Question

Which of the following statements is false? (a) An Arrhenius base increases the concentration of OH \(^{-}\) in water. (b) A Bronsted-Lowry base is a proton acceptor. (c) Water can act as a Bronsted-Lowry acid. (d) Water can act as a Bronsted-Lowry base. (e) Any compound that contains an \(-\)OH group acts as a Bronsted-Lowry base.

Short answer

The false statement is (e) - Any compound that contains an \(-\)OH group acts as a Bronsted-Lowry base. Not all compounds with an \(-\)OH group act as Bronsted-Lowry bases, such as alcohols.

Step-by-step solution

Identify Arrhenius base properties

According to the Arrhenius theory, a base is a substance that increases the concentration of hydroxide ions (OH\(^⁻\)) in water when dissolved. That means an Arrhenius base makes the solution more alkaline.

Identify Bronsted-Lowry base and acid properties

The Bronsted-Lowry theory defines a base as a proton (H\(^+\)) acceptor and an acid as a proton donor. In this theory, bases and acids can only react with each other in a process called proton transfer.

Recognize the role of water in the Bronsted-Lowry theory

According to the Bronsted-Lowry theory, water can indeed act as both an acid and a base. For example, when an acid dissolves in water, water will act as a base by accepting a proton. Conversely, when a base dissolves in water, water will act as an acid by donating a proton.

Considering compounds with an \(-\)OH group

Compounds with an \(-\)OH group can sometimes act as Bronsted-Lowry bases by accepting a proton. However, not all compounds with an \(-\)OH group will necessarily act as a Bronsted-Lowry base. For example, alcohols such as ethanol (C\(_2\)H\(_5\)OH) have an \(-\)OH group but do not behave as Bronsted-Lowry bases in reactions.

Verify each statement from the exercise

(a) An Arrhenius base increases the concentration of OH\(^⁻\) in water - This statement is true. (b) A Bronsted-Lowry base is a proton acceptor - This statement is also true. (c) Water can act as a Bronsted-Lowry acid - This statement is true. (d) Water can act as a Bronsted-Lowry base - This statement is true. (e) Any compound that contains an \(-\)OH group acts as a Bronsted-Lowry base - This statement is false, as not all compounds with an \(-\)OH group act as Bronsted-Lowry bases, such as alcohols. The false statement is (e).

Key concepts

Arrhenius Base

The concept of an Arrhenius base is rooted in one of the earliest theories of acids and bases. According to Svante Arrhenius, a base is any substance that, when dissolved in water, results in an increase in the hydroxide ion (OH\(^-\)) concentration. This definition highlights the importance of water as the solvent in which this behavior occurs.
Arrhenius bases typically include substances like sodium hydroxide (NaOH) and potassium hydroxide (KOH). When these compounds dissolve in water, they dissociate to release hydroxide ions into the solution, making it more alkaline:

• NaOH (aq) → Na\(^+\) (aq) + OH\(^-\) (aq)
• KOH (aq) → K\(^+\) (aq) + OH\(^-\) (aq)

The presence of high concentrations of hydroxide ions increases the pH, indicating basicity. This simplicity in definition is what makes the Arrhenius theory often used in introductory chemistry discussions about bases.

Bronsted-Lowry Theory

The Bronsted-Lowry theory broadens the concept of acids and bases beyond the limits of water-based solutions. This theory defines bases as proton (H\(^+\)) acceptors and acids as proton donors. This is more inclusive than the Arrhenius theory.
A typical reaction demonstrating the Bronsted-Lowry theory is the interaction between ammonia (NH\(_3\)) and water:

• NH\(_3\) + H\(_2\)O → NH\(_4\)\(^+\) + OH\(^-\)

In this reaction, ammonia acts as the base by accepting a proton from water, which in turn behaves as the acid by donating a proton. The flexibility of the Bronsted-Lowry theory allows it to explain acid-base reactions that don't occur in aqueous solutions, expanding the understanding of chemical reactions.

Hydroxide Ion

The hydroxide ion (OH\(^-\)) is fundamental to many acid-base reactions, central to both the Arrhenius and Bronsted-Lowry theories. It consists of one oxygen atom bonded to one hydrogen atom, carrying a negative charge. This ion is crucial in determining the properties of a solution.
In terms of the Arrhenius theory, the generation of hydroxide ions is a defining feature of a base. As they accumulate in solution, they increase the pH value, signifying that the solution is basic rather than acidic.
However, not all compounds with an \(-\)OH group will release hydroxide ions liberally. For example, alcohols (like methanol or ethanol) contain \(-\)OH groups but remain neutral in water without significant release of OH\(^-\), because their structure doesn't favor easy ionization of this group. It’s this nuance that clarifies that not every \(-\)OH-containing compound can be classified as a Bronsted-Lowry base.