15.2
Water: A Bronsted-Lowry Acid and Base
The reaction between a Brønsted-Lowry acid and water is called acid ionization. For example, when hydrogen fluoride dissolves in water and ionizes, pr…
An amphoteric molecule is capable of acting as both an acid and a base. Water is an example of an amphoteric molecule.
In an aqueous solution of ammonia, water can act as an acid by donating a proton to ammonia, a base. In contrast, water acts as a base in an aqueous solution of hydrochloric acid by accepting a proton from the acid.
Because it is amphoteric, water can undergo autoionization. In this process, one molecule of water acts as the acid by donating a proton to another molecule of water that acts as a base by accepting that proton.
This results in the production of a hydronium ion and a hydroxide ion.
The equilibrium constant for the autoionization of water, KW, also known as the ion-product constant for water, has a value of 1 × 10−14. Its equilibrium expression is written as the concentration of the hydronium ions times the concentration of the hydroxide ions, with no denominator as both reactants are liquids.
In pure water at 25 °C, the concentrations of hydronium and hydroxide ions are equal—that is 1 × 10−7 M.
KW can be used to calculate the concentration of hydronium and hydroxide ions in a solution and to determine whether a solution is acidic or basic.
If a solution is acidic, the concentration of hydronium ions will be higher than hydroxide ions. In contrast, if a solution is basic, the concentration of hydroxide ions will be higher than the hydronium ions.
If the concentration of hydronium ions in solution is 5 × 10−7 M, the hydroxide ion concentration can be determined using KW.
As the values of KW and hydronium ions are known, the concentration of hydroxide ions can be calculated to be 2 × 10−8 M.
As the hydronium ion concentration, 5 × 10−7 M, is higher than the hydroxide ion concentration, 2 × 10−8 M, the solution is acidic.
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Q1: What does it mean for water to be amphoteric?
Water is amphoteric because it can act as both an acid and a base. In acidic solutions, water accepts protons from acids, functioning as a base. In basic solutions, water donates protons to bases, functioning as an acid. This dual capability allows water to participate in acid-base reactions regardless of the solution composition.
Q2: How does water autoionization work?
Water autoionization occurs when one water molecule donates a proton to another water molecule. The donating molecule acts as an acid, while the accepting molecule acts as a base. This produces a hydronium ion and a hydroxide ion. Although the process occurs to only a slight extent, it establishes the ion-product constant for water, KW, which equals 1 × 10−14 at 25 °C.
Q3: What is the ion-product constant for water and why is it important?
The ion-product constant for water, KW, equals the concentration of hydronium ions multiplied by the concentration of hydroxide ions, with a value of 1 × 10−14 at 25 °C. KW is important because it allows calculation of either ion concentration when the other is known, and it determines whether a solution is acidic, basic, or neutral based on relative ion concentrations.
Q4: How do you determine if a solution is acidic or basic using ion concentrations?
A solution is acidic when hydronium ion concentration exceeds hydroxide ion concentration. A solution is basic when hydroxide ion concentration exceeds hydronium ion concentration. A solution is neutral when both concentrations are equal. Using KW and the known concentration of one ion, you can calculate the other and classify the solution accordingly.
Q5: What are the hydronium and hydroxide ion concentrations in pure water at 25 °C?
In pure water at 25 °C, the concentrations of hydronium ions and hydroxide ions are equal, each at 1 × 10−7 M. This equality results from water's autoionization, which produces one hydronium ion for every hydroxide ion formed. These equal concentrations make pure water neutral.
Q6: How can you calculate hydroxide ion concentration if you know the hydronium ion concentration?
Use the ion-product constant expression: KW = [H3O+][OH−]. Rearrange to solve for hydroxide ion concentration: [OH−] = KW / [H3O+]. For example, if hydronium concentration is 5 × 10−7 M, then [OH−] = (1 × 10−14) / (5 × 10−7) = 2 × 10−8 M, indicating an acidic solution.
Q7: Does the autoionization of water change with temperature?
Yes, water autoionization is endothermic, so it increases with temperature. At 25 °C, KW equals 1 × 10−14, but at 100 °C, KW increases to approximately 5.6 × 10−13, roughly 50 times larger. This temperature dependence means hydronium and hydroxide ion concentrations increase at higher temperatures.