Descriptive Chemistry 3
Acids-Bases - Read Chapter 8 pp.315- 330 (paying special attention to all discussions of acids
and bases), Chapter 15 pp. 620 - 626, and refer to the other portions of your text referred to
below while answering the following:
1. Define the terms metal, nonmetal, & metalloid (you may need to consult other sections).
Metal: a substance having characteristic luster, ductility, malleability, and high electrical
conductivity. Low ionization energies and electronegativities when compared to
nonmetals.
Nonmetal: an element having characteristic brittleness, lack of luster, poor electrical
conductivity, etc. High ionization energies and electronegativities when compared to
metals.
2. Describe the characteristic properties of metals and nonmetals (refer to pp. 323 - 326 in
your text).
See #1 (above) and consult your text.
3. Describe the periodic trends in metallic and nonmetallic character. How do these relate to
trends in ionization energy, electron affinity, and electronegativity (see Section 8.7, 8.8
and 9.7 in your text).
The most metallic elements are found in the lower left corner of the periodic table where
IE is low, EN is low, and EA is unfavorable. The most nonmetallic elements are found in
the upper right hand corner of the periodic table where IE is high, EN is high, and EA is
favorable.
4. Consider the following list of elements: O, Ba, Co, Be, Br, Se. From this list select the
element that (a) is most electronegative, (b) exhibits a maximum oxidation state of +7, (c)
loses an electron most readily, (d) forms pi bonds most readily, (e) is a transition metal.
•
•
•
•
•
most electronegative: O
maximum oxidation state = +7: Br
loses electron most readily: Ba
forms pi bonds most readily: O
is a transition metal: Co
5. Discuss why NH3 is a Bronsted-Lowry base while H2S and HCl are Bronsted-Lowry
acids.
General trends in compounds of X with H are: left to right across a period increasing
acidity, and top to bottom in the group also increases acidity. The reasons for these trends
are: increasing bond polarity (due to increase in the electronegativity of X) across a
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period and decreasing bond strength down a group.
For example, consider the following compounds . . .
basic
amphoteric
NH3 ----->
H2O
|
|
|
H2S
acidic
----->
HF
|
|
|
more
HCl
acidic
6. (Read Section 15.2 in your text to answer the next few questions.) Why is HF a weak
acid whereas HCl, HBr, and HI are all strong acids?
As mentioned above, the general periodic trend is increasing acid strength as one
proceeds down a group. As bond strength decreases it is easier to separate the H. As a
bond becomes more polar the H will carry a greater partial positive charge (be more like
a proton, H+). The most acidic compounds would be those with highly polarized very
weak bonds between H and X. However, as one can see from the following data HF is the
most polar bond but also the strongest. Therefore, HF is a weak acid due to the large
bond strength.
HF HCl HBr
E.N. X minus E.N. H
Bond Strength in kJ/mole
HI
1.9 0.9 0.7 0.4
565 427 363 295
NOTE: E.N. of H = 2.1
7. Nonmetal oxides are sometimes referred to as acid anhydrides (see p. 329). What does
this name mean? Write a reaction for a nonmetal oxide that illustrates acid anhydride
behavior.
Acid anhydride means acid minus water
Example 1 CO2 (g) + H2O (l) ---> H2CO3 (aq)
Example 2 SO3 (s) + H2O (l) ---> H2SO4 (aq)
In general: acid anhydride + water ---> acid
8. Why does acid strength increase in the order HClO < HClO2 < HClO3 < HClO4 .
This is due to the inductive effect. The more O (without attached H) attached to the
central atom in the oxyacid, the more electron density is withdrawn from the O-H bond
making it weaker (and making the H more positive . . . \"protonlike\"). Another way of
explaining the trend is by considering the stability of the conjugate base (the anion)
created when the oxyacid loses a proton. The species with the most oxygen atoms
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attached to the central atom will be the most capable of stabilizing the negative charge on
the anion.
9. Why does acid strength increase in the order HIO < HBrO < HClO ?
As the halogen becomes more electronegative, more electron density is withdrawn from
the H-O bond increasing acid strength. Cl is more electronegative than bromine, which is
more electronegative than iodine.
10. Which of the following organic acids should be the strongest? The weakest?
CH3COOH
CH2ClCOOH
CHCl2COOH
CCl3COOH
Again, inductive effects create the difference in acid strength. Electron density shifts the
O-H bond toward the end of the molecules with the Cl. The more Cl present on the
adjacent carbon, the greater amount of electron density is withdrawn, and the O-H bond
has more acidic character.
So the strongest acid from this list is CCl3COOH and the weakest acid from this list is
CH3COOH.
(in order from strongest to weakest CCl3COOH > CHCl2COOH > CH2ClCOOH >
CH3COOH)
11. (Refer to Section 15.6 in your text). Identify whether the following salts should behave
like acids or bases AND write reactions illustrating the rationale for your choice.
Na2CO3
K3PO4
Species
Na2CO3
K3PO4
NH4Cl
AlCl3
NaF
NH4HCO3
Kb > Ka
so it is basic
NH4Cl
Acidic
or
Basic
basic
basic
acidic
acidic
basic
basic
AlCl3
NaF
NH4HCO3
Reaction
CO32-(aq) + H2O (l) ---> HCO3-(aq) + OH-(aq)
PO43-(aq) + H2O (l) ---> HPO42-(aq) + OH-(aq)
NH4+(aq) + H2O (l) ---> NH3 (aq) + H3O+(aq)
Al3+(aq) + H2O (l) ---> AlOH2+(aq) + H+(aq)
OR
3+
Al(H2O)6 (aq) + H2O (l) ---> Al(H2O)5OH2+(aq) + H3O+(aq)
F-(aq) + H2O (l) ---> HF(aq) + OH-(aq)
NH4+(aq) + H2O (l) ---> NH3 (aq) + H3O+(aq)
Ka = 5.6 x 10-10
HCO3-(aq) + H2O (l) ---> H2CO3 (aq) + OH-(aq)
Kb = 2.3 x 10-8
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