CMB1004-L22: acids, bases and buffers

What is the dissociation equation for water?

• 2H₂O ⇌ H₃O⁺ + OH⁻

• Water dissociates into hydronium (H₃O⁺) and hydroxide (OH⁻) ions.

how is a buffer prepared

• can be prepared in 2 ways:

1. mixing a large volume of a weak acid with its conjugate base - eg acetic acid and acetate ion

2. making a large volume of weak base with its conjugate acid- eg ammonia and ammonium ion

Why is the concentration of water treated as a constant in aqueous solutions?

• Water is present in large excess in almost all solutions, so its concentration ([H₂O]) does not change significantly during reactions.

What is the water dissociation constant (Kw)?

• Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ at 25°C

• This constant represents the equilibrium between water dissociation into hydronium and hydroxide ions.

What is the definition of an acid and a base?

• Acid: A proton (H⁺) donor.

• Base: A proton (H⁺) acceptor or hydroxide (OH⁻) donor.

How is pH calculated?

• pH = -log₁₀[H⁺]

• pH measures the acidity of a solution, based on the concentration of hydrogen ions (H⁺).

What is a buffer?

• A buffer is a solution that resists changes in pH when small amounts of acid or base are added.

• Formed by mixing a weak acid and its conjugate base or a weak base and its conjugate acid.

How do buffers work when a strong base is added?

The weak acid in the buffer reacts with OH⁻ (from the strong base), consuming the hydroxide and only slightly changing the pH.

How do buffers work when a strong acid is added?

The conjugate base in the buffer reacts with H⁺ (from the strong acid), consuming the hydronium ion and only slightly changing the pH.

What is Ka and what does it represent?

• Ka is the acid dissociation constant, representing the equilibrium between a weak acid (HA) and its conjugate base (A⁻).

• Ka = [H⁺][A⁻] / [HA]

• A higher Ka indicates a stronger acid.

What is pKa and how is it related to Ka?

• pKa = -log₁₀Ka

• pKa expresses the strength of an acid; lower pKa means a stronger acid.

What does pKa tell us about an acid?

• If pKa < 3, the acid is strong.

• If 3 < pKa < 7, the acid is weak.

• If pKa > 7, the substance is a weak base.

How is pKa used in the Henderson-Hasselbalch equation?

• The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution.

• pH = pKa + log([A⁻]/[HA])

• pKa helps predict the pH based on the ratio of the conjugate base ([A⁻]) to the weak acid ([HA]).

What is the carbonic acid/bicarbonate buffer system?

• The system involves the equilibrium:
  H₂CO₃ ⇌ H⁺ + HCO₃⁻

• It stabilizes pH by absorbing excess H⁺ or OH⁻, using Le Chatelier’s principle to shift the equilibrium as needed.