Problem 1 - Chemistry: Principles and Reactions

Question

For each of the following reactions, indicate the Brønsted-Lowry acids and bases. What are the conjugate acid/base pairs? (a) \(\mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{CN}^{-}(a q) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{HNO}_{2}(a q)+\mathrm{OH}^{-}(a q) \rightleftharpoons \mathrm{NO}_{2}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{HCHO}_{2}(a q)+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CHO}_{2}^{-}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q)\)

Step-by-Step Solution

Answer

Question: Identify the Brønsted-Lowry acids and bases, and their conjugate pairs in the following reactions: (a) $\mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{CN}^{-}(a q) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{H}_{2} \mathrm{O}$ (b) $\mathrm{HNO}_{2}(a q)+\mathrm{OH}^{-}(a q) \rightleftharpoons \mathrm{NO}_{2}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}$ (c) $\mathrm{HCHO}_{2}(a q)+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CHO}_{2}^{-}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q)$ Answer: (a) Acids: $\mathrm{H}_{3} \mathrm{O}^{+}$; Bases: $\mathrm{CN}^{-}$; Conjugate pairs: $\mathrm{H}_{3} \mathrm{O}^{+}/\mathrm{H}_{2} \mathrm{O}$ and $\mathrm{CN}^{-}/\mathrm{HCN}$. (b) Acids: $\mathrm{HNO}_{2}$; Bases: $\mathrm{OH}^{-}$; Conjugate pairs: $\mathrm{HNO}_{2}/\mathrm{NO}_{2}^{-}$ and $\mathrm{OH}^{-}/\mathrm{H}_{2} \mathrm{O}$. (c) Acids: $\mathrm{HCHO}_{2}$; Bases: $\mathrm{H}_{2} \mathrm{O}$; Conjugate pairs: $\mathrm{HCHO}_{2}/\mathrm{CHO}_{2}^{-}$ and $\mathrm{H}_{2} \mathrm{O}/\mathrm{H}_{3} \mathrm{O}^{+}$.

Step 1: Reaction (a) - Identifying Acids and Bases

In the reaction: $$\mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{CN}^{-}(a q) \rightleftharpoons \mathrm{HCN}(a q)+\mathrm{H}_{2} \mathrm{O}$$ The proton donor is \(\mathrm{H}_{3} \mathrm{O}^{+}\), making it the Brønsted-Lowry acid. On the other hand, \(\mathrm{CN}^{-}\) accepts a proton, so it's the Brønsted-Lowry base.

Step 2: Reaction (a) - Conjugate Acid/Base Pairs

The conjugate base of the Brønsted-Lowry acid \(\mathrm{H}_{3} \mathrm{O}^{+}\) is \(\mathrm{H}_{2} \mathrm{O}\), and the conjugate acid of the Brønsted-Lowry base \(\mathrm{CN}^{-}\) is \(\mathrm{HCN}\). Thus, the conjugate pairs are \(\mathrm{H}_{3} \mathrm{O}^{+}/\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{CN}^{-}/\mathrm{HCN}\).

Step 3: Reaction (b) - Identifying Acids and Bases

In the reaction: $$\mathrm{HNO}_{2}(a q)+\mathrm{OH}^{-}(a q) \rightleftharpoons \mathrm{NO}_{2}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}$$ The proton donor is \(\mathrm{HNO}_{2}\), making it the Brønsted-Lowry acid. On the other hand, \(\mathrm{OH}^{-}\) accepts a proton, so it's the Brønsted-Lowry base.

Step 4: Reaction (b) - Conjugate Acid/Base Pairs

The conjugate base of the Brønsted-Lowry acid \(\mathrm{HNO}_{2}\) is \(\mathrm{NO}_{2}^{-}\), and the conjugate acid of the Brønsted-Lowry base \(\mathrm{OH}^{-}\) is \(\mathrm{H}_{2} \mathrm{O}\). Thus, the conjugate pairs are \(\mathrm{HNO}_{2}/\mathrm{NO}_{2}^{-}\) and \(\mathrm{OH}^{-}/\mathrm{H}_{2} \mathrm{O}\).

