Question

Write a balanced net ionic equation for the principal reaction in solutions of each of the following salts. In each case, identify the Brensted-Lowry acids and bases and the conjugate acid-base pairs. (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (c) \(\mathrm{NaCl}\) (d) \(\mathrm{ZnCl}_{2}\)

Short answer

(a) \( \mathrm{CO}_{3}^{2-} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{HCO}_{3}^{-} + \mathrm{OH}^{-} \); (b) \( \mathrm{NH}_{4}^{+} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{NH}_{3} + \mathrm{H}_{3} \mathrm{O}^{+} \); (c) No reaction; (d) \( \mathrm{Zn}^{2+} + 2\mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{Zn(OH)}^{+} + \mathrm{H}_{3} \mathrm{O}^{+} \).

Step-by-step solution

Dissociate the Salts (Part a)

For \( \mathrm{Na}_{2} \mathrm{CO}_{3} \), it dissociates into \( 2\mathrm{Na}^{+} \) and \( \mathrm{CO}_{3}^{2-} \) in solution. Since \( \mathrm{CO}_{3}^{2-} \) can react with water, we write the net ionic equation for this interaction: \[ \mathrm{CO}_{3}^{2-} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{HCO}_{3}^{-} + \mathrm{OH}^{-} \]. Here, \( \mathrm{H}_{2} \mathrm{O} \) acts as a Bronsted-Lowry acid, donating a proton to \( \mathrm{CO}_{3}^{2-} \).

Identify Conjugate Pairs (Part a)

In the equation \( \mathrm{CO}_{3}^{2-} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{HCO}_{3}^{-} + \mathrm{OH}^{-} \), the conjugate acid-base pairs are \( \mathrm{H}_{2} \mathrm{O} \) and \( \mathrm{OH}^{-} \), and \( \mathrm{CO}_{3}^{2-} \) and \( \mathrm{HCO}_{3}^{-} \). The reaction is a base-acid reaction.

Dissociate the Salts (Part b)

\( \mathrm{NH}_{4} \mathrm{NO}_{3} \) dissociates into \( \mathrm{NH}_{4}^{+} \) and \( \mathrm{NO}_{3}^{-} \). The \( \mathrm{NH}_{4}^{+} \) ion can interact with water: \[ \mathrm{NH}_{4}^{+} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{NH}_{3} + \mathrm{H}_{3} \mathrm{O}^{+} \]. \( \mathrm{NH}_{4}^{+} \) is the acid here, donating a proton.

Identify Conjugate Pairs (Part b)

In the reaction \( \mathrm{NH}_{4}^{+} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{NH}_{3} + \mathrm{H}_{3} \mathrm{O}^{+} \), the conjugate pairs are \( \mathrm{NH}_{4}^{+} \) and \( \mathrm{NH}_{3} \), \( \mathrm{H}_{2} \mathrm{O} \) and \( \mathrm{H}_{3} \mathrm{O}^{+} \). \( \mathrm{NH}_{4}^{+} \) acts as a Bronsted-Lowry acid.

Dissociate the Salts (Part c)

\( \mathrm{NaCl} \) dissociates into \( \mathrm{Na}^{+} \) and \( \mathrm{Cl}^{-} \). Neither ion reacts with water to form another acid or base, so no principal reaction occurs, and both ions are spectator ions.

No Reaction for Part c

Since neither \( \mathrm{Na}^{+} \) nor \( \mathrm{Cl}^{-} \) acts as an acid or base, no net ionic equation occurs in this case.

Dissociate the Salts (Part d)

\( \mathrm{ZnCl}_{2} \) dissociates into \( \mathrm{Zn}^{2+} \) and \( 2\mathrm{Cl}^{-} \). \( \mathrm{Zn}^{2+} \) can hydrolyze in water, reacting with \( \mathrm{H}_{2} \mathrm{O} \): \[ \mathrm{Zn}^{2+} + 2\mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{Zn(OH)}^{+} + \mathrm{H}_{3} \mathrm{O}^{+} \].

