November 25, 2014
Bronstead-Lowry
Acid Base Definitions
Empirical Definitions of Acids and Bases
Acids
–pH <7
–Turn blue litmus red
–React with metals to form H2 gas
–Taste sour
–electrolytes
Bases
–pH>7
–Turn red litmus blue
–Slippery
–Taste bitter
–electrolytes
November 25, 2014
Bronstead - Lowry definition of acids and bases (1923)
+
acid: a proton donor (loses a H )
base: a proton acceptor (gains an H+)
An acid base reaction is a chemical equilibrium having a
forward and reverse reaction involving the transfer of a
proton
November 25, 2014
Example
NH3 + HCO3-
H O + HNO
2 (l)
2(aq)
NH4+ + CO32-
H O+
+ NO 3 (aq)
2 (aq)
Conjugate Acids and Bases
-a conjugate acid base pair is an acid and base that
only differ by a proton
Example
What is the conjugate acid of SO32-?
What is the conjugate base of HCN(aq)
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in any acid base reaction, there are two conjugate acid
base pairs
Consider the following equilibrium:
HCN(aq) + H2O(l)
+
H3O (aq) + CN (aq)
Predicting Acid Base Equilibrium
- an acid base equilibrium favours the weaker acid and base
-remember the acid base table lists acids and bases in order
of strength
Example
HCN(aq) + H2O(l)
H3O+(aq) + CN-(aq)
Are products or reactants favored?
November 25, 2014
NH + HCO
3
3
-
NH
+
+ CO
4
3
2-
Are products or reactants favoured?
Buffer
A solution made by mixing a weak acid and conjugate base
together
Buffers resist changes in pH when small amounts of acid or
base are added to the buffer
November 25, 2014
Example
What would sodium nitrite be mixed with to form a buffer
solution?
NaNO2(aq) ----> Na+(aq) + NO2-(aq)
What would hydrofluoric acid be mixed with to form a
buffer solution?
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What could sodium dihydrogen phosphate be mixed with to
form a buffer solution?
Biological Buffers
Blood
carbonic acid / hydrogen carbonate
Cells
dihydrogen phosphate/ hydrogen phosphate
November 25, 2014
Use page 688 of your textbook to fill in the following
definitions:
a) amphiprotic substances
b) polyprotic acid
c) polyprotic base
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November 25, 2014