Unformatted text preview: s of the "hydrogen economy." As fossil
fuel supplies dwindle, hydrogen gas could take on more of a role as a fuel. Atomic energy could be used to
generate electricity which in turn could be used to electrolyze water to obtain hydrogen.
1 electrical energy + H2 O(l) → H2 (g) + 2 O 2 (g)
The combustion of hydrogen as a fuel would release energy.
1 H2 (g) + 2 O 2 (g) → H2 O(l)
∆ H° = –285.8 kJ
EXAMPLE 21.1 Hydrogen Compounds
Give an example of an ionic hydride and an example of a covalent hydride. What is the oxidation number of
hydrogen in each?
•Method of Solution
In ionic compounds, hydrogen has an oxidation number of –1. The hydrides of the alkali metals and some of the
alkaline earth metals are ionic hydrides. CaH2 is an example of an ionic hydride. Hydrogen has an
electronegativity of 2.1 and that of calcium is 1.0. Thus hydrogen has an oxidation number of –1 in CaH2 .
H2 S is a covalent hydride. The electronegativity of sulfur is 2.5. The lower polarity of the H—S bond is
indicated by a ∆ value of only 0.4. The oxidation number of hydrogen is +1 in H2 S.
EXAMPLE 21.2 Comparison of Ionic and Covalent Hydrids
Compare the reactions of water with covalent hydrides and ionic hydrides.
•Method of Solution
Covalent hydrides either do not react with water at all, which is the case with CH4 , or are acids in water. One of
them, ammonia, is a weak base.
H2 S(aq) + H2 O(l) HS– (aq) + H3 O+(aq) Ionic hydrides are strong Brønsted bases; they readily accept a proton from water.
H– (aq) + H2 O(l) → H2 (g) + OH– (aq) Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 4 18 / Nonmetallic Elements and Their Compounds
Lithium hydride is an ionic hydride.
LiH(s) + H 2 O(l) → LiOH(aq) + H2 (g)
4. Write an equation to show how aluminum can be used to prepare hydrogen in the laboratory.
What is the oxidation number of hydrogen in AlH3 , H 2 S?
Give an example of an ionic hydride.
Describe the hydride ion as a Brønsted acid or a base? Write an equation for the reaction of the hydride ion
with water. CARBON
2. Describe the chemical and physical properties of carbon.
Describe some of the inorganic compounds of carbon. Properties of Carbon. Carbon makes up only 0.09% of the Earth's crust by mass. Carbon occurs in
nature as the free element in the allotropic forms graphite and diamond. Carbon occurs as CO2 in the
atmosphere and as the carbonate in limestone and chalk. Fossil fuels contain a large percentage of carbon.
Carbon is the essential element in living organisms.
Pure carbon in the form of diamond is the hardest substance known. The microscopic structure of diamond
is such that each carbon atom is covalently linked to four other carbon atoms (see Figu...
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