Problem 32 - Chemistry and Chemical Reactivity

Question

Write an equation that describes the equilibrium that exists when the weak acid benzoic acid \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{H}\right)\) dissolves in water. Identify each of the four species in solution as either Bronsted acids or Bronsted bases. Does the equilibrium favor the products or the reactants? (In acting as an acid, the \(-\mathrm{CO}_{2} \mathrm{H}\) group supplies \(\left.\mathrm{H}^{+} \text {to } \mathrm{form} \mathrm{H}_{3} \mathrm{O}^{+} .\right)\)

Step-by-Step Solution

Answer

Reactants are favored; benzoic acid is the Bronsted acid, and water is the Bronsted base.

Step 1: Writing the Dissolution Equation

Start by writing the chemical equation for the dissolution of benzoic acid in water. Benzoic acid \((\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H})\) donates a proton \((\mathrm{H}^+)\) to water, which acts as a Bronsted base, forming hydronium \(\mathrm{H}_3\mathrm{O}^+\) and benzoate ion \(\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2^-\). The equation is: \[ \mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H}(aq) + \mathrm{H}_2\mathrm{O}(l) \rightleftharpoons \mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2^-(aq) + \mathrm{H}_3\mathrm{O}^+(aq) \]

Step 2: Identify Bronsted Acids and Bases

In the equation, identify each species as a Bronsted acid or base. - Benzoic acid \(\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H} \) acts as a Bronsted acid because it donates a proton. - Water \(\mathrm{H}_2\mathrm{O}\) acts as a Bronsted base because it accepts a proton to form \(\mathrm{H}_3\mathrm{O}^+\). - Benzoate ion \(\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2^-\) is a Bronsted base because it results from the acid donating a proton. - Hydronium ion \(\mathrm{H}_3\mathrm{O}^+\) acts as a Bronsted acid in the reverse reaction because it can donate a proton.

Step 3: Determining Equilibrium Favor

To determine which side the equilibrium favors, consider the strength of the acids and bases. - Benzoic acid is a weak acid, so it does not dissociate completely in solution, indicating the reactants are favored. - The equilibrium, therefore, largely remains towards the left side (reactants).

Key Concepts

Bronsted acids

In the world of chemistry, understanding the nature of acids and bases is crucial. Bronsted acids, named after the scientist Johannes Bronsted, are substances that can donate a proton ( H^+ ) to another substance. This concept is fundamental when discussing equilibrium reactions. When benzoic acid ( C_6H_5CO_2H ) dissolves in water, it acts as a Bronsted acid by donating a proton to a water molecule. This ability to donate a proton is what characterizes a Bronsted acid.

• In the reaction, benzoic acid is the proton donor.
• Water, which accepts the proton, plays the role of a Bronsted base in this context.

Through these interactions, the products of the reaction include a benzoate ion ( C_6H_5CO_2^- ) and a hydronium ion ( H_3O^+ ), both of which arise from the proton exchange between the acid and base.

Weak acids

Weak acids, like benzoic acid, do not fully dissociate in water. This incomplete dissociation is a key feature that differentiates them from strong acids.

• Weak acids establish an equilibrium in solution, meaning they partially ionize.
• This results in a mixture of ions and un-ionized acid molecules in the solution.

For benzoic acid, the equilibrium can be represented by the equation: \[ \mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2\mathrm{H}(aq) + \mathrm{H}_2\mathrm{O}(l) \rightleftharpoons \mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2^-(aq) + \mathrm{H}_3\mathrm{O}^+(aq) \] In this equilibrium, the reaction does not go to completion, which means that in any given sample, there will be a significant amount of benzoic acid molecules still intact. This is in contrast to strong acids which completely dissociate in water, leaving almost no original acid molecules present.

Chemical equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, leading to continuous dynamic balance. In the equilibrium reaction involving benzoic acid, both the forward reaction (formation of benzoate and hydronium ions) and the reverse reaction (recombination to form benzoic acid and water) occur simultaneously.

• The equilibrium is said to favor the reactants because benzoic acid is a weak acid.
• As a result, there is more undissociated benzoic acid compared to the ions produced.

The position of equilibrium is determined by the relative strengths of the acids and bases involved. Since weak acids do not dissociate completely, the equilibrium lies to the left, indicating that reactants are present in higher concentrations than the products.

Weak bases

In an equilibrium involving a weak acid like benzoic acid, the concept of weak bases also plays a crucial role. A weak base is a species that does not completely accept a proton in a solution. This is observed in benzoate ion ( C_6H_5CO_2^- ), which is formed as a conjugate base during the dissociation of benzoic acid in water.

• Benzoate ion acts as a weak base because it can accept a proton from hydronium ions to reform benzoic acid.
• In such reactions, both weak acids and weak bases exist in equilibrium with their conjugates.

The concept of weak bases is closely linked with that of weak acids, as they both illustrate the incomplete nature of proton transfer reactions. In this way, they establish a balance that defines chemical equilibrium in weak acid-base reactions.