R3.1.13 pH curves of Acid-Base combinations (Higher Level Only) Notes

General Shapes of pH Curves

1. You are conducting a titration experiment, carefully adding a base to an acid (or vice versa).
2. Each drop of titrant tells part of a story, and when you plot the pH against the volume of titrant added, you reveal the full narrative: a pH curve.

Key Features of pH Curves

Before diving into specific examples, let’s define the key features of a pH curve:

1. Intercept with the pH axis:
   1. This represents the initial pH of the solution, determined by the strength and concentration of the analyte (the substance being titrated).
2. Equivalence point:
   1. The point where the moles of acid equal the moles of base added.
   2. The pH at this point depends on the strength of the acid and base.
3. Buffer region:
   1. A relatively flat section of the curve, observed in titrations involving weak acids or bases, where the solution resists changes in pH.
4. Points where $ \text{pH} = \text{pKa} $ or $ \text{pOH} = \text{pKb} $:
   1. These occur halfway to the equivalence point in weak acid-base titrations and are critical for identifying the acid/base strength.

Strong Acid + Strong Base

1. The curve starts at a low initial pH(around 1 or lower for a 0.1 M strong acid) because strong acids like HCl or HNO(_3) fully dissociate, producing a high concentration of $ \text{H}^+ $ ions.
2. The pH rises gradually at first, then increases sharply near the equivalence point.
3. At the equivalence point, the pH is exactly 7, as the neutralization produces only water and a neutral salt (e.g., NaCl from HCl and NaOH).
4. After the equivalence point, the pH levels off at a high value(around 13–14) due to the excess strong base.

Weak Base + Strong Acid

1. The curve begins at a high initial pH because the weak base (e.g., ammonia, $NH_3$) only partially ionizes, producing a low concentration of hydroxide ions $OH^-$.
2. As the strong acid is added, the pH drops gradually since the weak base reacts incompletely with the $H^+$ ions, forming its conjugate acid (e.g., $NH_4^+$).
3. The equivalence point occurs at a pH less than 7 because the conjugate acid formed (e.g., $NH_4^+$) hydrolyzes, releasing additional $H^+$ ions, making the solution slightly acidic.
4. Beyond the equivalence point, the pH decreases sharply as excess strong acid dominates the solution, contributing a high concentration of free $H^+$ ions.