Question

For each of the following aqueous reactions, identify the acid, the base, the conjugate base, and the conjugate acid. a. \(\operatorname{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}(\mathrm{OH})^{2+}\) b. \(\mathrm{H}_{2} \mathrm{O}+\mathrm{HONH}_{3}^{+} \rightleftharpoons \mathrm{HONH}_{2}+\mathrm{H}_{3} \mathrm{O}^{+}\) c. \(\mathrm{HOCl}+\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \rightleftharpoons \mathrm{OCl}^{-}+\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3}^{+}\)

Short answer

a. Acid: Al(H2O)6^3+, Base: H2O, Conjugate Base: Al(H2O)5(OH)2+, Conjugate Acid: H3O+ b. Acid: HONH3+, Base: H2O, Conjugate Base: HONH2, Conjugate Acid: H3O+ c. Acid: HOCl, Base: C6H5NH2, Conjugate Base: OCl-, Conjugate Acid: C6H5NH3+

Step-by-step solution

Identify acid and base on the left side of the reaction

To identify the acid and base, we need to find which species is donating a proton and which species is accepting a proton. Looking at the products, we can see that H3O+ is formed, which indicates that H2O is the base and Al(H2O)6^3+ is the acid.

Identify conjugate base and conjugate acid on the right side of the reaction

As we have identified the acid and base, we can now identify the conjugate base and conjugate acid. The conjugate base is formed when the acid loses a proton, so Al(H2O)5(OH)2+ is the conjugate base. The conjugate acid is formed when the base gains a proton, so H3O+ is the conjugate acid. #b. H2O + HONH3+ ⇌ HONH2 + H3O+ #

Identify acid and base on the left side of the reaction

Similarly, as in the previous reaction, we can look at the products to determine which species is donating and accepting protons. H3O+ is formed, indicating that H2O is acting as the base again, and HONH3+ is the acid.

Identify conjugate base and conjugate acid on the right side of the reaction

The conjugate base is formed when the acid loses a proton, so HONH2 is the conjugate base. The conjugate acid is formed when the base gains a proton, so H3O+ is the conjugate acid again. #c. HOCl + C6H5NH2 ⇌ OCl- + C6H5NH3+ #

Identify acid and base on the left side of the reaction

By inspecting the products, we can see that C6H5NH3+ is formed, which indicates that C6H5NH2 is the base and HOCl is the acid.

Identify conjugate base and conjugate acid on the right side of the reaction

The conjugate base is formed when the acid loses a proton, so OCl- is the conjugate base. The conjugate acid is formed when the base gains a proton, so C6H5NH3+ is the conjugate acid.

Key concepts

Conjugate Acid-Base Pairs

When we discuss acid-base reactions, one important concept is the idea of conjugate acid-base pairs. A conjugate acid-base pair consists of two chemical species which differ by the presence or absence of a proton (H+). For every acid in a reaction, there is a corresponding conjugate base, and for every base, there is a conjugate acid.
For instance, in the aqueous reaction

• \( ext{Al(H}_2 ext{O})_6^{3+} + ext{H}_2 ext{O} \rightleftharpoons ext{H}_3 ext{O}^+ + ext{Al(H}_2 ext{O})_5( ext{OH})^{2+}\),

• the acid \( ext{Al(H}_2 ext{O})_6^{3+}\) donates a proton and forms the conjugate base \( ext{Al(H}_2 ext{O})_5( ext{OH})^{2+}\).
• The base \( ext{H}_2 ext{O}\) accepts a proton to become the conjugate acid \( ext{H}_3 ext{O}^+\).

In each scenario of an acid-base reaction, identifying these pairs helps understand the transfer of protons across the reactants and products. Observing changes in chemical structures from reactants to products allows chemists to determine which species are conjugates.

Proton Donor and Acceptor

The roles of proton donor and acceptor play a central part in acid-base reactions. A proton donor is referred to as an acid, while a proton acceptor is known as a base. By analyzing the reactants and their resulting products, one can establish which compound donates or accepts a proton.
Consider the reaction:

• \( ext{H}_2 ext{O} + ext{HONH}_3^+ \rightleftharpoons ext{HONH}_2 + ext{H}_3 ext{O}^+\)

• \( ext{HONH}_3^+\) acts as the proton donor, causing the acid to release a proton, transforming into \( ext{HONH}_2\).
• \( ext{H}_2 ext{O}\) functions as the proton acceptor, gaining a proton to become \( ext{H}_3 ext{O}^+\).

This process involves a simple exchange: giving and receiving of protons between molecules. Recognizing these roles is essential to comprehend how substances interact with each other during chemical reactions.

Aqueous Chemical Reactions

In aqueous chemical reactions, substances react in water, utilizing it as a solvent. This environment enables participants in the reaction to move freely and interchange protons. Understanding reactions in such solutions is paramount in studying chemistry, as it is one of the most common mediums.
During reactions,

• All reactants and products exist in the aqueous phase.
• Protons (H+) are exchanged, mediated by a water environment.

Take the reaction:

• \( ext{HOCl} + ext{C}_6 ext{H}_5 ext{NH}_2 \rightleftharpoons ext{OCl}^- + ext{C}_6 ext{H}_5 ext{NH}_3^+\)

• In this water-based solution, \( ext{HOCl}\) gives away a proton (acting as an acid), while \( ext{C}_6 ext{H}_5 ext{NH}_2\) accepts it to become \( ext{C}_6 ext{H}_5 ext{NH}_3^+\) (acting as a base).

Water provides the medium necessary for such reactions to take place efficiently, facilitating the rearrangement and transformation of protons and other reactive substances.