as fossil fuel supplies dwindle hydrogen gas could

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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) _______________________________________________________________________________ EXERCISES 1. 2. 3. 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 STUDY OBJECTIVES 1. 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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This note was uploaded on 09/15/2009 for the course CHEM 102 taught by Professor Bastos during the Spring '08 term at Adelphi.

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