6.1.8Additional lessons
pH Curves — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 122 of 172
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Part 1 of 4Features of a Curve
A pH curve plots pH against the volume of titrant added during a titration.
Axes
- pH is plotted on the vertical axis against the volume of solution added from the burette on the horizontal axis.
Starting pH
- The initial pH shows the strength of the substance in the flask;
- a strong acid starts low and a weak acid starts higher.
Steep section
- Near the equivalence point there is a rapid vertical change of several pH units from a tiny volume of added solution.
Equivalence point
- This is the volume at which the acid and base are in exact stoichiometric amounts, found at the middle of the steep section.
Part 2 of 4The Four Combinations
The Four Combinations
Strong acid and strong base
- The equivalence point is at pH 7, with a long vertical section from about pH 3 to pH 11.
Weak acid and strong base
- The equivalence point is above pH 7 because the conjugate base of the acid hydrolyses water.
Strong acid and weak base
- The equivalence point is below pH 7 because the conjugate acid of the base is acidic.
Weak acid and weak base
- There is no sharp vertical section, so no indicator can show the end point clearly.
| Titration | pH at equivalence |
|---|---|
| Strong acid + strong base | 7 |
| Weak acid + strong base | Greater than 7 |
| Strong acid + weak base | Less than 7 |
| Weak acid + weak base | About 7 (depends on Ka and Kb), no sharp change |
Part 3 of 4Drawing a Curve
1
Mark the starting pH on the vertical axis using the strength and concentration of the solution in the flask.
2
Draw a gradual rise (or fall) at first, since the added solution is largely consumed by the reaction.
3
Draw a steep vertical section centred on the equivalence volume at the correct pH.
4
Finish with a gentle approach to the pH of the solution in the burette.
5
Label the equivalence point and the volume at which it occurs, which is found from n = cV.
Part 4 of 4Exam Checklist & Tips
6.1.8 – Key Points for the Exam
- pH curve A graph showing how the pH changes as a titrant is added.
- Equivalence point The point at which the acid and base have reacted in exactly the ratio of the equation.
- End point The point at which the indicator changes colour, ideally the same as the equivalence point.
- Titrant The solution added from the burette.
Common Mistakes
- Placing the equivalence point at pH 7 for all titrations.
- Drawing the steep section as a gradual slope.
- Confusing end point with equivalence point.
- Starting a weak acid curve at the same pH as a strong acid of equal concentration.
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