Transcript Chapter 6 Hydrides and Oxides Notes

Hydrides
Most metals react with acid to produce hydrogen
gas.
For example:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
This is how hydrogen was first discovered by
Henry Cavendish. Hydrogen can also be made
from the electrolysis of water and the reaction of
hot carbon or methane with water.
Hydrogen has 1+ and 1- ionic charges possible. When
reacting with any 1A-5A elements it will take the 1form and be written as the anion “hydride”. When
reacting with any 6A-7A elements it will take the 1+
form and be the cation. Make note of the common
hydrogen compounds in the chart below for the
exceptions of the typical criss-cross.
LiH BeH2 B2H6 CH4 NH3 H2O HF
NaH MgH2 AlH3 SiH4 PH3 H2S HCl
KH CaH2 Ga2H6 GeH4 AsH3 H2Se HBr
RbH SrH2 InH3 SnH4 SbH3 H2Te HI
CsH BaH2
TlH PbH4 BiH3 H2Po HAt
Ionic hydrides (metal + hydrogen) are considered
basic because addition of water to an ionic hydride
forms a base:
CaH2(s) + H2O(l) → Ca(OH)2 + 2H2(g)
Oxides
Oxygen is reactive enough to form compounds with
all elements except the noble gases and Au, Pd, and
Pt.
That means all elements will oxidize, which we call
rust when it happens to iron.
Oxygen was first discovered by Carl Scheele, first
written about by Joseph Priestly (who called it
dephlogisticated air because it helped things to burn
well) and named oxygen by Antoine Lavoisier for the
French words for “acid former” (think how many
oxyacids there are).
When oxygen combines with alkali metals there are
three possibilities. With alkaline earth metals only
two:
1. It could form an oxide. Oxides have a -2 oxidation
state on the oxygen, for example: K2O.
2. It could form a peroxide. Peroxides have a -1
oxidation state on the oxygen, for example: K2O2.
3. It could form a superoxide. Superoxides have a oxidation state on the oxygen, for example: KO2.
The following table summarizes the possibilities with
the first two groups of the periodic table. You need
to know which ones will form which oxide.
Li
Na
K
Oxide
Li2O
Na2O
K2O
Peroxide
Li2O2
Na2O2 K2O2
Rb2O2 Cs2O2
NaO2
RbO2
Superoxide
KO2
Rb
Rb2O
Cs
Be
Mg
Ca
Sr
Ba
Cs2O
BeO
MgO
CaO
SrO
BaO
CaO2
SrO2
BaO2
CsO2
Notice the group IIA metals do not readily form
peroxides and none form superoxides. Only at
extremely excessive amounts of oxygen would
calcium through barium form the peroxides.
Almost all other metals form oxides as well, but as
they have multiple oxidation states they can make
multiple varieties of oxides, but no peroxides or
superoxides would be formed. With multiple
oxidation state metals, the resulting valence of the
metal depends entirely on the amount of oxygen
present for the reaction. For example:
4Cu + limited O2 → 2Cu2O (a red oxide)
2Cu + excess O2 → 2CuO (a black oxide)
It should be logical that the more negative oxygen
there is the higher the positive oxidation state will
be on the metal.
Reactions with Oxygen
Metallic oxides are considered basic oxides because
they will react with water to form a base (-OH).
Sometimes these are called basic anhydrides,
meaning they don’t yet have the water to form a
base with.
Na2O + H2O → 2NaOH
Nonmetallic oxides are considered acidic because
they will react with water to form an oxyacid (H_O).
Sometimes these are called acidic anhydrides.
SO3 + H2O → H2SO4
(note: B2O3 and SiO2 are not soluble so would not
combine with water)
Metallic oxides and nonmetallic oxides react with
each other to form oxy-salts.
MgO + CO2 → MgCO3
Combustion reactions involve combining oxygen
with hydrocarbons to form carbon dioxide and
water. Some metals (like magnesium) also combust,
but the products would most certainly NOT form
carbon dioxide and water.
C6H12 + 9O2 → 6CO2 + 6H2O
2Mg + O2 → 2MgO
(technically a combination reaction)
If not enough oxygen is available incomplete
combustion occurs and forms carbon monoxide and
water.
2C8H18 + 17O2 → 16CO + 18H2O
In a car the catalytic converter is a special material
that carbon monoxide would cling to and combine
with oxygen so only carbon dioxide exits. Both are
poisonous and dangerous to the environment, but
carbon monoxide is much more poisonous than
carbon dioxide.