Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille, Chemistry , 2007 (John Wiley) ISBN: 9 78047081 0866 . CHEM1002 [Part 2]. A/Prof Adam Bridgeman (Series 1) Dr Feike Dijkstra (Series 2) Weeks 8 – 13
Unless otherwise stated, all images in this file have been reproduced from:
Blackman, Bottle, Schmid, Mocerino and Wille,Chemistry, 2007 (John Wiley) ISBN: 9 78047081 0866
Acids & Bases 3 reproduced from:
Reproduced from ‘The Extraordinary Chemistry of Ordinary Things, C.H. Snyder, Wiley, 2002(Page 245)
Is a solution of reproduced from:NaCN acidic or basic?
NaCN is the salt of NaOH (strong base) and HCN (weak acid). The base “wins”: pH > 7
Overall reaction is
H2O(aq) + CN–(aq) OH–(aq) + HCN(aq)
Does a solution of NH4Cl have pH > 7 or < 7?
Salt of NH4OH (weak base) and HCl (strong acid)
acid “wins”: pH < 7 and reaction is
H2O(aq) + NH4+(aq) NH3(aq) + H3O+(aq)Salts of Weak Acids and Bases
CH3COOH(aq) + H2O(l) CH3COO-(aq)+ H3O+(aq)
A solution containing reproduced from:both a weak acid and its salt withstands pH changes when acid or base (limited amounts) are added.
Buffer with equalconcentrations of conjugate base and acid
Buffer after addition of H3O+
Buffer after addition of OH-
H2O + CH3COOH H3O+ + CH3COO-
CH3COOH + OH- H2O+ CH3COO-
Consider change in pH of pure water (pH = 7) if we add an equal amount of 10–3 M HCl:
[H+ ] = 1 x 10–3 M
pH goes from 7 to 3!Buffer systems and pH Change
Huge change!What about buffers?
HAc(aq) H+(aq) + Ac–(aq) Ka = 10-4.7
0.1 - 10-3
cf slide 6
x equal amount of 10 = 1.02 Ka = 0.000020 << 0.001
pH = – log x = 4.69
Buffer systems and pH Change
the pH hardly changes from 4.7!
Solution is buffered against pH change
Buffer Capacity equal amount of 10
Buffer capacity is related to the amount of strong acid or base that can be added without causing significant pH change.
Depends on amount of acid & conjugate base in solution:
highest when [HA] and [A–] are large.
highest when [HA] [A–]
Buffer Preparation and Capacity
Buffer Preparation and Capacity equal amount of 10
Most effective buffers have acid/base ratio less than 10 and more than 0.1 pH range is ±1
Biological systems, equal amount of 10e.g. blood, contain buffers: pH control essential because biochemical reactions are very sensitive to pH.
Human blood is slightly basic, pH 7.39 – 7.45.
In a healthy person, blood pH is never more than 0.2 pH units from its average value.
pH < 7.2, “acidosis”; pH > 7.6, “alkalosis”.
Death occurs if pH < 6.8 or > 7.8.Buffers in Natural Systems
“Extracellular” buffer (outside cell) equal amount of 10Buffer System in Blood
H+(aq) + HCO3–(aq)H2CO3(aq)
H2CO3(aq) H2O(l) + CO2 (g)
H2CO3(aq) + OH–(aq) HCO3–(aq) + H2O(l)
In the H3PO4 / NaH2PO4 / Na2HPO4 / Na3PO4 system, how could you make up a buffer with a pH of 7.40?
DATA: Ka1 = 7.2 x 10-3, Ka2 = 6.3 x 10-8, Ka3 = 4.2 x 10-13
pKa1 = 2.14, pKa2 = 7.20, pKa3 = 12.38
must use mixture of H2PO4- and HPO4-
original equal amount of 10
Practice Examples equal amount of 10
Summary: Acids & Bases 3 equal amount of 10