Question

Identify the Brønsted-Lowry acid-base pairs in each of the following equations: a. \(\mathrm{H}_{2} \mathrm{CO}_{3}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftarrows \mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{HCO}_{3}^{-}(a q)\) b. \(\mathrm{NH}_{4}^{+}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftarrows \mathrm{H}_{3} \mathrm{O}^{+}(a q)+\mathrm{NH}_{3}(a q)\) c. \(\mathrm{HCN}(a q)+\mathrm{NO}_{2}^{-}(a q) \rightleftarrows \mathrm{HNO}_{2}(a q)+\mathrm{CN}^{-}(a q)\)

Short answer

a) \(\text{H}_{2}\text{CO}_{3} \backslash \text{HCO}_{3}^{-}\) and \(\text{H}_{2}\text{O} \backslash \text{H}_{3}\text{O}^{+}\); b) \(\text{NH}_{4}^{+} \backslash \text{NH}_{3}\) and \(\text{H}_{2}\text{O} \backslash \text{H}_{3}\text{O}^{+}\); c) \(\text{HCN} \backslash \text{CN}^{-}\) and \(\text{NO}_{2}^{-} \backslash \text{HNO}_{2}\).

Step-by-step solution

Identify the Brønsted-Lowry Acid and Base for Equation (a)

In the reaction \(\text{H}_{2}\text{CO}_{3}(aq) + \text{H}_{2}\text{O}(l) \rightleftharpoons \text{H}_{3}\text{O}^{+}(aq) + \text{HCO}_{3}^{-}(aq)\), the acid is the species that donates a proton (\text{H}^{+}) and the base is the species that accepts the proton. \(\text{H}_{2}\text{CO}_{3}\) is the acid because it donates a proton to \(\text{H}_{2}\text{O}\). \(\text{H}_{2}\text{O}\) is the base because it accepts the proton to form \(\text{H}_{3}\text{O}^{+}\). Thus: \(\text{H}_{2}\text{CO}_{3} \rightarrow \text{HCO}_{3}^{-} + \text{H}^{+}\) (acid) and \(\text{H}_{2}\text{O} + \text{H}^{+} \rightarrow \text{H}_{3}\text{O}^{+}\) (base).

Identify the Conjugate Acid-Base Pairs in Equation (a)

The conjugate base of \(\text{H}_{2}\text{CO}_{3}\) is \(\text{HCO}_{3}^{-}\), and the conjugate acid of \(\text{H}_{2}\text{O}\) is \(\text{H}_{3}\text{O}^{+}\). So, the pairs are: \(\text{H}_{2}\text{CO}_{3} \backslash \text{HCO}_{3}^{-}\) and \(\text{H}_{2}\text{O} \backslash \text{H}_{3}\text{O}^{+}\).

Identify the Brønsted-Lowry Acid and Base for Equation (b)

In the reaction \(\text{NH}_{4}^{+}(aq) + \text{H}_{2}\text{O}(l) \rightleftharpoons \text{H}_{3}\text{O}^{+}(aq) + \text{NH}_{3}(aq)\), \(\text{NH}_{4}^{+}\) is the acid because it donates a proton to \(\text{H}_{2}\text{O}\). \(\text{H}_{2}\text{O}\) is the base because it accepts the proton to form \(\text{H}_{3}\text{O}^{+}\). Thus: \(\text{NH}_{4}^{+} \rightarrow \text{NH}_{3} + \text{H}^{+}\) (acid) and \(\text{H}_{2}\text{O} + \text{H}^{+} \rightarrow \text{H}_{3}\text{O}^{+}\) (base).

Identify the Conjugate Acid-Base Pairs in Equation (b)

The conjugate base of \(\text{NH}_{4}^{+}\) is \(\text{NH}_{3}\), and the conjugate acid of \(\text{H}_{2}\text{O}\) is \(\text{H}_{3}\text{O}^{+}\). So, the pairs are: \(\text{NH}_{4}^{+} \backslash \text{NH}_{3}\) and \(\text{H}_{2}\text{O} \backslash \text{H}_{3}\text{O}^{+}\).

Identify the Brønsted-Lowry Acid and Base for Equation (c)

In the reaction \(\text{HCN}(aq) + \text{NO}_{2}^{-}(aq) \rightleftharpoons \text{HNO}_{2}(aq) + \text{CN}^{-}(aq)\), \(\text{HCN}\) is the acid because it donates a proton to \(\text{NO}_{2}^{-}\). \(\text{NO}_{2}^{-}\) is the base because it accepts the proton to form \(\text{HNO}_{2}\). Thus: \(\text{HCN} \rightarrow \text{CN}^{-} + \text{H}^{+}\) (acid) and \(\text{NO}_{2}^{-} + \text{H}^{+} \rightarrow \text{HNO}_{2}\) (base).

Identify the Conjugate Acid-Base Pairs in Equation (c)

The conjugate base of \(\text{HCN}\) is \(\text{CN}^{-}\), and the conjugate acid of \(\text{NO}_{2}^{-}\) is \(\text{HNO}_{2}\). So, the pairs are: \(\text{HCN} \backslash \text{CN}^{-}\) and \(\text{NO}_{2}^{-} \backslash \text{HNO}_{2}\).

Key concepts

Acid-Base Reactions

H₂CO₃ donates a proton to H₂O to form H₃O⁺ and HCO₃^-, showcasing a classic acid-base reaction.
Understanding these transfers allows us to predict the behavior and products of many chemical reactions involving acids and bases.

Conjugate Acid-Base Pairs

HCN acts as the acid, losing a proton to become CN^-, while NO₂^- acts as the base to form HNO₂.

The conjugate pairs in this reaction are:

• HCN / CN^-
• NO₂^- / HNO₂

The concept of conjugate acid-base pairs helps us understand the reversible nature of acid-base reactions and predict the products of such interactions.

Proton Donor and Acceptor

NH₄⁺ donates a proton to H₂O, acting as the acid, while H₂O accepts the proton, acting as the base to form H₃O⁺ and NH₃.

This simple notion of proton transfer is foundational in understanding how acids and bases interact, affecting everything from industrial processes to biological systems. Recognizing substances as proton donors or acceptors enables us to classify and predict their behaviors in various chemical contexts.