STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 10)

by BerryBerryTeacher

in Berry Reference (Notes)

Titration curve is something fearful to nearly half of all STPM Chemistry students. Despite the simplicity of the concept, its concept is something which many Berry Readers will fail to grasp at first try. But fear not, this post, Part 10 of Berry Berry Easy’s notes on Ionic Equilibrium for STPM Chemistry Form 6 will set things right. All you really really need to know in essence for this topic is the basic definition of a titration curve, the two types of titrations (strong-strong or strong-weak reagents), the shape of a titration curve and its important features and also the three types of titration curve. After knowing it, you should have no fear for it. Even the practical application/test of it should be of no trouble.

[Tips: Brush up your mathematics knowledge on sigmoid as it'll come in handy to fully understand a titration curve. When possible, try to understand the end point and equivalence point of titration. If it is too confusing, just remember that end point is when indicator changes colour while equivalence point means that solution is in the proportion as prescribe by the chemical equation. The two need not coincide and this is an important practical consideration in real-life. Also, try to draw as many titration curve as possible and label the important features.]

STPM Chemistry Form 6 Notes – Ionic Equilibrium (Part 10)

Titration curves

  • quantitative reactions
  • acid-base titration – quantitative addition of a titrate acid to a base in solution (or of a titrate base to an acid in solution
  • equivalence point – when the moles of acid (H3O+) are equal to the moles of titrate base (OH-)
  • end point of an acid-base titration – the point at which the indicator changes colour

Two versions of titration

Version 1: titration of a strong reagent by a strong reagent.

Example: strong acid by strong base / strong base by strong acid

Point of curve Species in solution pH range pH calculation (Extra notes)
Initial Pure strong acid pH << 7 pH = -lg[HX]initial
Before equivalence H3O+ pH < 7 pH = -lg[H3O+]
At equivalence H2O & salt pH = 7 pH = 7
Past equivalence OH- pH > 7 pOH = -lg[OH-]excess

Version 2: titration of a weak reagent by strong reagent.

Example: weak acid by strong base / weak base by strong acid

Point of curve Species in solution pH range pH calculation (Extra notes)
Initial Pure weak acid pH < 7 pH = ½ pKa – ½ lg[HA]
Before equivalence HA and A- (Buffer) pKa ± 1 pH = pKa + lg ([A- ] / [HA])
At equivalence A- (diluted) pH > 7 pOH = ½ pKb – ½ lg[A- ]
Past equivalence OH- pH > 7 pOH = -lg[OH- ]excess

(there is no titration of a weak reagent with a weak reagent – two weak reagent do not react with one another)

Important point in curve shapes

Case 1: titration of a strong acid by a strong base / titration of a strong base by a strong acid.

  • the shape of the curve – sigmoidal
  • equivalence point is at pH = 7.0 (product is neutral salt and water)
  • pH is measured on a log scale
  • each change of 1.0 in the pH requires ten times less titrate. Example: pH = 1 to pH = 2, the hydronium concentration goes from 0.10M to 0.01M, a change of 0.09M H3O+
sigmoid

Example of sigmoid

Case 2: titration of a weak acid by a strong base / titration of a weak base by a strong acid

  • titration curves exhibit an initial cusp when the reagent being titrated is weak (a lip-o-weakness)
  • pH at half-equivalence point = pKa
  • titration of a weak acid by a strong base, the equivalence pH > 7.0 (because the neutralised product is a weak base / the conjugate base of the weak acid)
  • titration of a weak base by a strong acid, the equivalence pH < 7.0 (because the neutralised product is a weak acid / the conjugate acid of the weak base)

Three types of titration curves

  1. concentration effect on titration curve shape
  2. strength effect on titration shape
  3. polyprotic acid titration curve

Now that you easily break down the essence of titration curve, you should be confident enough to tackle the next sub-topic of Indicators, organic compounds with extended conjugation. Luckily we end our post here, and our lesson on indicators will only resume in Part 11 of Berry Berry Easy’s Ionic Equilibrium notes for STPM Form 6 students.

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