Ionisation of water and pH

For any Bronsted conjugate Acid-Base pair

14pOHpH

K10(55.4)K OH H Product Ion

101.8055.4

1010

OH

OH HKK

tionautoionisa HOHaq.OH

w14

a

1677

2ac

2

pH concept

pH = -log[H+]

pX = -logX

pH scale

[H+] > 10-7M, pH < 7

ACIDIC

[H+] < 10-7M, pH > 7

BASIC

[H+] = 10-7M, pH = 7

NEUTRALKa . Kb = Kw

BuffersEvery life form is extremely sensitive to slight pH changes. Human

blood for example needs to remain within the range 7.38-7.42.

Buffers: buffer the system against extreme changes in pH

Buffer solutions normally consist of two solutes: a weak Bronsted acid and its conjugate base

CH3COOH H++CH3COO-

-

3

3a

3

-3

a

COOCH

COOHCHKH

COOHCH

COOCH HK

COOHCH

COOCHlogpKpH

3

-3

a

BuffersIn general for: HAA- + H+

HA

AlogpKpH

-

a

Henderson-Hasselbach Equation

Buffer capacity

Q. If we generate 0.15mol H+ in a reaction vessel of 1L (with no accompanying volume change) containing 1mol each of CH3COOH and CH3COO-, what will the solution pH change be?

For the same reaction in water what is the pH change?

Acid-Base Reactions Acid/Base reactions are reactions that involve the neutralisation of an acid through the use of a base.

HCl + NaOH NaCl + H2OIn this reaction, the Na+ and the Cl- are called spectator ions because they play no role in the overall outcome of the reaction. The only thing that reacts is the H+ (from the HCl) and the OH- (from the NaOH). So the reaction that actually takes place is:

H+ + OH- H2O If in the end, the OH- was the limiting reagent and there are H+'s still left in the solution then the solution is acidic, but if the H+ was the limiting reagent and OH-'s were left in the solution then the solution is basic.  

TitrationTitration is the process of mixing acids and bases to analyse one of the solutions. For example, if you were given an unknown acidic solution and a 1 molar NaOH solution, titration could be used to determine what the concentration of the other solution was.

Acid-Base TitrationsThe goal of titration is to determine the equivalence point. The equivalence point is the point in which all the H+ and the OH- ions have been used to produce water. Titration also usually involves an indicator. An indicator is a liquid that turns a specific colour at a specific pH. (Different indicators change colours at different pH's). Indicators are chosen to allow a colour change at the equivalence point.

Titration of a strong acid with a strong base

50.00mL of 0.020M HCl with 0.100M NaOHH+ + OH- H2O Kc=1/Kw=1014

at equivalence pt.: nb mol HCl = nb mol NaOH

0.02mol/L x 50/1000 L = 0.1mol/L x Ve/1000 L

Ve = 0.001mol HCl (0.1mol/L x 1/1000 L) = 10 mL

pH determined by dissociation of H20: Kw = [H+][OH-] = 10-14

[H+] = 10-14 = 10-7 mol/L => pH = 7.00

Acid-Base TitrationsTitration of a strong acid with a strong base

Initial pH: 0.02mol/L strong acid. pH = 1.70

before equivalence pt.: when 3.00mL of NaOH has been added

L0.0132mol/350

500.02mol/L

10

310][H

Fraction of H+ remaining

Dilution factorInitial conc.

pH = 1.88

after equivalence pt.: 10.1mL NaOH added

l/L0.000166mo10.150

0.10.1mol/L][OH

Initial conc. of base Dilution factor

pOH = 3.78

pH = 10.22

Titration Curves

-2 0 2 4 6 8 10 12 14 160

2

4

6

8

10

12

Equivalence pt.

pH

Volume NaOH added (mL)

Titration curve of a strong acid with a strong base

Titration of a weak acid with a strong base

Take the example of a titration of 50.0mL 0.020M CH3COOH (Ka = 1.8 x 10-5) with 0.10M NaOH

CH3COOH + NaOH CH3COONa + H2O

Initial pH: a weak acid equilibrium problem

x-0.02K

COOHCH

COOCH HK

2

a

3

-3

a

x

x = 6 x 10-4, pH = 3.22

CH3COOH H++CH3COO-

0.02-x x x

Ve = 10mL (as before)Reaction is the reverse of Kb for CH3COO- base

K = 1/Kb = 1/(Kw / Ka) = 1.8 x 109

Titration of a weak acid with a strong base

Before eq. pt.: buffer system HA

AlogpKpH

-

a

Imagine we have added 3.00mLs of base

CH3COOH + NaOH CH3COONa + H2ORelative Initial: 1 3/10Relative final: 7/10 3/10

One of the simplest ways to treat these problems is to evaluate the quotient in the log using relative concentration before and after the reaction.

37.47/10

3/10log74.4pH

Titration of a weak acid with a strong base

Kb = (Kw / Ka) = 5.56 x 10-10 = x2/(F-x)

x = 3.05 x 10-6, pOH = 5.52, pH=8.48 (BASIC)

When volume of base added = 1/2Ve

apK4.745/10

5/10log4.74pH

at equivalence pt.: we have a solution of base in water

CH3COONa + H2O CH3COOH + OH-

F-x x x

L0.0167mol/1050

500.02mol/LF

Titration of a weak acid with a strong baseafter equivalence pt.: pH is determined by excess base added

For 10.1mL base added in total

l/L0.000166mo1.0150

0.10.10mol/L][OH

pOH = 3.78

pH = 10.22

-2 0 2 4 6 8 10 12 14 162

4

6

8

10

12

equivalence pt.

pH

Volume of NaOH added

Acid-Base IndicatorsUsually dyes that are weak acids and display different

colours in protonated/deprotonated forms.

HIn(aq.) H+ (aq.) +In- (aq.)

In general we seek an indicator whose transition range (±1pH unit from the indicator pKa) overlaps the steepest part of the titration curve as closely as possible

HIn

In HK

-

a

Acid-base indicatorsIndicator pH range pKa Acid Form Base Form

methyl violet 0.0- 1.6 0.8 yellow blue

thymol blue 1.2- 2.8 1.6 red yellow

methyl yellow 2.9- 4.0 3.3 red yellow

methyl orange 3.1- 4.4 4.2 red yellow

bromocresol green 3.8- 5.4 4.7 yellow blue

methyl red 4.2- 6.2 5.0 red yellow

bromothymol blue 6.0- 7.6 7.1 yellow blue

phenol red 6.4- 8.0 7.4 yellow red

thymol blue 8.0- 9.6 8.9 yellow blue

phenolphthalein 8.0- 9.8 9.7 colourless red

thymolphthalein 9.3-10.5 9.9 colourless blue

alizarin yellow R 10.1-12.0 11.0 yellow red

indigo carmine 11.4-13.0 12.2 blue yellow