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Hydrofluoric acid is unique in its reactions with sand (silicon dioxide) and with glass, which is a mixture of silicates:

SiO 2 ( s ) + 4HF ( a q ) SiF 4 ( g ) + 2H 2 O ( l )
CaSiO 3 ( s ) + 6HF ( a q ) CaF 2 ( s ) + SiF 4 ( g ) + 3H 2 O ( l )

The volatile silicon tetrafluoride escapes from these reactions. Because hydrogen fluoride attacks glass, it can frost or etch glass and is used to etch markings on thermometers, burets, and other glassware.

The largest use for hydrogen fluoride is in production of hydrochlorofluorocarbons for refrigerants, in plastics, and in propellants. The second largest use is in the manufacture of cryolite, Na 3 AlF 6 , which is important in the production of aluminum. The acid is also important in the production of other inorganic fluorides (such as BF 3 ), which serve as catalysts in the industrial synthesis of certain organic compounds.

Hydrochloric acid is relatively inexpensive. It is an important and versatile acid in industry and is important for the manufacture of metal chlorides, dyes, glue, glucose, and various other chemicals. A considerable amount is also important for the activation of oil wells and as pickle liquor—an acid used to remove oxide coating from iron or steel that is to be galvanized, tinned, or enameled. The amounts of hydrobromic acid and hydroiodic acid used commercially are insignificant by comparison.

Key concepts and summary

Hydrogen is the most abundant element in the universe and its chemistry is truly unique. Although it has some chemical reactivity that is similar to that of the alkali metals, hydrogen has many of the same chemical properties of a nonmetal with a relatively low electronegativity. It forms ionic hydrides with active metals, covalent compounds in which it has an oxidation state of 1− with less electronegative elements, and covalent compounds in which it has an oxidation state of 1+ with more electronegative nonmetals. It reacts explosively with oxygen, fluorine, and chlorine, less readily with bromine, and much less readily with iodine, sulfur, and nitrogen. Hydrogen reduces the oxides of metals with lower reduction potentials than chromium to form the metal and water. The hydrogen halides are all acidic when dissolved in water.

Chemistry end of chapter exercises

Why does hydrogen not exhibit an oxidation state of 1− when bonded to nonmetals?

The electronegativity of the nonmetals is greater than that of hydrogen. Thus, the negative charge is better represented on the nonmetal, which has the greater tendency to attract electrons in the bond to itself.

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The reaction of calcium hydride, CaH 2 , with water can be characterized as a Lewis acid-base reaction:
CaH 2 ( s ) + 2H 2 O ( l ) Ca ( OH ) 2 ( a q ) + 2H 2 ( g )

Identify the Lewis acid and the Lewis base among the reactants. The reaction is also an oxidation-reduction reaction. Identify the oxidizing agent, the reducing agent, and the changes in oxidation number that occur in the reaction.

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In drawing Lewis structures, we learn that a hydrogen atom forms only one bond in a covalent compound. Why?

Hydrogen has only one orbital with which to bond to other atoms. Consequently, only one two-electron bond can form.

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What mass of CaH 2 is necessary to react with water to provide enough hydrogen gas to fill a balloon at 20 °C and 0.8 atm pressure with a volume of 4.5 L? The balanced equation is:
CaH 2 ( s ) + 2H 2 O ( l ) Ca ( OH ) 2 ( a q ) + 2H 2 ( g )

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What mass of hydrogen gas results from the reaction of 8.5 g of KH with water?
KH + H 2 O KOH + H 2

0.43 g H 2

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Questions & Answers

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sodium hydroxide (NaOH)
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a hydrocarbon contains 7.7 percent by mass of hydrogen and 92.3 percent by mass of carbon
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TYPES OF COVALENT BOND-POLAR BOND-NON POLAR BOND-DOUBLE BOND-TRIPPLE BOND. There are three types of covalent bond depending upon the number of shared electron pairs. A covalent bond formed by the mutual sharing of one electron pair between two atoms is called a "Single Covalent bond.
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coordination number is the number of atoms or ions immediately surrounding a central atom in a complex or crystal
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Antoine Lavoisier. Father of modern chemistry
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geometric isomerism are molecules that are locked into their spatial position with respect to one another due to a double Bond or ring structure
Chromatography is a physical method of seperation where by mixtures that are in two phrases are separated
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Practice Key Terms 3

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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