Question

Which of the following are Brønsted-Lowry bases but not Arrhenius bases? (a) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{HCO}_{3}^{-}\) (d) \(\mathrm{NaOH}\)

Short answer

\(\mathrm{NH}_{3}\) and \(\mathrm{HCO}_{3}^{-}\) are the Brønsted-Lowry bases that are not Arrhenius bases.

Step-by-step solution

Understand Arrhenius Base Definition

An Arrhenius base is a substance that increases the concentration of hydroxide ions (\(OH^-\)) when dissolved in water. Essentially, it must release \(OH^-\) ions into solution.

Understand Brønsted-Lowry Base Definition

A Brønsted-Lowry base is a molecule or ion that can accept a proton (\(H^+\)) from another substance. This definition is broader than the Arrhenius definition.

Analyze the Compounds One by One

Examine each compound to see if it fits the criteria for only a Brønsted-Lowry base and not an Arrhenius base.

Evaluate \(\mathrm{Ca(OH)_{2}}\)

\(\mathrm{Ca(OH)_{2}}\) is an Arrhenius base because it dissociates in water to release hydroxide ions. Therefore, it is not only a Brønsted-Lowry base.

Evaluate \(\mathrm{NH}_{3}\)

\(\mathrm{NH}_{3}\) (ammonia) is a Brønsted-Lowry base because it can accept a proton from water to form \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{OH}^{-}\), but it does not release \(\mathrm{OH}^{-}\) directly into water, qualifying it not to be an Arrhenius base.

Evaluate \(\mathrm{HCO}_{3}^{-}\)

\(\mathrm{HCO}_{3}^{-}\) (bicarbonate) can accept a proton to form \(\mathrm{H_2CO_3}\), acting as a Brønsted-Lowry base. It does not produce \(\mathrm{OH}^{-}\) directly, so it is not an Arrhenius base.

Evaluate \(\mathrm{NaOH}\)

\(\mathrm{NaOH}\) is an Arrhenius base because it dissociates in water to release \(\mathrm{OH}^{-}\). Therefore, it is also a Brønsted-Lowry base, but not only a Brønsted-Lowry base.

Select the Correct Answers Based on Analysis

Based on the analysis, \(\mathrm{NH}_{3}\) and \(\mathrm{HCO}_{3}^{-}\) are Brønsted-Lowry bases but not Arrhenius bases.

Key concepts

Arrhenius bases

Arrhenius bases are substances that increase the concentration of hydroxide ions (\( OH^- \)) in a solution when they are dissolved in water. This is a relatively narrow definition compared to others. For something to be classified as an Arrhenius base, it must have a direct contribution of \( OH^- \) ions to the solution:

• This means the substance dissociates in water to release \( OH^- \)
• Examples include substances like sodium hydroxide (\( \text{NaOH} \)) and calcium hydroxide (\( \text{Ca(OH)}_2 \))
• These substances are typical of strong bases, which ionize completely in water

Arrhenius's theory was historically significant, helping to identify and classify different bases, but there are limitations to its use, especially when considering chemical reactions that don’t occur in aqueous solutions.

proton acceptors

Brønsted-Lowry bases, often known as proton acceptors, encompass a broader range of substances compared to Arrhenius bases. A Brønsted-Lowry base is any species that can accept a proton (\( H^+ \)). This definition doesn’t focus on the hydroxide ions but rather the ability to gain protons, which allows for a greater diversity of base candidates. Here’s what sets them apart:

• They can be molecules or ions – think ammonia (\( NH_3 \)), which accepts protons to form amines.
• They don't need to contain or produce \( OH^- \)
• This concept applies in both aqueous and non-aqueous environments, broadening its application to many chemical reactions

Because of this broad definition, Brønsted-Lowry's theory is often preferred when explaining more complex chemical behavior where proton exchange is involved.

hydroxide ions in solution

Hydroxide ions (\( OH^- \)) play a crucial role in the classification of Arrhenius bases. These ions are formed when certain bases dissociate in water. The presence of hydroxide ions is what makes solutions basic, and their concentration determines the pH of the solution:

• The dissociation of substances like \( \text{NaOH} \) in water directly increases \( OH^- \)
• In an Arrhenius context, the focus is on this production of hydroxide ions.
• Strong bases that dissociate completely contribute significantly to the concentration of \( OH^- \)

While the presence of hydroxide ions is crucial for Arrhenius bases, it cannot always explain reactions where no water is involved or where compounds do not directly release these ions. Understanding hydroxide ions is essential in grasping the characteristics of classic bases but knowing their limitations is just as important.