Question

(a) Identify the Bronsted-Lowry acid and the BrønstedLowry base in the following reaction: (b) Identify the Lewis acid and the Lewis base in the reaction. [Sections \(16.2\) and \(16.11]\)

Short answer

In the reaction NH3(aq) + H2O(l) ⇌ NH4⁺(aq) + OH⁻(aq), the Bronsted-Lowry Acid is H2O, the Bronsted-Lowry Base is NH3, the Lewis Acid is H2O, and the Lewis Base is NH3.

Step-by-step solution

Identify the Bronsted-Lowry Acid and Base

In this step, we determine which species donates a proton and which species accepts a proton. In the given reaction, NH3 accepts a proton from H2O to form NH4⁺, while H2O donates a proton to form OH⁻. Thus: - Bronsted-Lowry Acid: H2O - Bronsted-Lowry Base: NH3

Identify the Lewis Acid and Base

In this step, we determine which species donates an electron pair and which species accepts an electron pair. Since NH3 has a lone pair of electrons, it donates an electron pair to H2O to share a proton and form the NH4⁺ ion. So: - Lewis Base: NH3 In the case of H2O sharing a proton with NH3, it can be considered as the species accepting an electron pair. Thus: - Lewis Acid: H2O To summarize, in the reaction NH3(aq) + H2O(l) ⇌ NH4⁺(aq) + OH⁻(aq): - Bronsted-Lowry Acid: H2O - Bronsted-Lowry Base: NH3 - Lewis Acid: H2O - Lewis Base: NH3

Key concepts

Brønsted-Lowry Acid-Base Theory

The Brønsted-Lowry theory is all about protons, or hydrogen ions ( H^+ ). This is a very handy way to look at reactions where proton transfer happens between molecules. In this theory, an acid is defined as a substance that donates a proton, and a base is a substance that accepts a proton. For example, in the reaction of NH_3 with H_2O , water ( H_2O ) acts as the acid. It donates a proton ( H^+ ) to ammonia ( NH_3 ), which plays the role of the base. So:

• Brønsted-Lowry Acid: H_2O (because it donates a proton)
• Brønsted-Lowry Base: NH_3 (because it accepts a proton)

Understanding this theory makes predicting the outcomes of reactions much simpler. Emphasizing proton movement helps clarify how substances change in these reactions.

Lewis Acid-Base Theory

In contrast to the Brønsted-Lowry theory, the Lewis acid-base theory focuses on electron pairs rather than protons. It describes acids and bases in terms of their ability to accept or donate electron pairs. Here, a Lewis acid is a species that can accept an electron pair, while a Lewis base is a species that can donate an electron pair. In the example reaction, ammonia ( NH_3 ) provides a lone pair of electrons to the water molecule. This donation of electrons allows NH_3 to connect to the H_2O molecule, forming NH_4^+ . Hence:

• Lewis Acid: H_2O (accepts an electron pair)
• Lewis Base: NH_3 (donates an electron pair)

This theory broadens our understanding of acid-base interactions beyond just protons by highlighting the importance of electron exchange.

Proton Transfer

Proton transfer is a crucial concept in acid-base chemistry, particularly in the Brønsted-Lowry framework. It involves the movement of a hydrogen ion ( H^+ ) from one molecule to another. This transfer of protons is what classifies substances as acids or bases in the Brønsted-Lowry theory.
In our particular reaction, proton transfer occurs when H_2O donates a proton to NH_3 , transforming into OH^- and forming NH_4^+ from NH_3 . This fundamental movement of protons is why such reactions are often called proton-exchange reactions.

• Proton Donor (Acid): H_2O
• Proton Acceptor (Base): NH_3

It's this transfer that enables acids and bases to neutralize one another and allows for the transformation of molecules.

Electron Pair Donation

Electron pair donation is a core concept in the Lewis theory of acids and bases. It describes how a base donates its pair of electrons to form a new bond with an acid. In our provided reaction, ammonia ( NH_3 ) acts as the Lewis base by donating an electron pair to H_2O .
The electron pair donation from NH_3 forms a bond between the nitrogen atom of ammonia and the hydrogen ion from water, resulting in the ammonium ion ( NH_4^+ ). This shows:

• Electron Pair Donor (Base): NH_3
• Electron Pair Acceptor (Acid): H_2O

This process is central in reactions where electron-sharing creates larger or more complex molecules. It's essential in understanding reactions beyond simple proton transfers.