Question

If a substance is an Arrhenius base, is it necessarily a Brønsted-Lowry base? Is it necessarily a Lewis base? Explain.

Short answer

An Arrhenius base is necessarily a Brønsted-Lowry base since it produces hydroxide ions in water and can accept a proton (H+) from water. On the other hand, an Arrhenius base is not necessarily a Lewis base, as it is characterized by increasing the hydroxide ion concentration in an aqueous solution and not necessarily by donating an electron pair to form a coordinate covalent bond.

Step-by-step solution

Define Arrhenius bases

An Arrhenius base is a substance that, when dissolved in water, increases the concentration of hydroxide ions (OH-) in the solution. It's characterized by producing hydroxide ions in aqueous solution.

Define Brønsted-Lowry bases

A Brønsted-Lowry base is a substance that can accept a proton (H+) from another substance (called a Brønsted-Lowry acid), forming a conjugate acid/base pair. A Brønsted-Lowry base doesn't need to be in aqueous solution to act as a base.

Define Lewis bases

A Lewis base is a substance that can donate an electron pair to another substance (called a Lewis acid), forming a coordinate covalent bond between them. Lewis bases don't need to be in aqueous solution or accept H+ ions to act as a base.

Compare Arrhenius and Brønsted-Lowry bases

Since an Arrhenius base produces hydroxide ions in water, it can accept a proton (H+) from water, which acts as a Brønsted-Lowry acid. In this reaction, the base and the water molecule form a conjugate acid/base pair. Therefore, an Arrhenius base can act as a Brønsted-Lowry base.

Compare Arrhenius and Lewis bases

An Arrhenius base is not necessarily a Lewis base since it's characterised by increasing the hydroxide ion concentration in an aqueous solution, not necessarily by donating an electron pair to form a coordinate covalent bond. Although some Arrhenius bases can also act as Lewis bases, it doesn't apply to all Arrhenius bases. In conclusion, an Arrhenius base is necessarily a Brønsted-Lowry base, but it is not necessarily a Lewis base.

Key concepts

Arrhenius Base

An Arrhenius base is a foundational concept in acid-base chemistry. According to Svante Arrhenius, a substance is deemed an Arrhenius base if it increases the concentration of hydroxide ions (OH\(^-\)) in an aqueous solution. This means that when you dissolve an Arrhenius base in water, it releases hydroxide ions into the solution, making the solution more basic.
The classic example of an Arrhenius base is sodium hydroxide (NaOH). When NaOH is dissolved in water, it dissociates completely to produce sodium ions (Na\(^+\)) and hydroxide ions (OH\(^-\)): \[ \text{NaOH (s)} \rightarrow \text{Na}^{+} (aq) + \text{OH}^{-} (aq) \] Some key points about Arrhenius bases include:

• Only applicable in aqueous solutions.
• They increase the pH of the solution by increasing the concentration of OH\(^-\).
• Useful in understanding simple acid-base reactions in water.

It's essential to realize that the Arrhenius definition is specific to water-based solutions and does not capture the full scope of how substances can behave as bases in non-aqueous environments.

Brønsted-Lowry Base

The Brønsted-Lowry model expands the concept of bases and is one of the most widely used in chemistry today. According to this theory, a base is any substance that can accept a proton (H\(^+\)) from another substance acting as an acid. This means a base does not rely solely on hydroxide ions, as in the Arrhenius definition.
Take for instance ammonia (NH\(_3\)), which can accept a proton to become ammonium (NH\(_4^+\)): \[\text{NH}_3 + \text{H}^+ \rightarrow \text{NH}_4^+\] Key characteristics of Brønsted-Lowry bases:

• Not limited to aqueous solutions; they can operate in various phases, including gases and solids.
• Focuses on proton transfer, allowing more reactions to be explained under this framework.
• Every Brønsted-Lowry base paired with an acid forms a conjugate acid-base pair.

By this definition, an Arrhenius base fits the Brønsted-Lowry criteria because hydroxide ions readily accept protons to form water, making every Arrhenius base a Brønsted-Lowry base.

Lewis Base

The Lewis base concept takes a different approach to defining bases by focusing on electron pairs instead of protons. According to Gilbert Lewis, a base is a substance that can donate an electron pair to form a covalent bond with a Lewis acid (an electron-pair acceptor).
Unlike Arrhenius and Brønsted-Lowry bases, Lewis bases do not require water or protons to perform their role. For example, consider the reaction between ammonia (NH\(_3\)) and boron trifluoride (BF\(_3\)). Here, ammonia donates its lone pair to the electron-deficient boron, forming a coordinate covalent bond: \[\text{NH}_3 + \text{BF}_3 \rightarrow \text{NH}_3\text{BF}_3\] Some pivotal attributes of Lewis bases include:

• Not limited by the need for protons; it's all about electron pair donation.
• Applicable to a vast range of reactions, including those in organic and inorganic chemistry.
• Reveal the mechanism of many complex reactions beyond simple acid-base interactions.

While some Arrhenius bases can act as Lewis bases if they can donate electron pairs, not all of them do, explaining why not every Arrhenius base qualifies as a Lewis base.