ion.

Step-by-Step Solution

Answer

The equation is \( \mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+} \rightleftharpoons \mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\left(\mathrm{OH}\right)^{+} + \mathrm{H}^{+} \).

Step 1: Recognize the Acid Species

The species \( \mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+} \) acts as an acidic cation in an aqueous solution. In this context, the water molecules coordinated to the copper ion can donate a proton \( (\mathrm{H}^+) \), because the central metal ion imparts a partial positive charge on the hydrogens of the water molecules.

Step 2: Write the Ionization Reaction

The process involves the loss of a proton \( \mathrm{H}^+ \) from one of the water molecules in the complex. The chemical reaction can be written as: \[ \mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+} \rightleftharpoons \mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\left(\mathrm{OH}\right)^{+} + \mathrm{H}^{+} \]

Step 3: Balance the Equation

The ionization reaction as written is already balanced, with one water molecule donating a proton \( \mathrm{H}^+ \) to form a hydroxido ligand \( \mathrm{OH}^- \) while the overall charge is reduced by one unit from the original \( 2+ \) to \( 1+ \).

Key Concepts

Acidic Cation

An acidic cation essentially refers to a positively charged ion that can donate a proton, often resulting in an acidic environment. In the case of the \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\) ion, the central copper ion is surrounded by water molecules, which influences the properties of this cation. The copper ion has a 2+ charge, creating a strong attraction between itself and the negative side of the water molecule's dipole.

This attraction leads to copper effectively pulling electron density away from the hydrogen atoms in the water, resulting in a positive charge on these hydrogens that makes them vulnerable to leaving as protons \((\mathrm{H}^+)\). As a result, the water molecules coordinated to the metal act as weak acids. The central metal, and the effect it has on bound ligands, determines the strength of the acidic cation in this kind of system.

Proton Donation

Proton donation is a core idea when it comes to understanding acids in chemistry. It describes the process whereby a chemical species, like our water molecules attached to the copper ion, release a proton \((\mathrm{H}^+)\). This ability to donate a proton classifies a species as an acid according to the Brønsted-Lowry acid definition.

In the ion \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\), there's significant attraction between the central copper ion and the water's electrons, polarizing the water molecules and making the hydrogen atoms more likely to leave. Once one loses a proton, the water's role shifts in the ionization reaction, resulting in a new complex with an OH ligand.

• The change from an acid to a conjugate base is an important concept when following proton donation.
• Understanding how the interaction between metal ions and bound water affects this behavior is crucial in complex ion analysis.

Ionization Reaction

An ionization reaction involves a neutral molecule or ion being transformed by gaining or losing a charged particle, like a proton. In this example, the ionization of \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\) is initiated by the loss of a proton from the coordinated water molecule. The reaction is depicted as follows:

\[ \mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+} \rightleftharpoons \mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\left(\mathrm{OH}\right)^{+} + \mathrm{H}^{+} \]

Here, one water molecule loses a hydrogen ion \(\mathrm{H}^+\) which results in the formation of a hydroxido ligand \(\mathrm{OH}^{-}\). The transition causes the overall charge of the complex to change from 2+ to 1+.

This reaction highlights how the structure of complex ions can change upon interacting with surrounding molecules and describes a form of equilibrium due to the reversible nature of the reaction.