Problem 7 - Chemistry and Chemical Reactivity

Question

In each of the following acid-base reactions, identify the Bronsted acid and base on the left and their conjugate partners on the right. (a) \(\mathrm{HCO}_{2} \mathrm{H}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell) \rightleftarrows \mathrm{HCO}_{2}^{-}(\mathrm{aq})+\mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})\). (b) \(\mathrm{NH}_{3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{S}(\mathrm{aq}) \rightleftarrows \mathrm{NH}_{4}^{+}(\mathrm{aq})+\mathrm{HS}^{-}(\mathrm{aq})\). (c) \(\mathrm{HSO}_{4}^{-}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}) \rightleftarrows \mathrm{SO}_{4}^{2-}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)\).

Step-by-Step Solution

Answer

(a) Acid: \( \mathrm{HCO}_{2}\mathrm{H} \), Base: \( \mathrm{H}_{2}\mathrm{O} \); (b) Acid: \( \mathrm{H}_{2}\mathrm{S} \), Base: \( \mathrm{NH}_{3} \); (c) Acid: \( \mathrm{HSO}_{4}^{-} \), Base: \( \mathrm{OH}^{-} \).

Step 1: Identify Parts in Reaction (a)

In the reaction \( \mathrm{HCO}_{2} \mathrm{H} + \mathrm{H}_{2} \mathrm{O} \rightleftarrows \mathrm{HCO}_{2}^{-} + \mathrm{H}_{3}\mathrm{O}^{+} \), \( \mathrm{HCO}_{2} \mathrm{H} \) donates a proton (\( \mathrm{H}^{+} \)) to \( \mathrm{H}_{2} \mathrm{O} \), making \( \mathrm{HCO}_{2} \mathrm{H} \) the Bronsted acid and \( \mathrm{H}_{2} \mathrm{O} \) the Bronsted base. The products \( \mathrm{HCO}_{2}^{-} \) and \( \mathrm{H}_{3}\mathrm{O}^{+} \) are the conjugate base and conjugate acid, respectively.

Step 2: Identify Parts in Reaction (b)

In the reaction \( \mathrm{NH}_{3} + \mathrm{H}_{2} \mathrm{S} \rightleftarrows \mathrm{NH}_{4}^{+} + \mathrm{HS}^{-} \), \( \mathrm{H}_{2} \mathrm{S} \) donates a proton to \( \mathrm{NH}_{3} \), thereby making \( \mathrm{H}_{2} \mathrm{S} \) the Bronsted acid and \( \mathrm{NH}_{3} \) the Bronsted base. The resulting \( \mathrm{NH}_{4}^{+} \) and \( \mathrm{HS}^{-} \) are the conjugate acid and conjugate base, respectively.

Step 3: Identify Parts in Reaction (c)

In the reaction \( \mathrm{HSO}_{4}^{-} + \mathrm{OH}^{-} \rightleftarrows \mathrm{SO}_{4}^{2-} + \mathrm{H}_{2}\mathrm{O} \), \( \mathrm{HSO}_{4}^{-} \) donates a proton to \( \mathrm{OH}^{-} \), making \( \mathrm{HSO}_{4}^{-} \) the Bronsted acid and \( \mathrm{OH}^{-} \) the Bronsted base. The products are \( \mathrm{SO}_{4}^{2-} \) and \( \mathrm{H}_{2}\mathrm{O} \), which act as the conjugate base and conjugate acid, respectively.

Key Concepts

Conjugate Acid-Base Pairs

In the realm of chemistry, conjugate acid-base pairs play a fundamental role in understanding reactions. These pairs are primarily linked through the donation and acceptance of protons.

A conjugate acid is formed when a base accepts a proton, while a conjugate base is what remains after an acid donates a proton. This relationship is beautifully demonstrated in the simplest acid-base reactions.

Consider the reaction:

• In reaction (a), - **Formic acid** \(\text{(HCO}_2\text{H)}\): This acts as the Bronsted acid, donating a proton to water.- **Water (H₂O)**: The proton acceptor, turns into the conjugate acid \(\text{(H}_3\text{O}^+)\).
• The conjugate base that corresponds to formic acid in this case is formate ion \(\text{(HCO}_2^-\).

These relationships help us predict how substances like acids and bases will act under various conditions. Recognizing conjugate pairs is key to mastering acid-base equilibrium and understanding how substances can transform in reversible reactions.

Proton Transfer

The Bronsted-Lowry theory of acids and bases hinges on one simple concept: proton transfer.

This theory assumes that acids are proton donors and bases are proton acceptors. During a chemical reaction, the movement of this very small \(\text{H}^+\) ion—or proton—between molecules defines many chemical processes.

Take, for example, the reaction (b):

• **Hydrogen sulfide (H₂S)**: Donates a proton to ammonia \(\text{(NH}_3\text{)}\), thereby acting as the Bronsted acid.
• **Ammonia (NH₃)**: Acts as the Bronsted base here, accepting the proton and forming the conjugate acid \(\text{(NH}_4^+)\).

Proton transfer influences not only the formation of products but also helps determine the strength and behavior of acids and bases in solutions. Understanding this flow of protons is crucial for predicting how acids and bases interact in any given scenario.

Acid-Base Reactions

Acid-base reactions are a cornerstone of chemistry, involving the exchange of protons between reactants.

These reactions are dynamic, often reaching a point of equilibrium where the forward and reverse reactions occur at the same rate.

For example, in reaction (c):

• **Bisulfate ion (HSO₄⁻)**: Donates a proton to hydroxide ion \(\text{(OH}^-\text{)}\) to form sulfate ion \(\text{(SO}_4^{2-}\text{)}\) and water.
• This reaction highlights how substances can shift roles between being acids and bases, depending on the context of the reaction setting.

To understand acid-base reactions, it's not just about identifying which is the acid or the base; it's about seeing the overall exchange and the resulting equilibrium. This broad perspective helps chemists understand how and why certain reactions proceed, and how they can be controlled or utilized in practical applications.