Question

Consider the following generic equation: $$For which of the following pairs would this be the correct prototype equation for the acid-base reaction in solution? If it is not correct, write the proper equation for the acid-base reaction between the pair. (a) nitric acid and calcium hydroxide (b) hydrochloric acid and \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (c) hydrobromic acid and aqueous ammonia (d) perchloric acid and barium hydroxide (e) sodium hydroxide and nitrous acid\mathrm{H}^{+}(a q)+\mathrm{B}^{-}(a q) \longrightarrow \mathrm{HB}(a q)$$

Short answer

Based on the analysis and solutions for each pair of substances, the general acid-base reaction equation correctly represents pairs (a), (d), and (e). For pairs (b) and (c), the base accepts the proton instead of donating it. The proper equations for these reactions are: (b) $$\mathrm{H}^{+}(a q)+\mathrm{CH}_{3}\mathrm{NH}_{2}(a q) \longrightarrow \mathrm{CH}_{3}\mathrm{NH}_{3}^{+}(a q)$$ (c) $$\mathrm{H}^{+}(a q)+\mathrm{NH}_{3}(a q) \longrightarrow \mathrm{NH}_{4}^{+}(a q)$$

Step-by-step solution

Understand the given pairs of substances

The given pairs are: (a) nitric acid and calcium hydroxide (b) hydrochloric acid and \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (c) hydrobromic acid and aqueous ammonia (d) perchloric acid and barium hydroxide (e) sodium hydroxide and nitrous acid For each pair, we will identify the acid, its conjugate base, and the proper equation for the acid-base reaction.

Evaluate pair (a)

(a) nitric acid and calcium hydroxide Nitric acid (HNO\(_3\)) will donate a proton (H\(^+\)) while calcium hydroxide (Ca(OH)\(_2\)) will donate the hydroxide ion (OH\(^-\)). The general equation for this reaction is: $$\mathrm{H}^{+}(a q)+\mathrm{OH}^{-}(a q) \longrightarrow \mathrm{H}_{2}\mathrm{O}(l)$$

Evaluate pair (b)

(b) hydrochloric acid and \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) Hydrochloric acid (HCl) will donate a proton (H\(^+\)) and \(\mathrm{CH}_{3}\mathrm{NH}_{2}\) (methylamine) will act as the base, accepting the proton. The general equation for this reaction is: $$\mathrm{H}^{+}(a q)+\mathrm{CH}_{3}\mathrm{NH}_{2}(a q) \longrightarrow \mathrm{CH}_{3}\mathrm{NH}_{3}^{+}(a q)$$

Evaluate pair (c)

(c) hydrobromic acid and aqueous ammonia Hydrobromic acid (HBr) will donate a proton (H\(^+\)) and aqueous ammonia (NH\(_3\)) will act as the base, accepting the proton. The general equation for this reaction is: $$\mathrm{H}^{+}(a q)+\mathrm{NH}_{3}(a q) \longrightarrow \mathrm{NH}_{4}^{+}(a q)$$

Evaluate pair (d)

(d) perchloric acid and barium hydroxide Perchloric acid (HClO\(_4\)) will donate a proton (H\(^+\)) while barium hydroxide (Ba(OH)\(_2\)) will donate the hydroxide ion (OH\(^-\)). The general equation for this reaction is: $$\mathrm{H}^{+}(a q)+\mathrm{OH}^{-}(a q) \longrightarrow \mathrm{H}_{2}\mathrm{O}(l)$$

Evaluate pair (e)

(e) sodium hydroxide and nitrous acid Sodium hydroxide (NaOH) will donate the hydroxide ion (OH\(^-\)) and nitrous acid (HNO\(_2\)) will donate a proton (H\(^+\)). The general equation for this reaction is: $$\mathrm{H}^{+}(a q)+\mathrm{OH}^{-}(a q) \longrightarrow \mathrm{H}_{2}\mathrm{O}(l)$$ In summary, each pair of substances can be correctly represented by the general acid-base reaction equation given in the exercise, with the exception of pairs (b) and (c) where the base accepts the proton rather than donating it.

Key concepts

Acid-Base Neutralization

Acid-base neutralization is a fundamental chemical process where an acid and a base react with each other to form water and a salt. This is the very essence of many reactions occurring in both natural environments and industrial processes. Chemistry students often encounter basic equations that describe these neutralizations. For example, when a strong acid, like hydrochloric acid (HCl), reacts with a strong base, such as sodium hydroxide (NaOH), the result is water (H₂O) and sodium chloride (NaCl), a salt:
\[\begin{equation}H^+(aq) + OH^-(aq) \rightarrow H_2O(l)\end{equation}\]
In the context of the given exercise, pairs (a), (d), and (e) demonstrated typical acid-base neutralization reactions, creating water as one of the products. It's essential for students to recognize that neutralization typically involves the transfer of a proton (H⁺) from the acid to the hydroxide ion (OH^-) provided by the base. Understanding this classic reaction helps in grasping more complex acid-base relationships and their representations in chemical equations.

Proton Transfer Reactions

Proton transfer reactions are a cornerstone of acid-base chemistry. They describe the process by which a proton (H⁺), or hydrogen ion, is transferred from an acid to a base. This type of reaction is also called a Brønsted-Lowry acid-base reaction, where an acid is a proton donor and a base is a proton acceptor.
Considering the textbook exercise, for pairs (b) and (c), the reactions between hydrochloric acid and methylamine, and hydrobromic acid and aqueous ammonia, are classic examples. The acid donates a proton to the base, forming a positively charged ammonium ion:\[\begin{equation}\text{H}^+(aq) + \text{NH}_3(aq) \rightarrow \text{NH}_4^+(aq)\end{equation}\]
These reactions are crucial because they are ubiquitous in chemical and biological systems. Recognizing a proton transfer reaction helps understand various mechanisms, such as how enzymes function in the body or how pollutants are neutralized in the environment.

Strong Acids and Bases

Strong acids and bases completely dissociate in water, providing the maximum number of hydronium ions (H₃O⁺) or hydroxide ions (OH^-) respectively. This characteristic makes them highly reactive and effective at neutralizing one another. Examples of strong acids include hydrochloric acid (HCl) and perchloric acid (HClO₄), while strong bases include sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)₂).
In our exercise, nitric acid, hydrochloric acid, hydrobromic acid, and perchloric acid are all strong acids, whereas calcium hydroxide and barium hydroxide are strong bases. These substances, when mixed together in proper stoichiometric proportions, form water, as seen in pairs (a) and (d). It is important for students to distinguish these substances from their weak counterparts because it has implications for reaction completeness and the pH of the resulting solution.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions where reactants are transformed into products. Writing accurate chemical equations requires knowledge of reactants, products, and their states of matter. The reactions must also adhere to the law of conservation of mass, meaning the number of atoms of each element must be balanced on both sides of the equation.

With respect to the textbook exercise, chemical equations were written for the acid-base reactions, showcasing the necessary changes to properly represent the process. For instance, the generalized equation provided in the question was suitable for neutralization reactions where a hydronium ion (H⁺) combines with a hydroxide ion (OH^-) to form water. However, for reactions involving a transfer of a proton to a base that doesn't yield hydroxide ions, the chemical equation was adjusted to reflect the creation of a new product, such as in cases (b) and (c) where an ammonium ion is formed. This process of tailoring the equation to fit the specific reactants is crucial in capturing the essence of the chemical phenomenon accurately.