Question

(a) What is the difference between the Arrhenius and the Brensted-Lowry definitions of a base? (b) Can a substance behave as an Arrhenius base if it does not contain an $\mathrm{OH}$ group? Explain.

Short answer

(a) The main difference between the Arrhenius and Brensted-Lowry definitions of a base is that the Arrhenius definition focuses on the formation of hydroxide ions (${\mathrm{OH}}^{-}$) in aqueous solutions, while the Brensted-Lowry definition is more general, as it applies to any substance that can accept a proton (${\mathrm{H}}^{+}$) regardless of the presence or absence of water. (b) A substance without an $\mathrm{OH}$ group would generally not be considered an Arrhenius base, as it is unlikely to increase the concentration of ${\mathrm{OH}}^{-}$ ions when dissolved in water. However, it can still be a Brensted-Lowry base if it can accept a proton.

Step-by-step solution

Understand the Arrhenius and the Brensted-Lowry definitions of a base

Let's first define what a base is according to both the Arrhenius and the Brensted-Lowry definitions. The Arrhenius definition: A base is a substance that increases the concentration of hydroxide ions (${\mathrm{OH}}^{-}$) when dissolved in water. The Brensted-Lowry definition: A base is a substance that can accept a proton (hydrogen ion, ${\mathrm{H}}^{+}$).

Determine the difference between the two definitions

Now, let's discuss the main difference between the Arrhenius and the Brensted-Lowry definitions of a base. The Arrhenius definition is limited to bases in aqueous solutions and focuses on the formation of hydroxide ions. In contrast, the Brensted-Lowry definition is more general, as it applies to any substance that can accept a proton, regardless of the presence or absence of water.

Can a substance behave as an Arrhenius base without an OH group?

Next, we need to determine if a substance can behave as an Arrhenius base without containing an $\mathrm{OH}$ group. According to the Arrhenius definition, a base must increase the concentration of hydroxide ions (${\mathrm{OH}}^{-}$) in the solution. If a substance does not contain an $\mathrm{OH}$ group, it is unlikely to increase the concentration of ${\mathrm{OH}}^{-}$ ions when dissolved in water. Therefore, a substance without an $\mathrm{OH}$ group would generally not be considered an Arrhenius base. So, a substance without an $\mathrm{OH}$ group can still be a Brensted-Lowry base, as it could accept a proton, but it would not be considered an Arrhenius base.

Key concepts

Arrhenius Definition

The Arrhenius definition of a base is grounded in the behavior of substances in water. When a base dissolves in water, it must increase the concentration of hydroxide ions (${\mathrm{OH}}^{-}$). This definition is very specific as it requires the presence of water, making it limited to aqueous solutions.
This means that for a substance to qualify as an Arrhenius base:

• It must dissolve in water.
• It must increase the concentration of hydroxide ions.

Typical examples of Arrhenius bases include sodium hydroxide ($\mathrm{NaOH}$) and potassium hydroxide ($\mathrm{KOH}$), both clearly providing ${\mathrm{OH}}^{-}$ ions in water. This specificity can be limiting as it doesn't account for base behavior in non-aqueous environments.

Brønsted-Lowry Definition

The Brønsted-Lowry definition broadens the concept of a base beyond substances that produce hydroxide ions. According to this definition, a base is any substance that can accept a proton (${\mathrm{H}}^{+}$). This is a more flexible approach, as it does not constrain the reaction environment to only aqueous solutions.
Key aspects of the Brønsted-Lowry definition include:

• The ability to accept a proton from another substance.
• Applicability in both aqueous and non-aqueous solutions.

Thus, even substances without $\mathrm{OH}$ groups, such as ${\mathrm{NH}}_3$ (ammonia), can act as Brønsted-Lowry bases by accepting protons. This definition is more widely applicable in various chemical contexts.

Hydroxide Ions

Hydroxide ions (${\mathrm{OH}}^{-}$) are pivotal in the Arrhenius definition of a base. These ions consist of an oxygen and a hydrogen atom, bearing a negative charge. In water, bases like $\mathrm{NaOH}$ dissociate to release ${\mathrm{OH}}^{-}$ ions, thus increasing their concentration. This increase is crucial as it's the defining feature of an Arrhenius base.
Hydroxide ions are essential in:

• Determining basicity in aqueous solutions.
• Influencing the pH of a solution.

Understanding how hydroxide ions work can help predict the behavior of many common household and industrial bases in solution.

Proton Acceptance

In the Brønsted-Lowry framework, accepting a proton is the hallmark of a base. This reaction can occur between a diverse range of chemical species. The ability to accept a proton makes a substance a base in this context, unlike the Arrhenius approach, where the existence of ${\mathrm{OH}}^{-}$ ions matters.
Important points about proton acceptance include:

• It doesn't rely on the substance having $\mathrm{OH}$ groups.
• It allows bases to function in various solvents, not just water.

Therefore, substances that may not be bases in the Arrhenius sense, like pyridine and ammonia, qualify as Brønsted-Lowry bases by accepting protons during reactions.

Aqueous Solutions

Aqueous solutions are where the Arrhenius definition comes into play. In these solutions, water is the solvent, and it's the medium in which bases dissociate to provide hydroxide ions. The water's role is crucial as it facilitates the dissociation process necessary for the Arrhenius definition.
Features of aqueous solutions include:

• Being a medium for dissociation of electrolytes.
• Contributing to pH and conductivity changes based on the dissolved substances.

While primarily important for the Arrhenius definition, understanding how bases behave in aqueous solutions helps grasp the basics of acid-base chemistry.