Identify Conjugate Pairs (Part d)

In the reaction \( \mathrm{Zn}^{2+} + 2\mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{Zn(OH)}^{+} + \mathrm{H}_{3} \mathrm{O}^{+} \), the pairs are \( \mathrm{H}_{2} \mathrm{O} \) and \( \mathrm{H}_{3} \mathrm{O}^{+} \), and \( \mathrm{Zn}^{2+} \) with \( \mathrm{Zn(OH)}^{+} \). \( \mathrm{Zn}^{2+} \) acts as a Bronsted-Lowry acid.

Key concepts

Bronsted-Lowry acid

When it comes to acid-base chemistry, a crucial concept is the Bronsted-Lowry theory. This theory describes acids as compounds that donate protons (H\(^+\)) and bases as those that accept protons. For example, in the reaction involving ammonium nitrate, \(\mathrm{NH}_{4}^{+} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{NH}_{3} + \mathrm{H}_{3} \mathrm{O}^{+}\), \(\mathrm{NH}_{4}^{+}\) is the Bronsted-Lowry acid as it donates a proton to water, forming \(\mathrm{NH}_{3}\) and \(\mathrm{H}_{3} \mathrm{O}^{+}\). Similarly, in the reaction with \(\mathrm{ZnCl}_{2}\), \(\mathrm{Zn}^{2+}\) acts as a Bronsted-Lowry acid by donating a proton to water, resulting in zinc hydroxide and hydronium ions.

conjugate acid-base pairs

In every acid-base reaction, conjugate acid-base pairs play a key role. These pairs result from the transfer of a proton from the acid to the base. Once an acid donates its proton, it forms its conjugate base. Conversely, the base that accepts a proton forms its conjugate acid.
For instance:

• In the reaction \(\mathrm{CO}_{3}^{2-} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{HCO}_{3}^{-} + \mathrm{OH}^{-}\), \(\mathrm{CO}_{3}^{2-}\) forms \(\mathrm{HCO}_{3}^{-}\) as its conjugate acid, and water forms \(\mathrm{OH}^{-}\) as its conjugate base.
• In \(\mathrm{NH}_{4}^{+} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{NH}_{3} + \mathrm{H}_{3} \mathrm{O}^{+}\), \(\mathrm{NH}_{4}^{+}\) becomes \(\mathrm{NH}_{3}\) and water forms \(\mathrm{H}_{3} \mathrm{O}^{+}\) as conjugate acid-base pairs.

Understanding these pairs helps to predict the direction of acid-base equilibrium reactions.

salt dissociation

Salt dissociation is a process where a salt compound breaks down into its component ions when dissolved in water. This step is crucial in determining whether a reaction will occur in an aqueous solution.

• For example, baking soda, \(\mathrm{Na}_{2} \mathrm{CO}_{3}\), dissociates into \(2\mathrm{Na}^{+}\) and \(\mathrm{CO}_{3}^{2-}\) ions.
• Ammonium nitrate, \(\mathrm{NH}_{4} \mathrm{NO}_{3}\), separates into \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{NO}_{3}^{-}\).
• Table salt, \(\mathrm{NaCl}\), breaks into \(\mathrm{Na}^{+}\) and \(\mathrm{Cl}^{-}\), although neither reacts further with water in a noteworthy manner.

Sourcing specific ions from a salt enables reactions like those between \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{CO}_{3}^{2-}\) with water to occur.

hydrolysis reactions

When ionic compounds dissolve and react with water, this is known as hydrolysis. Hydrolysis reactions are an interaction where either a cation or anion from a salt reacts with water to affect the solution's pH.
For example, the \(\mathrm{CO}_{3}^{2-}\) ion from Sodium carbonate reacts with water in the equation \(\mathrm{CO}_{3}^{2-} + \mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{HCO}_{3}^{-} + \mathrm{OH}^{-}\). This increases the hydroxide ion concentration, making the solution more basic. Similarly, in the case of \(\mathrm{ZnCl}_{2}\), \(\mathrm{Zn}^{2+}\) undergoes hydrolysis: \(\mathrm{Zn}^{2+} + 2\mathrm{H}_{2} \mathrm{O} \leftrightarrow \mathrm{Zn(OH)}^{+} + \mathrm{H}_{3} \mathrm{O}^{+}\). This reaction introduces more \(\mathrm{H}_{3} \mathrm{O}^{+}\), making the solution more acidic. Hydrolysis explains why some solutions from salt dissolutions become acidic or basic.