10.4: Water - Both an Acid and a Base
- Page ID
- 218361
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Learning Objectives
- To write chemical equations for water acting as an acid and as a base.
Water (H2O) is an interesting compound in many respects. Here, we will consider its ability to behave as an acid or a base.
In some circumstances, a water molecule will accept a proton and thus act as a Brønsted-Lowry base. We saw an example in the dissolving of HCl in H2O:
\[\rm{HCl + H_2O_{(ℓ)} \rightarrow H_3O^+_{(aq)} + Cl^−_{(aq)}} \label{Eq1}\]
In other circumstances, a water molecule can donate a proton and thus act as a Brønsted-Lowry acid. For example, in the presence of the amide ion (see Example 4 in Section 10.2), a water molecule donates a proton, making ammonia as a product:
\[H_2O_{(ℓ)} + NH^−_{2(aq)} \rightarrow OH^−_{(aq)} + NH_{3(aq)} \label{Eq2}\]
In this case, NH2− is a Brønsted-Lowry base (the proton acceptor).
So, depending on the circumstances, H2O can act as either a Brønsted-Lowry acid or a Brønsted-Lowry base. Water is not the only substance that can react as an acid in some cases or a base in others, but it is certainly the most common example—and the most important one. A substance that can either donate or accept a proton, depending on the circumstances, is called an amphiprotic compound.
A water molecule can act as an acid or a base even in a sample of pure water. About 6 in every 100 million (6 in 108) water molecules undergo the following reaction:
\[H_2O_{(ℓ)} + H_2O_{(ℓ)} \rightarrow H_3O^+_{(aq)} + OH^−_{(aq)} \label{Eq3}\]
This process is called the autoionization of water (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 comparison, liquid ammonia undergoes autoionization as well, but only about 1 molecule in a million billion (1 in 1015) reacts with another ammonia molecule.)
Example \(\PageIndex{1}\)
Identify water as either a Brønsted-Lowry acid or a Brønsted-Lowry base.
- H2O(ℓ) + NO2−(aq) → HNO2(aq) + OH−(aq)
- HC2H3O2(aq) + H2O(ℓ) → H3O+(aq) + C2H3O2−(aq)
Solution
- In this reaction, the water molecule donates a proton to the NO2− ion, making OH−(aq). As the proton donor, H2O acts as a Brønsted-Lowry acid.
- In this reaction, the water molecule accepts a proton from HC2H3O2, becoming H3O+(aq). As the proton acceptor, H2O is a Brønsted-Lowry base.
Exercise \(\PageIndex{2}\)
Identify water as either a Brønsted-Lowry acid or a Brønsted-Lowry base.
- HCOOH(aq) + H2O(ℓ) → H3O+(aq) + HCOO−(aq)
- H2O(ℓ) + PO43−(aq) → OH−(aq) + HPO42−(aq)
Concept Review Exercises
- Explain how water can act as an acid.
- Explain how water can act as a base.
Answers
- Under the right conditions, H2O can donate a proton, making it a Brønsted-Lowry acid.
- Under the right conditions, H2O can accept a proton, making it a Brønsted-Lowry base.
Key Takeaway
- Water molecules can act as both an acid and a base, depending on the conditions.