Acid-base titrations

Simulation: http://users.skynet.be/eddy/titratie.swfStrong acid/ strong base

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)

End points need not be “neutral‘ due to the phenomenon of salt hydrolysis

Equivalence point: the point at which sufficient base has been added to just neutralise the acid (or vice-versa).

strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)

pH 1 at the start due to 0.1M HCl

(strong monoprotic acid)

strong acidstrong acid (HC (HCll) ) 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 (HC (HCll) ) 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 (HC (HCll) ) 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)

strong acidstrong acid (HC (HCll) ) 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

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 (HC (HCll) ) v. weak basev. weak base (NH (NH33))

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)

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)

4.3 exercise 4

Other pH curves - Other pH curves - acid v. carbonate

Sodium carbonate reacts with hydrochloric acid in two steps...

Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl

Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2

Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2

Other pH curves - Other pH curves - acid v. carbonate

There are two sharp pH changes

The second addition of HCl is exactly the same as the first because the number of moles of HCl which react with the NaHCO3 is the same as that reacting with the Na2CO3.

17.50cm3 35.00cm3

Sodium carbonate reacts with hydrochloric acid in two steps...

Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl

Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2

Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2

Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))

Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...

Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O

Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O

Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O

Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))

There are three sharp pH changes

Each successive addition of NaOH is the same as equal

number of moles are involved.

Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...

Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O

Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O

Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O

Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))

pH of H3PO4 = 1.5

Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...

Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O

Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O

Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O

Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))

pH of NaH2PO4 = 4.4

pH of H3PO4 = 1.5

Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...

Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O

Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O

Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O

Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))

pH of Na2HPO4 = 9.6

pH of NaH2PO4 = 4.4

pH of H3PO4 = 1.5

Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...

Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O

Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O

Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O

Other pH curves - Other pH curves - polyprotic acids (Hpolyprotic acids (H33POPO44))

pH of Na3PO4 = 12

pH of Na2HPO4 = 9.6

pH of NaH2PO4 = 4.4

pH of H3PO4 = 1.5

Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...

Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O

Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O

Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O

Page 50, Question 4a), 4bi, 4bii