Chapter 18: Problem 9
Identify the conjugate acid-base pairs in the following equation. $$\mathrm{HNO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{NO}_{2}^{-}+\mathrm{H}_{3} \mathrm{O}^{+}$$
Short Answer
Expert verified
The conjugate pairs are \(\mathrm{HNO}_{2}/\mathrm{NO}_{2}^{-}\) and \(\mathrm{H}_{2}
\mathrm{O}/\mathrm{H}_{3}
\mathrm{O}^{+}\).
Step by step solution
01
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 \((\mathrm{H}^{+})\).
02
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 \(\mathrm{HNO}_{2}\).
03
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.
04
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.
05
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.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
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.
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 \( \mathrm{H}_3 \mathrm{O}^+ \).
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.
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, \( \mathrm{NO}_2^- \).
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.
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 \( \mathrm{H}_3 \mathrm{O}^+ \).