Equilibrium in Acid- Base Systems 16.4b: pH curve shape vs. acid-base strength

Equilibrium in Acid-Equilibrium in Acid-Base SystemsBase Systems

16.4b: pH curve shape vs. 16.4b: pH curve shape vs. acid-base strengthacid-base strength

Acid-Base StrengthAcid-Base Strength

• quantitative reactions are not limited to SA-SB– they can occur between SA-WB and WA-SB, but

not between WA-WB

• equivalence pt. pH for a SA-SB is 7• equivalence pt. pH for a quantitative SA-WB

is <7• equivalence pt. pH for a quantitative WA-SB

is >7• WA-WB reactions do not have a detectable

equivalence pt. since they are usually not quantitative

pH curvespH curves

Types There are four types of acid-base titration; each has a characteristic curve.

strong acid (HCl) v. strong base (NaOH) weak acid (CH3COOH) v. strong alkali (NaOH)

strong acid (HCl) v. weak base (NH3)

weak acid (CH3COOH) v. weak base (NH3)

In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M)

strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)


Very little pH change during the initial 20cm3





Very sharp change in pH over the addition of less than half a drop of NaOH





Very sharp change in pH over the addition of less than half a drop of NaOH

Curve levels off at pH 13 due to excess 0.1M NaOH

(a strong alkali)





Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE


Sharp change in pH over the addition of less than

half a drop of NH3

Curve levels off at pH 10 due to excess 0.1M NH3

(a weak alkali)


strong acidstrong acid (HCl) (HCl) v. weak basev. weak base (NH (NH33))

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

strong acid strong acid (HCl) (HCl) v. weak basev. weak base (NH (NH33))

Only methyl orange is suitable - it is the only one to change in the ‘vertical’ portion

Steady pH change

Sharp change in pH over the addition of less than

half a drop of NaOH

Curve levels off at pH 13 due to excess 0.1M NaOH

(a strong alkali)

pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

weak acid weak acid (CH(CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

Only phenolphthalein is suitable - it is the only one to change in the ‘vertical’ portion

weak acidweak acid (CH (CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)

weak acidweak acid (CH (CH33COOH) COOH) v. weak basev. weak base (NH (NH33))

Types

Steady pH change

pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

NO SHARPCHANGE IN pH

Curve levels off at pH 10 due to excess 0.1M NH3

(a weak alkali)

PHENOLPHTHALEIN

LITMUS

METHYL ORANGE

NOTHING SUITABLE

There is no suitable indicator- none change in the ‘vertical’ portion.The end point can be detected by plotting a curve using a pH meter.

weak acidweak acid (CH (CH33COOH) COOH) v. weak base v. weak base (NH(NH33))

Buffer CapacityBuffer Capacity

• amount of Hamount of H++ or OH or OH-- the solution can the solution can absorb without a significant change in pHabsorb without a significant change in pH

• determined by the size of [HA] and [Adetermined by the size of [HA] and [A--]]

• if the amounts are equal and have large if the amounts are equal and have large concentrations, it will have the largest concentrations, it will have the largest buffer capacitybuffer capacity

HomeostasisHomeostasis

(Increase breathing rate)

HomeworkHomework

• Textbook p762 #11-13(a,c), 14

• Textbook p767 #1,3,5,7

• LSM D,F,H

• LSM K,L summary

Finished Chem 30. Congrats and study for your Diploma!

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Equilibrium in Acid- Base Systems 16.4b: pH curve shape vs. acid-base strength