Question

Classify the following as Arrhenius, Bronsted-Lowry, or Lewis acid-base reactions. A reaction may fit all, two, one, or none of the categories: (a) \(\mathrm{Cu}^{2+}+4 \mathrm{Cl}^{-} \rightleftharpoons \mathrm{CuCl}_{4}^{2-}\) (b) \(\mathrm{Al}(\mathrm{OH})_{3}+3 \mathrm{HNO}_{3} \rightleftharpoons \mathrm{Al}^{3+}+3 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{NO}_{3}^{-}\) (c) \(\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightleftharpoons 2 \mathrm{NH}_{3}\) (d) \(\mathrm{CN}^{-}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HCN}+\mathrm{OH}^{-}\)

Short answer

(a) Lewis. (b) Arrhenius and Bronsted-Lowry. (c) None. (d) Bronsted-Lowry.

Step-by-step solution

- Analyze reaction (a)

Consider the reaction: \textbf{(a) } \text{ \( \mathrm{Cu}^{2+}+4 \mathrm{Cl}^{-} \rightleftharpoons \mathrm{CuCl}_{4}^{2-} \)} Here, \( \mathrm{Cu}^{2+} \) accepts pairs of electrons from \( \mathrm{Cl}^{-} \) ions to form \( \mathrm{CuCl}_{4}^{2-} \). This fits the definition of a Lewis acid (electron pair acceptor) and a Lewis base (electron pair donor). It is not an Arrhenius or Bronsted-Lowry reaction because there is no proton transfer or production of \( \mathrm{H}^{+} \).

- Analyze reaction (b)

Consider the reaction: \textbf{(b) } \text{ \( \mathrm{Al}(\mathrm{OH})_{3}+3 \mathrm{HNO}_{3} \rightleftharpoons \mathrm{Al}^{3+}+3 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{NO}_{3}^{-} \)} In this reaction, \( \mathrm{HNO}_{3} \) provides \( \mathrm{H}^{+} \) ions, which react with \( \mathrm{OH}^{-} \) ions from \( \mathrm{Al}(\mathrm{OH})_{3} \) to form \( \mathrm{H}_{2} \mathrm{O} \). This process involves proton transfer, fitting both Arrhenius and Bronsted-Lowry acid-base reaction definitions. \( \mathrm{HNO}_{3} \) is the Arrhenius acid (provides \( \mathrm{H}^{+} \)) and Bronsted-Lowry acid (proton donor).

- Analyze reaction (c)

Consider the reaction: \textbf{(c) } \text{ \( \mathrm{N}_{2}+3 \mathrm{H}_{2} \rightleftharpoons 2 \mathrm{NH}_{3} \)} In this reaction, nitrogen and hydrogen gases react to form ammonia. There is no transfer of protons or electron pairs between species; thus, it does not fit Arrhenius, Bronsted-Lowry, or Lewis acid-base definitions.

- Analyze reaction (d)

Consider the reaction: \textbf{(d) } \text{ \( \mathrm{CN}^{-}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HCN}+\mathrm{OH}^{-} \)} Here, \( \mathrm{CN}^{-} \) accepts a proton from \( \mathrm{H}_{2} \mathrm{O} \), generating \( \mathrm{HCN} \) and \( \mathrm{OH}^{-} \). This involves proton transfer, making it a Bronsted-Lowry acid-base reaction. \( \mathrm{CN}^{-} \) is a Bronsted-Lowry base (proton acceptor), and \( \mathrm{H_{2}O} \) is a Bronsted-Lowry acid (proton donor). It does not fit the Arrhenius or Lewis definitions.

Key concepts

Arrhenius acid

An Arrhenius acid is a substance that increases the concentration of hydrogen ions (\( \text{H}^{+} \)) when dissolved in water. For example, when hydrochloric acid (\( \text{HCl} \)) is added to water, it dissociates to form \( \text{H}^{+} \) and \( \text{Cl}^{-} \) ions. This increase in \( \text{H}^{+} \) ions makes \( \text{HCl} \) an Arrhenius acid. Arrhenius acids typically produce sour tastes, act as electrolytes, and react with bases to produce water and a salt. Examples include common acids like:

• Hydrochloric acid (\( \text{HCl} \))
• Sulfuric acid (\( \text{H}_2\text{SO}_4 \))
• Nitric acid (\( \text{HNO}_3 \))

Understanding the Arrhenius concept is helpful for simple acid-base reactions in aqueous solutions. However, it does have limitations, especially when dealing with non-aqueous systems.

Bronsted-Lowry acid

A Bronsted-Lowry acid is defined as a proton donor. This concept broadens the definition of acids and bases beyond just aqueous solutions. When a substance donates a proton (\( \text{H}^{+} \)), it is considered a Bronsted-Lowry acid.
For instance, in the reaction: \text{\( \text{NH}_4^+ + \text{H}_2\text{O} \rightleftharpoons \text{NH}_3 + \text{H}_3\text{O}^+ \)}
Here, \( \text{NH}_4^+ \) donates a proton to water (\( \text{H}_2\text{O} \)), forming ammonia (\( \text{NH}_3 \)) and hydronium ion (\( \text{H}_3\text{O}^+ \)). Because \( \text{NH}_4^+ \) donates a proton, it is a Bronsted-Lowry acid. The Bronsted-Lowry concept is particularly useful because it applies to a wider range of chemical reactions, including those not in an aqueous medium. Examples include:

• Ammonium ion (\( \text{NH}_4^+ \))
• Acetic acid (\( \text{CH}_3\text{COOH} \))

It allows us to understand acid-base reactions in gases and other phases.

Lewis acid

A Lewis acid is defined as an electron pair acceptor. This definition expands the concept of acids to include more than just proton donors or substances that increase \( \text{H}^{+} \) concentration. For instance, in the reaction:
\( \text{BF}_3 + \text{NH}_3 \rightleftharpoons \text{F}_3\text{B}-\text{NH}_3 \)
Here, \( \text{BF}_3 \) accepts an electron pair from \( \text{NH}_3 \) to form a bond. Therefore, \( \text{BF}_3 \) is a Lewis acid because it accepts an electron pair. Lewis acids are commonly found in many reactions involving metals and ligands. Examples of Lewis acids include:

• Boron trifluoride (\( \text{BF}_3 \))
• Aluminum chloride (\( \text{AlCl}_3 \))
• Iron (III) chloride (FeCl\text{\( _3 \)})

The Lewis concept allows the classification of more complex chemical interactions, significantly beyond the aqueous reactions that Arrhenius acids cover.