Problem 9 - Chemistry Matter and Change

Question

Identify the conjugate acid-base pairs in the following equation. $$\mathrm{HNO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{NO}_{2}^{-}+\mathrm{H}_{3} \mathrm{O}^{+}$$

Step-by-Step Solution

Answer

The conjugate pairs are \(\mathrm{HNO}_{2}/\mathrm{NO}_{2}^{-}\) and \(\mathrm{H}_{2} \mathrm{O}/\mathrm{H}_{3} \mathrm{O}^{+}\).

Step 1: Identify the Acid in the Reactants

In the forward reaction direction, compare the reactants. Here, \(\mathrm{HNO}_{2}\) acts as an acid because it donates a proton \((\mathrm{H}^{+})\).

Step 2: Identify the Base in the Reactants

Next, look at the other reactant, \(\mathrm{H}_{2} \,\mathrm{O}\). It acts as a base because it accepts a proton \((\mathrm{H}^{+})\) from \(\mathrm{HNO}_{2}\).

Step 3: Identify the Conjugate Base in the Products

In the reaction products, \(\mathrm{NO}_{2}^{-}\) is the conjugate base since it is formed from \(\mathrm{HNO}_{2}\) after losing a proton.

Step 4: Identify the Conjugate Acid in the Products

Finally, \(\mathrm{H}_{3} \,\mathrm{O}^{+}\) is the conjugate acid in the product side because it is formed when \(\mathrm{H}_{2}\mathrm{O}\) gains a proton.

Step 5: Pair the Conjugates

Now pair the species: \(\mathrm{HNO}_{2}\) and \(\mathrm{NO}_{2}^{-}\) form one conjugate acid-base pair; \(\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{H}_{3}\mathrm{O}^{+}\) form the second pair.

Key Concepts

Conjugate Acid-Base Pairs

In the context of acid-base reactions, a key concept is conjugate acid-base pairs. These pairs consist of two species that transform into each other by the gain or loss of a proton \(\text{(} H^+ \text{)}\). This transformation is crucial in reversible reactions where equilibrium is established between reactants and products.

• For instance, in the reaction \( \mathrm{HNO}_2 + \mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{NO}_2^- + \mathrm{H}_3 \mathrm{O}^+ \), two conjugate acid-base pairs are formed.
• \( \mathrm{HNO}_2 \) is an acid and its conjugate base is \( \mathrm{NO}_2^- \) after it donates a proton.
• The base \( \mathrm{H}_2 \mathrm{O} \) turns into the conjugate acid \( \mathrm{H}_3 \mathrm{O}^+ \) upon accepting a proton.

In every acid-base reaction, recognition of these pairs helps in understanding proton transfers and predicting reaction behaviors.

Proton Transfer

Proton transfer is a fundamental aspect of acid-base reactions. During such reactions, the movement or transfer of a proton \(H^+\) from one molecule to another defines whether a molecule acts as an acid or a base.

• In our given example, \( \mathrm{HNO}_2 \) donates a proton to \( \mathrm{H}_2 \mathrm{O} \), which accepts it.
• This transfer converts \( \mathrm{HNO}_2 \) into \( \mathrm{NO}_2^- \) and \( \mathrm{H}_2 \mathrm{O} \) into \( \mathrm{H}_3 \mathrm{O}^+ \).

Thinking about proton transfers allows students to view reactions in terms of *who* is "giving" and "receiving" a proton and understand the balance of acids and bases involved. Recognizing these actions can simplify predicting how the molecules will change and interact.

Acid

An acid is a substance capable of donating a proton \(H^+\) to another substance in a reaction. In Brønsted-Lowry theory, acids are defined by this ability to donate protons.
The stronger the tendency to donate a proton, the stronger the acid is considered to be.

• For example, in our reaction \( \mathrm{HNO}_2 \) acts as the acid because it loses one proton to \( \mathrm{H}_2 \mathrm{O} \).
• This act of donating forms the conjugate base, \( \mathrm{NO}_2^- \).

Understanding the concept of acids helps students predict which molecules will donate protons in reactions and form conjugate bases, a crucial step in balancing and analyzing reactions.

Base

A base is a substance that can accept a proton \(H^+\) from another substance. According to the Brønsted-Lowry definition, a base accepts protons, which is the key behavior distinguishing it from an acid.
In any acid-base reaction, identifying the base involves finding the proton acceptor.

• Here, \( \mathrm{H}_2 \mathrm{O} \) serves as the base because it gains a proton from \( \mathrm{HNO}_2 \).
• Upon accepting the proton, \( \mathrm{H}_2 \mathrm{O} \) becomes \( \mathrm{H}_3 \mathrm{O}^+ \).

Recognizing bases within reactions helps students understand how molecules change form, highlighting the importance of proton acceptance in determining a base’s identity.