Problem 70 - Chemistry The Molecular Nature of Matter

Question

The ion \(\mathrm{Hg}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}\) acts as an acid. Write an equation showing its behavior as a Brønsted-Lowry acid in water.

Step-by-Step Solution

Answer

The ion \(\mathrm{Hg}\left(\mathrm{H}_2 \mathrm{O}\right)_{6}^{2+}\) acts as a Brønsted-Lowry acid by donating a proton to a water molecule, resulting in the equation: \(\mathrm{Hg}\left(\mathrm{H}_2 \mathrm{O}\right)_{6}^{2+} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Hg}\left(\mathrm{H}_2 \mathrm{O}\right)_{5}\left(\mathrm{OH}\right)^{+} + \mathrm{H}_3\mathrm{O}^+\).

Step 1: Identify the Bronsted-Lowry acid

According to the Brønsted-Lowry theory, an acid is a substance that can donate a proton (hydrogen ion, \(H^+\)). In this case, the complex ion \(\mathrm{Hg}\left(\mathrm{H}_2 \mathrm{O}\right)_{6}^{2+}\) is the acid because it has the ability to donate a proton.

Step 2: Write the ionization equation

Write the chemical equation showing the acid \(\mathrm{Hg}\left(\mathrm{H}_2 \mathrm{O}\right)_{6}^{2+}\) donating a proton to water (\(\mathrm{H}_2\mathrm{O}\)) to form the conjugate base and a hydronium ion (\(\mathrm{H}_3\mathrm{O}^+\)).

Step 3: Balance the equation

Ensure that the equation is balanced, with the same number of each type of atom on both sides of the equation, and that charge is conserved. In this case, the charge decreases by one on the product side, due to the loss of a positive proton from the acid, forming one \(\mathrm{H}_3\mathrm{O}^+\) ion.

Key Concepts

Acid-Base Reactions

Understanding acid-base reactions is crucial for grasping the fundamental concepts in chemistry. An acid-base reaction involves the transfer of protons from one reactant (the acid) to another (the base). According to the Brønsted-Lowry theory, an acid is characterized by its ability to donate a proton (H⁺), whereas a base is a substance capable of accepting a proton.

For example, in the reaction of the complex ion Hg(H₂O)₆²⁺ acting as an acid, it donates a proton to water molecules acting as the base. This proton transfer results in the formation of a hydronium ion (H₃O⁺) and a conjugate base, highlighting the dynamic nature of acid-base reactions.

In essence, acid-base reactions are about the movement of protons, and this is also what dictates the direction of the reaction. The proton donor (the acid) and the proton acceptor (the base) form what are known as conjugate pairs. Identifying these pairs is a fundamental skill in evaluating and predicting the outcomes of acid-base chemistry.

Proton Donation

Proton donation is the hallmark of an acid's behavior in chemical reactions, as posited by the Brønsted-Lowry acid theory. When we talk about proton donation, we're referring to the release of a hydrogen ion—essentially a hydrogen atom devoid of its electron, represented as H⁺.

Take, for instance, our textbook example of Hg(H₂O)₆²⁺. This complex ion can be considered a proton donor because it has hydrogen ions to spare, and when placed in water, it exhibits its acidic properties by releasing one of these protons. The resulting products, after the proton is donated, are what we refer to as the conjugate base of the original acid and the hydronium ion, proving the initial substance’s acidic nature.

Proton donation is not a one-sided affair. The substance that accepts the proton—the base—plays an equally significant role. In many reactions, water acts as this acceptor, transforming into the hydronium ion upon acceptance of the proton. This interplay is crucial in all acid-base chemistry and dictates the reactivity and strength of acids and bases.

Chemical Equilibrium

Chemical equilibrium is a state in acid-base reactions, and indeed all reversible chemical reactions, where the rate of the forward reaction equals the rate of the reverse reaction. This balance means that the concentrations of reactants and products remain constant over time, although they are not necessarily equal.

In the context of the ionization of a Brønsted-Lowry acid such as Hg(H₂O)₆²⁺ in water, chemical equilibrium is reached when the rate at which the complex ion donates protons to water matches the rate at which the conjugate base re-accepts protons to form the original acid. The equilibrium constant, denoted as K_eq, quantifies the concentrations of the reactants and products at equilibrium.

It’s important to understand that even at equilibrium, the reactions continue to occur; they’re just happening at the same rate in both directions, resulting in no net change in concentration. Analyzing how different factors like concentration, temperature, and pressure affect chemical equilibrium is fundamental for chemists to control and predict the outcome of reactions.