Upload
leonard-carter
View
214
Download
0
Embed Size (px)
Citation preview
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