Weak acid equilibria | Acids and bases | AP Chemistry | Khan Academy

TL;DR

Strong acids fully ionize in solution, while weak acids only partially ionize.

Transcript

• [Instructor] Before we get into the topic of weak acid equilibria, let's look at a strong acid first. So let's say that HA is a strong acid and reacts with water to produce the hydronium ion and A-minus, the conjugate base to HA. Technically, the reaction comes to an equilibrium. However, the equilibrium favors the products so much that we don't ... Read More

Key Insights

• 💪 Strong acids fully ionize in solution, while weak acids only partially ionize.
• 😑 The equilibrium constant expression for acid ionization is [H3O+][A-]/[HA].
• 💪 The Ka value represents the acid ionization constant and is greater than one for strong acids and less than one for weak acids.

Questions & Answers

Q: What is the difference between a strong acid and a weak acid?

Strong acids completely ionize in solution, while weak acids only partially ionize. Strong acids have a high concentration of hydronium ions and conjugate base, while weak acids have a smaller concentration of these ions and a larger concentration of the undissociated acid.

Q: How can you determine if an acid is strong or weak?

The magnitude of the acid's Ka value can indicate its strength. Strong acids have Ka values much greater than one, while weak acids have Ka values less than one. The higher the Ka value, the stronger the acid.

Q: What is the equilibrium constant expression for the ionization of an acid?

The equilibrium constant expression is [H3O+][A-]/[HA], where [H3O+], [A-], and [HA] are the equilibrium concentrations of hydronium ions, conjugate base, and acid, respectively. Pure water is excluded from the expression.

Q: How can the concentration of hydronium ions be used to calculate percent ionization?

Percent ionization can be calculated by dividing the equilibrium concentration of hydronium ions by the initial concentration of the acid HA and multiplying by 100%. This gives the percentage of the acid that has ionized.

Summary & Key Takeaways

• Strong acids, such as HA, completely ionize in water, leading to a high concentration of hydronium ions (H3O+) and conjugate base (A-) in solution.

• Weak acids, on the other hand, only partially ionize, resulting in a small amount of hydronium ions and conjugate base and a larger amount of the undissociated acid.

• The equilibrium constant expression for both strong and weak acids is the same, but the Ka value (acid ionization constant) is much greater for strong acids and less than one for weak acids.