Problem 31 - Chemistry in Focus

Question

Identify the Bronsted-Lowry acid and base in each of the following reactions: a. \(\mathrm{HClO}_{2}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{ClO}_{2}^{-}\) b. \(\mathrm{CH}_{3} \mathrm{NH}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{OH}^{-}+\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\) c. \(\mathrm{HF}+\mathrm{NH}_{3} \rightleftharpoons \mathrm{NH}_{4}^{+}+\mathrm{F}^{-}\)

Step-by-Step Solution

Answer

a. Acid: \(\mathrm{HClO}_2\), Base: \(\mathrm{H}_2\mathrm{O}\) b. Acid: \(\mathrm{CH}_3\mathrm{NH}_2\), Base: \(\mathrm{H}_2\mathrm{O}\) c. Acid: \(\mathrm{HF}\), Base: \(\mathrm{NH}_3\)

Step 1: Understanding the Bronsted-Lowry Acid-Base Theory

The Bronsted-Lowry theory defines an acid as a proton (H+) donor and a base as a proton acceptor. In each reaction, identify the species that donates a proton and the one that accepts it.

Step 2: Identifying the Acid and Base in Reaction a

Examine the reaction \(\mathrm{HClO}_{2}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{H}_{3}\mathrm{O}^{+}+\mathrm{ClO}_{2}^{-}\). The acid is the species that donates a hydrogen ion (proton), which is \(\mathrm{HClO}_2\) because it loses a hydrogen ion to form \(\mathrm{ClO}_2^{-}\). The base is \(\mathrm{H}_2\mathrm{O}\) because it accepts a hydrogen ion to form \(\mathrm{H}_3\mathrm{O}^{+}\).

Step 3: Identifying the Acid and Base in Reaction b

Look at the reaction \(\mathrm{CH}_{3} \mathrm{NH}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{OH}^{-}+\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\). The acid here is \(\mathrm{CH}_3\mathrm{NH}_2\) as it donates a proton to become \(\mathrm{CH}_3\mathrm{NH}_3^{+}\). The base is \(\mathrm{H}_2\mathrm{O}\) because it accepts a proton and becomes \(\mathrm{OH}^{-}\).

Step 4: Identifying the Acid and Base in Reaction c

Consider the reaction \(\mathrm{HF}+\mathrm{NH}_{3} \rightleftharpoons \mathrm{NH}_{4}^{+}+\mathrm{F}^{-}\). \(\mathrm{HF}\) is the acid because it donates a proton to become \(\mathrm{F}^{-}\). The base is \(\mathrm{NH}_{3}\) as it accepts the proton to form \(\mathrm{NH}_{4}^{+}\).

Key Concepts

Acid-Base Reactions

The dance of protons between substances defines acid-base reactions, a fundamental aspect of chemical processes. According to the Bronsted-Lowry theory, these reactions involve a transfer of protons (H⁺) from one substance to another. In simplistic terms, when an acid and a base come into contact, the acid will donate a proton to the base. For instance, in the reaction of hydrochloric acid (HCl) with water (H₂O), HCl donates a proton to become chloride (Cl⁻), and water accepts this proton, transforming into hydronium (H₃O⁺).

To improve understanding, visualizing the proton transfer as a 'hand-off' in a relay race can be helpful. The substance 'holding' the proton initially is the acid, and it 'passes' this proton to the base. Analyzing the direction of this proton transfer can clarify which is the acid and which the base in a reaction. Moreover, recognizing that these reactions can be reversible is crucial. The products can act as an acid or base themselves, potentially donating or accepting protons in a dynamic equilibrium.

Proton Donor and Acceptor

In the narrative of acid-base reactions, key players are the proton donors and acceptors. A proton donor, which we refer to as an acid, has a penchant for giving away a proton, while the proton acceptor, or base, has a tendency to receive it. These roles are not random but are determined by the molecular structure and the presence of replaceable hydrogen atoms for acids or lone pairs of electrons for bases.

Imagine a proton donor as a gracious host at a party, willing to give a gift (the proton) to a guest—the proton acceptor. A solid grasp of this concept allows one to dissect a reaction, such as \(\mathrm{HClO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_3\mathrm{O}^+ + \mathrm{ClO}_2^{-}\), and understand that \(\mathrm{HClO}_2\) is the donor turning to \(\mathrm{ClO}_2^{-}\) after the 'handoff,' while water, by accepting, transforms into \(\mathrm{H}_3\mathrm{O}^+\).

To make this even clearer, you could imagine a label that read 'proton donor' suddenly jumping from one molecule to the other, signifying the moment the identity of the acid and base is exchanged during the reaction.

Chemical Equilibrium

When acid-base reactions are reversible, they reach a state known as chemical equilibrium. This doesn't mean that the reactions have stopped; instead, they're occurring at equal rates in both directions, balancing out overall effects. At equilibrium, the ratio of the concentrations of products to reactants remains constant. It's like a bustling marketplace where the amount of goods being bought is equal to the amount being sold.

Looking at the reaction \(\mathrm{CH}_3\mathrm{NH}_2 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{OH}^- + \mathrm{CH}_3\mathrm{NH}_3^+\), both forward and backward reactions are occurring simultaneously. In the beginning, the forward reaction might be more prominent as the reactants are abundant. Over time, as products accumulate, the reverse reaction becomes more frequent until a balance is achieved. This equilibrium can be represented by a double arrow, signifying the continuous but balanced exchange. Understanding equilibrium is essential for predicting how a reaction will respond to changes in conditions like concentration or temperature, governed by Le Chatelier's principle. This principle helps us predict that, if more acid were added to this system, the reaction would shift to produce more products to restore equilibrium – a testament to the adaptable nature of chemical systems.