Acid-Base Titrations (College Board AP® Chemistry): Flashcards
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Define equivalence point.
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Define equivalence point.
The point in a titration at which the moles of titrant added are exactly equal to the moles of analyte originally present in solution.
True or False?
For a strong acid–strong base titration, the pH at the equivalence point is 7.
True.
When a strong acid and strong base react in equal moles, the resulting salt solution is neutral, giving a pH of exactly 7 at the equivalence point.
Why does a titration curve show an S-shaped profile rather than a straight line?
At the start and end of the titration the solution contains mostly one species (excess acid or base), so pH changes slowly. Near the equivalence point, even a tiny addition of titrant converts the last traces of analyte, causing a sudden large change in [H+] and therefore a steep vertical section — producing the characteristic S-shape.
True or False?
The equivalence point always occurs at pH 7 in any acid-base titration.
False.
The equivalence point is pH 7 only for strong acid–strong base titrations. For a weak acid–strong base titration the equivalence point is above 7, and for a weak base–strong acid titration it is below 7.
What four pieces of information can be read directly from a titration curve?
The initial pH of the acid (y-intercept).
The pH at the equivalence point (midpoint of the steep vertical section).
The volume of base required to reach the equivalence point (x-value at the equivalence point).
The pH range across the vertical (steep) section of the curve.
In a titration, the .......... point is where the midpoint of the inflection of the pH curve is located, and it represents the point at which the moles of titrant equal the moles of .......... originally present.
In a titration, the equivalence point is where the midpoint of the inflection of the pH curve is located, and it represents the point at which the moles of titrant equal the moles of analyte originally present.
Define half-equivalence point.
The stage in a titration at which exactly half the weak acid (or weak base) has been neutralised, so the concentrations of the weak acid and its conjugate base are equal.
True or False?
At the half-equivalence point of a weak acid–strong base titration, pH = pKa.
True.
At the half-equivalence point, [HA] = [A-]. Substituting into the Henderson-Hasselbalch equation gives pH = pKa + log(1) = pKa.
Why is a weak acid–weak base titration not routinely performed in practice?
The pH change around the equivalence point is very gradual with no steep vertical section. This means no indicator undergoes a sharp colour change at the equivalence point, making it impossible to determine the end point accurately.
True or False?
The equivalence point for a weak base–strong acid titration occurs above pH 7.
False.
For a weak base–strong acid titration the equivalence point is below pH 7, because the resulting solution contains the conjugate acid of the weak base, which is itself weakly acidic.
How does a buffer region form during a weak acid–strong base titration, and what is its significance?
As the strong base neutralises the weak acid, conjugate base (A-) accumulates alongside the remaining weak acid (HA). This mixture resists pH change because A- reacts with any added H+ and HA reacts with any added OH-. The half-equivalence point lies at the centre of this buffer region where [HA] = [A-] and pH = pKa.
At the half-equivalence point of a weak acid–strong base titration, [HA] = [A-] and pH = .........., while the equivalence point occurs at a pH .......... 7.
At the half-equivalence point of a weak acid–strong base titration, [HA] = [A-] and pH = pKa, while the equivalence point occurs at a pH above 7.
Define polyprotic acid.
An acid that contains more than one ionisable hydrogen atom and therefore donates protons in a series of stepwise ionisation reactions (e.g. H2SO4, H3PO4).
True or False?
The titration curve of H3PO4 with NaOH has three equivalence points.
True.
Phosphoric acid is triprotic and ionises in three steps, each requiring an equal quantity of NaOH to neutralise. The curve therefore shows three equivalence points.
Why does the pH at the half-equivalence point of each ionisation step equal the pKa for that step?
At each half-equivalence point the concentration of the conjugate acid and conjugate base for that step are equal. The Ka expression simplifies to Ka = [H+], so taking −log of both sides gives pH = pKa for that particular ionisation step.
True or False?
All ionisation steps of a polyprotic acid release their protons simultaneously.
False.
Polyprotic acids ionise in stepwise fashion. The first proton is released most readily (largest Ka1); each successive step has a smaller Ka because removing a proton from an increasingly negative ion is progressively harder.
What species are present in the buffer regions between the equivalence points of a polyprotic acid titration?
Each buffer region lies between two successive equivalence points and contains a conjugate acid–base pair from adjacent ionisation steps — for example, H3PO4/H2PO4- in the first region and H2PO4-/HPO42- in the second. At the midpoint of each region, [conjugate acid] = [conjugate base] and pH = pKa for that step.
For a polyprotic acid titrated against a strong base, the number of .......... points equals the number of ionisable protons, and at the midpoint of each buffer region pH = .......... for that ionisation step.
For a polyprotic acid titrated against a strong base, the number of equivalence points equals the number of ionisable protons, and at the midpoint of each buffer region pH = pKa for that ionisation step.
Define acid-base indicator.
A weak acid (or weak base) whose protonated and deprotonated forms are different colours. The colour observed depends on the relative concentrations of HIn and In-, which are controlled by the pH of the solution.
True or False?
Phenolphthalein is the most suitable indicator for a weak acid–strong base titration.
True.
Phenolphthalein has a pKa of 9.3 and changes colour between pH 8.3 and 10.0. This falls within the rapid pH change that occurs at the equivalence point (above pH 7) in a weak acid–strong base titration.
Why is there no suitable indicator for a weak acid–weak base titration?
In a weak acid–weak base titration there is no sharp, steep pH change at the equivalence point — pH changes slowly and gradually throughout. Indicators only signal the end point reliably when the pH changes rapidly across the indicator's transition range. Without a steep change, any indicator would change colour gradually over a wide volume range, making the end point indeterminate.
True or False?
At the end point of an indicator, [HIn] = [In-] and pH = pKa of the indicator.
True.
When [HIn] = [In-], the Ka expression gives Ka = [H+]. Taking −log of both sides: pKa = pH. This is the endpoint where the colour is an equal mixture of both forms.
How do you select the most appropriate indicator for a given titration?
Identify the pH range of the rapid change at the equivalence point for the titration type, then choose an indicator whose pKa (and colour-change range of approximately pKa ± 1) falls within that region. An indicator whose transition range lies outside the steep section will give an inaccurate end point.
An acid-base indicator changes colour over approximately the range pKa .......... 1, and the most appropriate indicator for a titration is chosen so that its colour-change range falls within the .......... pH change at the equivalence point.
An acid-base indicator changes colour over approximately the range pKa ± 1, and the most appropriate indicator for a titration is chosen so that its colour-change range falls within the rapid pH change at the equivalence point.
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