AP Chemistry · Unit 8 · Topic 8.4
Acid-Base Reactions and Buffers
Mixing an acid and a base does not automatically make a buffer. The decisive question is what remains after the fast neutralization step.
Perform stoichiometry before equilibrium
When strong hydroxide reacts with a weak acid, \(HA+OH^-\rightarrow A^-+H_2O\) is usually effectively complete under ordinary aqueous conditions. Count moles of each reactant and determine the limiting reagent. Only after this reaction should you decide which equilibrium controls the pH. An ICE table begun with the unreacted mixing amounts can describe the wrong chemical situation.
Recognize a working conjugate pair
A buffer contains appreciable amounts of a weak acid and its conjugate base. HA can consume added hydroxide, while A⁻ can accept added hydronium. The species must be conjugate partners; a random weak acid and unrelated salt do not necessarily provide this two-way response. “Appreciable” is relative to the acid or base disturbance the solution must resist.
Classify the mixture before choosing a formula
| OH⁻ added | Major acid–base species afterward | Appropriate model |
|---|---|---|
| 2.00 mmol | 4.00 mmol HA; 2.00 mmol A⁻ | Buffer |
| 6.00 mmol | 6.00 mmol A⁻ | Weak-base hydrolysis |
| 8.00 mmol | 6.00 mmol A⁻; 2.00 mmol excess OH⁻ | Excess strong base dominates |
A worked buffer adjustment
An original buffer contains 6.00 mmol HA and 4.00 mmol A⁻, with pKa = 4.60. Add 1.00 mmol NaOH. Neutralization changes the amounts to 5.00 mmol HA and 5.00 mmol A⁻. For a sufficiently concentrated, approximately ideal buffer,
Before addition, pH was \(4.60+\log(4/6)=4.42\). The modest increase follows from the change in the conjugate-pair ratio.
Explain the response at particle level
Added hydroxide does not simply float unchanged beside HA. Proton transfer converts much of it into water while converting HA to A⁻. The final pH still increases because the base-to-acid ratio increases. A buffer therefore reduces a pH change; it does not make added acid or base disappear without altering the solution's composition.
Compare buffered and unbuffered samples
Add equal small amounts of teacher-approved dilute acid to equal volumes of buffer and water, recording initial and final pH. The buffer should show the smaller change if its capacity is sufficient. Use the same delivered moles, not merely the same number of drops from different solutions. Temperature and probe calibration should be controlled.
Know when the buffer formula fails
If neutralization exhausts HA or A⁻, their ratio is no longer a valid ordinary buffer approximation. At equivalence, use the surviving conjugate species' hydrolysis equilibrium. Beyond equivalence, first calculate excess strong acid or base. Very dilute mixtures also require attention to water autoionization and actual equilibrium concentrations.
Practice with answers
- Mix 5 mmol HA with 2 mmol OH⁻. Is a buffer formed? Yes: approximately 3 mmol HA and 2 mmol A⁻ remain.
- Add 7 mmol OH⁻ instead. What controls pH? The 2 mmol excess strong hydroxide, after dilution is included.
- Which component consumes added strong acid? The conjugate base A⁻.
Factual references: OpenStax Chemistry 2e, 14.6 and 14.7. All mixture amounts and instructional examples are original.
Official curriculum reference: College Board AP Chemistry course page. The explanation and worked example are independently written for this study site.