pH Curves of Acid–Base Reactions HL Only

Full Notes

pH curves graph the change in pH as one reagent is added to another during a titration. The shape of the curve depends on the strength of the acid and base involved.

Types of Titration Curves

Here’s a breakdown of the common acid–base titrations you need to know:

Strong Acid vs. Strong Base (e.g. HCl + NaOH)

• Initial pH: Very low (~1, strong acid).
• Rapid rise around pH 7 (equivalence point).
• Final pH: Very high (~13, strong base).
• Suitable indicators: Phenolphthalein or Methyl Orange.

Strong Acid vs. Weak Base (e.g. HCl + NH₃)

• Initial pH: Very low (~1, strong acid).
• Equivalence point: Below pH 7 (~5).
• Final pH: ~9 to 10, weak base.
• Suitable indicator: Methyl Orange.

Weak Acid vs. Strong Base (e.g. CH₃COOH + NaOH)

• Initial pH: Moderate (~4, weak acid).
• Buffer zone: After a small quantity of base is added, a buffer system is formed meaning the pH change is minimised until overwhelmed.
• Equivalence point: Above pH 7 (~9).
• pH at half-neutralisation = pK_a
• Final pH: Very high (~13, strong base).
• Suitable indicator: Phenolphthalein.

Weak Acid vs. Weak Base (CH₃COOH + NH₃)

• Initial pH: Moderate (~4, weak acid).
• No sharp pH change: No clear equivalence point.
• pH meter required (no suitable indicator).

Interpreting the Curve

• Initial pH: Depends on strength of acid or base.
• Buffer region (in weak acid + strong base): gradual pH rise due to partial neutralisation.
• Vertical section: where pH changes rapidly — marks the equivalence point.
• Final pH: set by excess base or acid added.

Key Points on Curves

• Intercept with pH axis: initial pH of acid (or base if titrating acid into base).
• Equivalence point: where moles of acid = moles of base added.
• Buffer region: gradual change in pH; presence of both weak acid/base and its conjugate.
• pH = pKa (or pOH = pKb): occurs at half-equivalence point for weak acid/base titrations.

Determining K_a Using a Titration Curve

The K_a of a weak acid can be found from its titration curve with a strong base (e.g. NaOH).

At the half-equivalence point, half of the acid has been neutralised
so: [HA] = [A⁻].

So: K_a = 10^−pH and pK_a = pH.

This makes the half-equivalence point a useful way to determine K_a or pK_a directly from a titration curve.

Determining K_b Using a Titration Curve

The K_b of a weak base can also be found from its titration curve with a strong acid.

In this titration, the base reacts to form its conjugate acid (BH⁺):

B + H₂O ⇌ BH⁺ + OH⁻

The equilibrium expression is:

K_b = [BH⁺][OH⁻] / [B]

At the half-equivalence point: [B] = [BH⁺].

So: K_b = [OH⁻], and taking logs: pK_b = pOH.

Since pH + pOH = 14, you can use the pH at this point to calculate pK_b.

Summary

• pH curves show neutralisation progress for different acid–base combinations.
• Equivalence points vary with acid and base strength.
• Buffer regions occur in weak acid/base titrations.
• K_a can be determined at half-equivalence (pH = pK_a).
• K_b can be determined at half-equivalence (pOH = pK_b).