equilibrium in acid- base systems 16.4b: ph curve shape vs. acid-base strength
TRANSCRIPT
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)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
Very little pH change during the initial 20cm3
Very sharp change in pH over the addition of less than half a drop of NaOH
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
Very little pH change during the initial 20cm3
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)
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
strong acidstrong acid (HCl) (HCl) v. strong base v. strong base (NaOH)(NaOH)
Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
Very little pH change during the initial 20cm3
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)
pH 1 at the start due to 0.1M HCl
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!