Problem 34 - General Chemistry: Principles and Modern Applications

Question

Write simple chemical equations to show how the complex ion \(\left[\mathrm{CrOH}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\right]^{2+}\) acts as \((\mathrm{a})\) an acid; (b) a base.

Step-by-Step Solution

Answer

The acid behavior: \([CrOH(H_{2}O)_{5}]^{2+} \rightarrow [Cr(H_{2}O)_{6}]^{3+} + OH^{-}\) and the base behavior: \([CrOH(H_{2}O)_{5}]^{2+} + H_{2}O \rightarrow [Cr(H_{2}O)_{6}]^{3+} + OH^{-}\).

Step 1: Proton Donation (Acidic Behavior)

The complex ion can donate a proton (from OH) to a water molecule. This donation is indicative of an acid. Thus the reaction can be represented as follows: \([CrOH(H_{2}O)_{5}]^{2+} \rightarrow [Cr(H_{2}O)_{6}]^{3+} + OH^{-}\)

Step 2: Proton Acceptance (Base Behavior)

The complex ion can also accept a proton from a water molecule. This is a characteristic of a base. The reaction becomes: \([CrOH(H_{2}O)_{5}]^{2+} + H_{2}O \rightarrow [Cr(H_{2}O)_{6}]^{3+} + OH^{-}\).

Key Concepts

Acid-Base Reactions

Acid-base reactions are fundamental chemical processes where an acid donates a proton (H+) and a base accepts a proton. This proton exchange plays a vital role in the chemistry of solutions and has wide applications, from digesting food to industrial chemical reactions. In the case of the complex ion \( [CrOH(H_{2}O)_{5}]^{2+} \), it can act as an acid by donating a proton, typically from its hydroxyl (OH) group, to another molecule like water. This donation forms the reaction product \( [Cr(H_{2}O)_{6}]^{3+} \) and an \( OH^- \) ion. This behavior illustrates the amphoteric nature of the complex ion, meaning it can act as either an acid or a base depending on the surrounding conditions.

Understanding acid-base reactions is crucial in explaining many physical and biological phenomena. For students tackling chemical equations, always look for the proton donor and acceptor to identify which species is the acid and which is the base.

Complex Ions

Defining Complex Ions

An in-depth understanding of complex ions is essential for students studying coordination chemistry. Complex ions consist of a central metal ion surrounded by molecules or ions, known as ligands. These ligands can donate electrons to the metal, forming coordinate covalent bonds. The example of the complex ion \( [CrOH(H_{2}O)_{5}]^{2+} \) shows chromium at the center with a hydroxide and five water molecules as its ligands.

The behavior of these ions in solutions, especially regarding their acid-base reactions, relies on the nature of the ligands and the central metal. Ligands that can donate or accept protons, like the \( OH^- \) in our example, give complex ions the ability to engage in acid-base chemistry.

Proton Transfer

Role of Proton Transfer

Proton transfer refers to the movement of a proton from one atom or molecule to another, which is the defining event in an acid-base reaction. When \( [CrOH(H_{2}O)_{5}]^{2+} \) acts as an acid, it undergoes proton transfer by releasing a proton from the OH group to become \( [Cr(H_{2}O)_{6}]^{3+} \) with the creation of an \( OH^- \) ion. Conversely, when it acts as a base, it receives a proton from \( H_2O \) to form the same \( [Cr(H_{2}O)_{6}]^{3+} \) complex. Proton transfer processes are ubiquitous in chemistry and are fundamental to our understanding of chemical reaction mechanisms, including those in biological systems.

Equilibrium in Aqueous Solutions

Understanding Equilibrium

Aqueous solutions often reach a state of equilibrium, where the rate of the forward reaction equals that of the reverse reaction, and the concentrations of reactants and products remain constant over time. In an acid-base reaction involving complex ions like \( [CrOH(H_{2}O)_{5}]^{2+} \), the establishment of equilibrium depends on the relative strength of the acid and the base, as well as the concentration of other ions in the solution. Factors such as pH and the presence of other potential ligands can shift the position of the equilibrium, influencing the behavior of the reaction. For students exploring these reactions, it is important to recognize how changes in the solution's composition can affect the equilibrium position and therefore the outcome of the reaction.