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CH 235-2F
Worksheet 6-Exam 2
September 29, 2014
Short Answer:
1.
What are the three definitions for acids/bases? Give an example of each. Which
one is most useful for organic chemistry?
Arrhenius theory-says that acids dissociate in water to give H3O+ and bases dissociate
in water to give OH- ex. NaOH and HCl
Bronsted Lowry Theory-says that an acid donates a H+ and a base accepts an H+
ex. HCl and NH3
Lewis Theory-acids are electron acceptors and bases are electron donors. This is the
most useful for organic chemistry. ex. NH3 and CH3COOH
2. Fill in the table below regarding factors that affect acidity/basicity.
Factor
s character
Arrange the following in
order of most s
character to least s
character: sp3, sp2, sp
sp > sp2 > sp3
Why does more s
character mean a strong
acid/weaker conjugate
base?
More s character means
the electrons are held
tighter to the nucleus so
this makes the atom
“feel” more
electronegative so the
conjugate base is more
stable (weaker)
Size
Follow this trend when
going down a
__________________.
Column/Group
Which is larger, Iodine
or bromine?
Iodine
According to your
answer in the previous
box, is HI a stronger
acid than HBr?
yes
Electronegativity
Follow this trend when
going across a
__________________.
Row/Period
True or False: HF is a
weaker acid than NH3.
False (Compare N vs.
F)
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Factor
Inductive Effect
This involves the pulling
of electron density
through
_____________
How does distance
affect this?
More distance means
less of an effect
true
____________. bonds
Resonance
This trend allows for
__________________
of charge.
True or false: more
electronegative atoms
will result in more of an
inductive effect.
Draw the resonance
structure for the
conjugate base of
CH3COOH
Delocalization
General Question: Does
equilibrium favor the
strong acid/strong base
or the weak acid/weak
base?
Weak acid/weak base
3. What is a nucleophile? What is an electrophile? Give examples of each. How do you
determine which species is a nucleophile/electrophile given a reaction?
Nucleophile- the “base” in the reaction, “nucleus loving”, usually has a lone pair of
electrons or a source of electrons that can be used to attack the electrophile. Ex. NH3
Electrophile-the “acid” of the reaction, “electron loving”, usually electron deficient. Ex.
HBr
4. For each reaction below, determine which species is the nucleophile and which
species is the electrophile.
H20 + NH3
CH3COOH + H2O
HPO42- + NH4+
NH4+ + OH-
________________
H2O is the Elec. NH3 is the Nuc.
CH3COO- + H3O+ CH3COOH is the Elec. H2O is the Nuc.
NH3 + H2PO4- ___ HPO42- is the Nuc. and NH4+ is the elec.
For this type of problem, look at what happens in the reaction. Something that
has an added proton in the product was a nucleophile in the reactants.
Something that lost a proton or gained an electron pair in the products was an
electrophile in the reactants.
Matching
A. Low pKa
B. High pKa
A, E 1. Strong Acid
C, G 2. Strong Base
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C.
D.
E.
F.
G.
H.
Low pKb
High pKb
High Ka
Low Ka
High Kb
Low Kb
B, F 3. Weak Acid
D, H 4. Weak Base
Remember, Ka/Kb is the equilibrium equation for the reaction. It is equal to
products over reactants. A strong acid will have a lot of product because it
readily dissociates; so, it will have a higher Ka. pKa is the (-)log of the Ka. A high
Ka means a small pKa. This same trend works for Kb/pKb.
5. Arrange the following in order of increasing acidity: HS, HF, HCl, HSe. What trend
did you use?
HF < HCl < HS < HSe
Use the size trend. Remember when going from left to right, atoms get smaller.
When going from top to bottom, atoms get bigger.
6. Arrange the following from weakest to strongest conjugate base (I have shown
the acid):
2
3
1
To get the conjugate base, simply remove the proton from the OH group. The first
molecule has resonance, but the middle molecule does not. The last structure has
resonance AND electronegative atoms, which allows for the inductive effect.
Remember: strong conjugate bases come from weak acids.
Multiple Choice
1.
Which of the following statements is true?
A. HCl is a stronger acid than HI.
B. A molecule that can delocalize charge due to resonance is a more stable conjugate
base than one with no resonance structures.
C. A carbon with sp2 hybridization has more s character than a carbon with sp
hybridization.
D. HF is a weaker acid than H2O.
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E. Both B and D.
2. Which of the hydrogens indicated below is most likely to be removed in an acid/base
reaction?
A.
B.
C.
D.
Green Arrow
Blue Arrow
Yellow Arrow
Red Arrow
Think about the resonance structures you can draw when this proton is taken off.
This means you can delocalize that charge instead of it “sitting” on one atom.