Question

Chloroacetic acid, ClCH2COOH, is a weak acid with a pKa of +2.86. (a) Write the balanced chemical equation that will have an equilibrium constant that is equal to Ka. Option 1: ClCH2COOH ⇌ ClCH2COO- + H+ Option 2: ClCH2COOH + H2O ⇌ ClCH2COO- + H3O+ Option 3: ClCH2COOH + OH- ⇌ ClCH2COO- + H2O Option 4: ClCH2COOH ⇌ ClCH2COO- + OH-

Answer 1

The correct balanced chemical equation for the ionization of chloroacetic acid that has an equilibrium constant equal to Ka is Option 2: ClCH2COOH + H2O ⇌ ClCH2COO- + H3O+. The equation involves chloroacetic acid reacting with water to produce its conjugate base and hydronium ion, which is the typical representation for weak acid ionization.

Step-by-step explanation:

To find the balanced chemical equation for chloroacetic acid with an equilibrium constant equal to Ka, we need to look at how it ionizes in water as a weak acid. The correct equation is one where chloroacetic acid donates a proton to water, resulting in the formation of the conjugate base and hydronium ion. Option 2: ClCH2COOH + H2O ⇌ ClCH2COO- + H3O+ is the balanced equation that fits this description.

Ka is the acid dissociation constant and corresponds to the ionization of weak acids in water. The pKa is related to Ka by the equation pKa = -log(Ka). An equilibrium equation for a weak acid typically includes water as one of the reactants and the hydronium ion, H3O+, as one of the products, indicating proton transfer to the water molecule. This reflects the fact that when a weak acid ionizes in water, it does so by donating a proton to water, not just by dissociation into H+ and its conjugate base in absence of water.

Answer 2

The correct balanced chemical equation for chloroacetic acid ionizing in water, with an equilibrium constant equal to Ka, is Option 2: ClCH2COOH + H2O ⇌ ClCH2COO- + H3O+. So, the best answer is option 2, ClCH2COOH + H2O ⇌ ClCH2COO- + H3O+

Step-by-step explanation:

The balanced chemical equation for chloroacetic acid, ClCH2COOH, that will have an equilibrium constant equal to Ka is Option 2: ClCH2COOH + H2O ⇌ ClCH2COO- + H3O+.

The ionization of a weak acid in water involves the transfer of a proton (H+) from the acid to water, forming the hydronium ion (H3O+).

A reaction involving water as a reactant and forming hydronium ions better represents the process that occurs in an aqueous solution, which leads to the establishment of acid equilibrium represented by Ka.

Option 1 lacks water involvement, implying a non-aqueous reaction, which isn't the typical expression for acid dissociation in solution.

Option 3 describes a reaction with hydroxide, which is more suited to the concept of the acid reacting with a base, rather than ionizing in water. Option 4 implies that the acid directly forms hydroxide ions which is not the case with acidic solutions.

Therefore, Option 2 is the correct representation of how the weak acid ionizes in water, coinciding with the concept of Ka as per the Arrhenius definition of acids and bases in aqueous solutions.

This equilibrium also explains how pH can be estimated based on the initial concentration of the acid (C₀) and Ka.

So, the best answer is option 2, ClCH2COOH + H2O ⇌ ClCH2COO- + H3O+