Autoionization of water | Acids and bases | AP Chemistry | Khan Academy

TL;DR

Water molecules can act as both an acid and a base through autoionization, resulting in the formation of hydronium and hydroxide ions. The equilibrium constant expression for the autoionization of water is expressed as KW, which is equal to 1.0 x 10^-14 at 25 degrees Celsius.

Transcript

• [Instructor] The autoionization of water refers to the reaction of water molecules to form two ions, the hydronium ion, which is H3O+, and the hydroxide ion, which is OH-. Water can function as an acid or a base, and in this reaction, one water molecule functions as a Bronsted-Lowry acid and donates a proton and another water molecule functions a... Read More

Key Insights

• 💁 Water can function as both an acid and a base through the autoionization process, resulting in the formation of hydronium and hydroxide ions.
• 😑 The equilibrium constant expression for the autoionization of water is KW, which represents the product of the concentrations of hydronium and hydroxide ions.
• 😐 Pure water at 25 degrees Celsius is a neutral substance, with equal concentrations of hydronium and hydroxide ions.
• 😑 KW is equal to 1.0 x 10^-14 at 25 degrees Celsius, indicating low concentrations of hydronium and hydroxide ions at equilibrium.
• 😑 An increase in temperature causes an increase in the value of KW, resulting in higher concentrations of hydronium and hydroxide ions at equilibrium.
• 💁 The autoionization of water is an endothermic reaction, as an increase in temperature causes a shift in the equilibrium towards the formation of more hydronium and hydroxide ions.
• 😐 KW can be used to determine the acidity or basicity of an aqueous solution, where equal concentrations of hydronium and hydroxide ions indicate a neutral solution.

Questions & Answers

Q: What is the autoionization of water?

The autoionization of water is the process in which water molecules react to form hydronium (H3O+) and hydroxide (OH-) ions. One water molecule functions as an acid and donates a proton, while another water molecule acts as a base and accepts the proton.

Q: How is the equilibrium constant expression written for the autoionization of water?

The equilibrium constant expression for the autoionization of water is KW, which is equal to the concentration of hydronium ions multiplied by the concentration of hydroxide ions. Since both ions have a coefficient of one in the balanced equation, their concentrations are raised to the first power.

Q: What is the significance of KW being equal to 1.0 x 10^-14 at 25 degrees Celsius?

The value of KW being very small indicates that at equilibrium, the concentrations of hydronium and hydroxide ions in pure water are extremely low. Most of the species present in water are H2O molecules.

Q: How can the concentration of hydronium and hydroxide ions be calculated?

By solving the KW equation, which is X^2 = 1.0 x 10^-14, the concentration of hydronium ions (X) is found to be 1.0 x 10^-7 M. Since the concentrations of hydronium and hydroxide ions are equal, the concentration of hydroxide ions is also 1.0 x 10^-7 M.

Summary & Key Takeaways

• The autoionization of water refers to the reaction of water molecules forming hydronium and hydroxide ions, with one water molecule acting as an acid and the other as a base.

• The equilibrium constant expression for the autoionization of water is KW, which does not include liquid water and is equal to 1.0 x 10^-14 at 25 degrees Celsius.

• The concentration of hydronium ions and hydroxide ions in pure water at 25 degrees Celsius is 1.0 x 10^-7 M, making water a neutral substance.