No2- Bronsted Acid Or Base

10.iii: Water - Both an Acid and a Base

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Learning Objectives

To write chemical equations for h2o acting as an acid and equally a base.

Water (H₂O) is an interesting chemical compound in many respects. Here, we will consider its ability to behave every bit an acid or a base.

In some circumstances, a water molecule will take a proton and thus act as a Brønsted-Lowry base of operations. We saw an case in the dissolving of HCl in H_iiO:

\[\rm{HCl + H_2O_{(ℓ)} \rightarrow H_3O^+_{(aq)} + Cl^−_{(aq)}} \label{Eq1} \]

In other circumstances, a h2o molecule tin can donate a proton and thus act as a Brønsted-Lowry acid. For example, in the presence of the amide ion (see Case iv in Section x.2), a water molecule donates a proton, making ammonia as a product:

\[H_2O_{(ℓ)} + NH^−_{two(aq)} \rightarrow OH^−_{(aq)} + NH_{iii(aq)} \label{Eq2} \]

In this case, NH₂ ⁻ is a Brønsted-Lowry base of operations (the proton acceptor).

So, depending on the circumstances, H₂O can act as either a Brønsted-Lowry acrid or a Brønsted-Lowry base of operations. H2o is not the but substance that tin react as an acrid in some cases or a base in others, simply it is certainly the almost mutual example—and the virtually of import i. A substance that can either donate or accept a proton, depending on the circumstances, is called an amphiprotic compound.

A water molecule tin can act as an acid or a base even in a sample of pure water. Most 6 in every 100 million (6 in x^eight) water molecules undergo the following reaction:

\[H_2O_{(ℓ)} + H_2O_{(ℓ)} \rightarrow H_3O^+_{(aq)} + OH^−_{(aq)} \label{Eq3} \]

This process is chosen the autoionization of h2o (Figure \(\PageIndex{1}\)) and occurs in every sample of water, whether it is pure or part of a solution. Autoionization occurs to some extent in any amphiprotic liquid. (For comparing, liquid ammonia undergoes autoionization as well, but but about i molecule in a million billion (1 in 10^xv) reacts with another ammonia molecule.)

Notation

It is rare to truly have pure water. Water exposed to air will usually be slightly acidic because dissolved carbon dioxide gas, or carbonic acid, decreases the pH slightly below 7. Alternatively, dissolved minerals, like calcium carbonate (limestone), can make water slightly basic.

Figure \(\PageIndex{ane}\) Autoionization. A modest fraction of water molecules—approximately vi in 100 meg—ionize spontaneously into hydronium ions and hydroxide ions. This picture necessarily overrepresents the amount of autoionization that really occurs in pure water.

Instance \(\PageIndex{1}\)

Identify h2o as either a Brønsted-Lowry acid or a Brønsted-Lowry base.

H_iiO(ℓ) + NO₂ ⁻(aq) → HNO₂(aq) + OH⁻(aq)
HC₂H₃O_two(aq) + H_iiO(ℓ) → H₃O⁺(aq) + C₂H_threeO₂ ⁻(aq)

Solution

In this reaction, the water molecule donates a proton to the NO₂ ⁻ ion, making OH⁻(aq). Every bit the proton donor, H₂O acts equally a Brønsted-Lowry acrid.
In this reaction, the water molecule accepts a proton from HC_twoH_iiiO₂, becoming H₃O⁺(aq). As the proton acceptor, H₂O is a Brønsted-Lowry base of operations.

Exercise \(\PageIndex{2}\)

Identify water as either a Brønsted-Lowry acid or a Brønsted-Lowry base.

HCOOH(aq) + H_iiO(ℓ) → H₃O⁺(aq) + HCOO⁻(aq)
H₂O(ℓ) + PO₄ ³ ⁻(aq) → OH⁻(aq) + HPO₄ ² ⁻(aq)

Answer

1. H₂O acts equally the proton acceptor (Brønsted-Lowry base)

two. H_twoO acts equally the proton donor (Brønsted-Lowry acid)

Cardinal Takeaway

H2o molecules can deed every bit both an acid and a base of operations, depending on the atmospheric condition.