Question

How can water be both a Bronsted base and a Lewis base? Can water be a Bronsted acid? A Lewis acid?

Short answer

Water can act as both a Brønsted base and acid, and as a Lewis base, but not as a Lewis acid.

Step-by-step solution

Understanding Brønsted-Lowry Theory

In the Brønsted-Lowry theory, a base is defined as a substance that can accept a proton (H⁺), while an acid is a substance that can donate a proton. Water can act as a Brønsted base by accepting a proton to form hydronium ion (H₃O⁺). Conversely, water can also act as a Brønsted acid by donating a proton to form hydroxide ion (OH⁻).

Understanding Lewis Theory

In the Lewis theory, a base is defined as a substance that can donate a pair of electrons to form a bond, while an acid is a substance that can accept a pair of electrons. Water can act as a Lewis base because it has lone pairs of electrons on the oxygen atom that can be donated to form bonds with Lewis acids.

Water as a Brønsted Base

As a Brønsted base, water accepts a proton: \[ \text{H}_2\text{O} + \text{H}^+ \rightarrow \text{H}_3\text{O}^+ \]Here, water gains a proton to form hydronium ion, demonstrating its role as a proton acceptor.

Water as a Brønsted Acid

As a Brønsted acid, water donates a proton: \[ \text{H}_2\text{O} \rightarrow \text{OH}^- + \text{H}^+ \]Here, water loses a proton to form a hydroxide ion, demonstrating its role as a proton donor.

Water as a Lewis Base

As a Lewis base, water donates electron pairs: When water donates its lone pair of electrons to form a coordinate bond with an acid, it acts as a Lewis base. For example, it can donate electrons to a hydrogen ion (H⁺), forming a bond in the hydronium ion.

Evaluating Water as a Lewis Acid

A Lewis acid is a molecule that can accept an electron pair. Water generally does not act as a Lewis acid because it does not have the capacity to accept electron pairs without first converting into a cation like H⁺.

Key concepts

Brønsted-Lowry Theory

The Brønsted-Lowry theory is a fundamental concept in acid-base chemistry. It defines acids and bases through the transfer of protons (H⁺ ions). According to this theory, an acid is a substance that donates a proton, while a base is a substance that accepts a proton. This approach broadens our understanding beyond just aqueous solutions to include non-water-based environments.
In this framework, water is a special case. It can behave both as an acid and a base, a property known as amphoterism.

• As a Brønsted base, water accepts a proton from an acid, transforming into a hydronium ion (\(\text{H}_3\text{O}^+\)).


• In contrast, when water acts as a Brønsted acid, it donates a proton, resulting in the formation of a hydroxide ion (\(\text{OH}^-\)).

This dual capability makes water a perfect medium for a multitude of reactions, illustrating its centrality in acid-base chemistry.

Lewis Theory

The Lewis theory expands the definition of acids and bases through a focus on electron pairs rather than protons. In this view, a base is an electron pair donor, while an acid is an electron pair acceptor. This broader perspective includes reactions where no protons are exchanged, emphasizing the role of electron sharing or transfer.
Water also plays a versatile role under the Lewis theory. The oxygen atom in a water molecule has two lone pairs of electrons that can be donated during the formation of coordinate covalent bonds. When water donates these electron pairs to another species, it functions as a Lewis base.
This ability to donate electron pairs makes water an effective participant in reactions with diverse compounds, highlighting its adaptability in various chemical environments.

Water as Brønsted Acid/Base

In the realm of Brønsted-Lowry theory, water's ability to act either as an acid or a base underscores its amphoteric nature. Water's dual behavior is essential for maintaining many biochemical processes, including those that occur in our own bodies.
When water accepts a proton (\( \text{H}^+ \)), it acts as a Brønsted base and forms a hydronium ion (\(\text{H}_3\text{O}^+\)). This action illustrates its role in receiving protons from acidic substances.
However, when water donates a proton, becoming a hydroxide ion (\(\text{OH}^-\)), it displays its function as a Brønsted acid. This behavior is critical in balancing the pH of solutions and facilitating various chemical reactions.
Understanding this duality helps us appreciate water's central position in both chemical and biological systems, where it seamlessly switches between these roles as interactions demand.

Water as Lewis Base

Under the Lewis theory, water is predominantly recognized for its role as a Lewis base. This occurs when it donates electron pairs to other molecules to form bonds. A common scenario is when water interacts with a hydrogen ion (\( \text{H}^+ \)), leading to the formation of a hydronium ion (\(\text{H}_3\text{O}^+\)).

• Here, the electron pairs from water's oxygen are instrumental in creating a covalent bond with the hydrogen ion.


• This electron sharing forms the basis of water's role as an electron pair donor, or Lewis base.

In many chemical reactions, this characteristic allows water to stabilize other ions and molecules, reinforcing its pivotal role in both organic and inorganic chemistry.
However, it should be noted that water is generally not recognized as a Lewis acid, as it lacks the ability to accept electron pairs unless transformed into a cation like \( \text{H}^+ \).