Problem 36 - Introductory Chemistry Atoms First

Question

If \(\mathrm{HCO}_{3}^{-}\) is considered a weak base, what is its conjugate acid?

Step-by-Step Solution

Answer

The conjugate acid of the weak base \(\mathrm{HCO}_{3}^{-}\) is \(\mathrm{H_{2}CO_{3}}\).

Step 1: Identify the weak base

The given weak base is \(\mathrm{HCO}_{3}^{-}\).

Step 2: Add a proton to the weak base

We will add a proton (H+) to our weak base, \(\mathrm{HCO}_{3}^{-}\). This can be represented as: $$\mathrm{HCO_{3}^{-}} + \mathrm{H^{+}}$$

Step 3: Find the conjugate acid

When the proton is added to the weak base, a new compound is formed. Combining the weak base and the proton, we get: $$\mathrm{HCO_{3}^{-}} + \mathrm{H^{+}} \rightarrow \mathrm{H_{2}CO_{3}}$$ The conjugate acid of the weak base \(\mathrm{HCO}_{3}^{-}\) is \(\mathrm{H_{2}CO_{3}}\).

Key Concepts

Weak Base

When we talk about a \weak base\, we're referring to a substance that only partially dissociates in water to produce hydroxide ions, \( OH^- \). This limited ability to donate their electrons leads to a less complete ionization in a solution. A \weak base\, like \( \mathrm{HCO}_{3}^{-} \), does not readily bind with \( \mathrm{H^+} \) ions, which are protons, to create water, as a strong base would.

A key point here is understanding that the strength of a base depends on its ability to accept protons. In the exercise, \( \mathrm{HCO}_{3}^{-} \) acted as a weak base because, in the context of the reaction, it will only accept a proton from a strong acid to a certain extent. This is unlike stronger bases which would almost completely neutralize the acid.

Proton Transfer

The concept of \proton transfer\ is pivotal to acid-base chemistry. It involves the movement of a proton, or \( \mathrm{H^+} \) ion, from one species to another. When \( \mathrm{HCO}_{3}^{-} \) accepts a proton, it undergoes a transformation into its \( \mathrm{H_2CO_{3}} \) form. This process is a cornerstone of the Brønsted-Lowry theory of acids and bases, defining acids as proton donors and bases as proton acceptors.

It's essential to realize that the proton does not exist freely in solution but is very quickly attracted to entities that can provide a pair of electrons to bond with. During this transfer, the weak base \( \mathrm{HCO}_{3}^{-} \) accepts a proton, illustrating the reversible nature of such interactions in many chemical reactions.

Chemical Equilibrium

The concept of \chemical equilibrium\ plays a crucial role in the understanding of reactions like the one involving a weak base and its conjugate acid. In a state of equilibrium, the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of the reactants and products over time. These reactions are dynamic, meaning that the exchange of particles occurs continuously but at a consistent rate in both directions.

With regard to our weak base, \( \mathrm{HCO}_{3}^{-} \), and its conjugate acid, \( \mathrm{H_2CO_{3}} \), the equilibrium in an aqueous solution signifies a balance between the two forms. This does not indicate that the amounts of the weak base and conjugate acid are the same, but that their ratio remains constant at equilibrium. Understanding this helps grasp why even 'spent' weak bases can sometimes continue to have effects in buffering pH, as their presence in a balanced state helps moderate changes in the system's acidity.