Step 5: Reaction (c) - Identifying Acids and Bases

In the reaction: $$\mathrm{HCHO}_{2}(a q)+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{CHO}_{2}^{-}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q)$$ The proton donor is \(\mathrm{HCHO}_{2}\), making it the Brønsted-Lowry acid. On the other hand, \(\mathrm{H}_{2} \mathrm{O}\) accepts a proton, so it's the Brønsted-Lowry base in this reaction.

Step 6: Reaction (c) - Conjugate Acid/Base Pairs

The conjugate base of the Brønsted-Lowry acid \(\mathrm{HCHO}_{2}\) is \(\mathrm{CHO}_{2}^{-}\), and the conjugate acid of the Brønsted-Lowry base \(\mathrm{H}_{2} \mathrm{O}\) is \(\mathrm{H}_{3} \mathrm{O}^{+}\). Thus, the conjugate pairs are \(\mathrm{HCHO}_{2}/\mathrm{CHO}_{2}^{-}\) and \(\mathrm{H}_{2} \mathrm{O}/\mathrm{H}_{3} \mathrm{O}^{+}\).

Key Concepts

Conjugate Acid/Base Pairs

Understanding the concept of conjugate acid/base pairs is essential in the study of acid-base reactions according to the Brønsted-Lowry theory. A conjugate acid/base pair is a set of two species that transform into each other by the gain or loss of a proton (H⁺). This transformation is most easily observed in reversible reactions.

For instance, when a Brønsted-Lowry acid donates a proton during a chemical reaction, it forms its conjugate base. Conversely, when a Brønsted-Lowry base accepts a proton, it forms its conjugate acid. Consider the reaction involving hydroxide (OH^-) and nitrous acid (HNO₂); OH^- accepts a proton to become water (H₂O), hence OH^-/H₂O is a conjugate pair. Meanwhile, HNO₂ donates a proton and becomes nitrite (NO₂^-), the conjugate base, establishing the conjugate pair HNO₂/NO₂^-.

• Each acid has a corresponding conjugate base.
• Each base has a corresponding conjugate acid.
• Conjugate pairs differ by one proton.

By recognizing these pairs, students can predict the outcome of acid-base reactions and understand reaction mechanisms more thoroughly.

Proton Donor

In the context of Brønsted-Lowry acid-base chemistry, the term proton donor refers to a substance that gives up a proton in the form of a hydrogen ion (H⁺) during a chemical reaction. This proton donation is the defining characteristic of what is known as a Brønsted-Lowry acid.

For example, in the reaction between hydroxonium (H₃O⁺) and cyanide (CN^-), the hydroxonium ion donates a proton to cyanide, forming water (H₂O) and hydrogen cyanide (HCN). Here, H₃O⁺ is the proton donor. The ability to donate a proton is a fundamental property that allows acids to react with bases.

• Proton donors have at least one removable hydrogen.
• The strength of an acid is related to its tendency to donate a proton.
• The proton donor, after losing a proton, becomes a conjugate base.

Identifying the proton donor in a reaction helps determine the direction of the reaction and the relative strengths of acid-base pairs.

Proton Acceptor

The term proton acceptor is synonymous with the definition of a Brønsted-Lowry base. It pertains to a substance that can accept a proton, typically from an acid, during a chemical reaction. This acceptance of a proton is what defines the behavior of bases in acid-base interactions.

Take, for instance, the interaction between formic acid (HCHO₂) and water (H₂O). In this case, water acts as a proton acceptor, gaining a proton from formic acid and thus forming the hydronium ion (H₃O⁺). Concomitantly, formic acid becomes its conjugate base, the formate ion (CHO₂^-).

• Proton acceptors have an available pair of electrons for bonding.
• The strength of a base is related to its tendency to accept a proton.
• The proton acceptor, after gaining a proton, becomes a conjugate acid.

Understanding proton acceptors is key to predicting the products of an acid-base reaction and assessing the relative strength of bases.