Transcript Previously in Chem104: structural characteristics of acids & bases

Previously in Chem104:
Today in Chem104:
• structural
• acid/base
characteristics of acids reactions: the easy,
& bases
the difficult, the
impossible
• calculations
• Polyprotic acids
Let’s do some chemistry:
Acid / Base Reactions
There’s 3 types of acid /base reaction problems:
Easy: strong acid + strong base
These reactions go to completion: 1 step
Difficult: strong acid + weak base
or (strong base) + (weak acid)
These reactions are neutral’n & equil’n: 2 steps
Impossible: weak acid + weak base
These reactions depend on magnitudes
of Ka and Kb.
Weak Acid /Weak Base Reactions
First type: you start on one side, with one acid and one base.
What does the reaction HAVE TO do?


methylamine
citric acid
methylammonium citrate
The reaction HAS TO go forward to
make some product until it reaches
equilibrium.
How much product it makes
depends on the Ka and Kb.
XX
Weak Acid /Weak Base Reactions
First type: you start on one side, with one acid and one base.
What does the reaction HAVE TO do?


methylamine
citric acid
methylammonium citrate
Calculation of amounts is very difficult unless you have information
on most of the species. You won’t have problems like that.
You will be asked about which direction the equilibrium favors in a
weak acid / weak base problem.
Weak Acid /Weak Base Reactions
Second type: you start with all four species, both acid/base pairs.
Which side does the equilibrium favor?


Methylamine
pKb 3.35
citric acid
pKa 3.0
methylammonium citrate
The equilibrium favors the side with weakest acid and base pair.
Or, the stronger acid and base push the reaction to the side with
weaker acid /base.
Example weak-weak problems
Sodium carbonate (0.1 mol) is dissolved in a solution of HF (0.01 M).
Describe the chemistry that occurs.
Example weak-weak problems
A solution of HF (0.01 M) is mixed with a solution of
hypochlorous acid (0.01 M).
Does anything happen?
Example weak-weak problems
A 1M solution of acetic acid is mixed with a 1 M solution of propanoic
acid acid.
Does anything happen?
Example weak-weak problems
Equimolar solutions of acetic acid, acetate, cyanide and hydrocyanic acid
are mixed. How will the equilibrium shift?
Polyprotic acids
…are acids that can donate more than one proton!
Phosphoric acid, H3PO4 … does having 3 H’s automatically mean it
acts as a polyprotic acid?
Step 1. H3PO4
+ H2O
H2PO4 - + H3O+
Ka1 = 7.6 x 10-3
Step 2. H2PO4-
+ H2O
HPO4 2- + H3O+
Ka2 = 6.2 x 10-8
Step 3. HPO42-
+ H2O
PO4 3- + H3O+
Ka3 = 2.12 x 10-13
Net. H3PO4
+ 3H2O
PO4 3- + 3H3O+
Keq = Ka1Ka2Ka3