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Hydrogen, Oxygen - General Chemistry and Qualitative Analysis - Lecture Handout, Exercises of Chemistry

Anand Agricultural UniversityChemistry

Some important topics from the course are Kinetics, Equilibrium, Acid-Base Chemistry, Applications of Aqueous Equilibria, hermodynamics, Electrochemistry and Organic Chemistry. Keywords in these lecture handout are: Hydrogen, Oxygen, Water Former, Electronic and Bonding Properties, Quantitative Application, Bond Dissociation Energy, Isotopes of Hydrogen, Isotope Effects, Synthesis of Hydrogen, Reactivity of Hydrogen, Binary Hydrides

Typology: Exercises

2012/2013

Uploaded on 08/31/2013

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Chapter 18, Hydrogen, Oxygen, & H2O
The previous chapters have significant theoretical components
and were oriented toward quantitative application. This chapter is
somewhat more descriptive in nature.
We will review:
1. Electronic & bonding properties of H and O.
2. Properties of common compounds formed by H & O.
3. A little bit about H2O. (Check out “photo,” p. 730.)
I. Hydrogen (“water former”)
A. Hydrogen is a diatomic gas (H2) under normal T & P.
1. The H!H covalent bond is very strong.
a) Bond dissociation energy = 436 kJ/mol
b) Even at 2000 K, only about 0.04% dissociates.
2. Does this strong bond seem inconsistent with the idea
that H2 is quite reactive?
B. Where is hydrogen found?
1. In stars (like our sun) and nebulae.
2. In the ocean and Lake Bowen. (9th most prevalent by
mass, 3rd most prevalent by number of atoms)
3. In many organic polymers. (95% of industrial H2 is
used for production of other chemicals)
4. In your body. (DNA, RNA, proteins, fats, etc.)
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Download Hydrogen, Oxygen - General Chemistry and Qualitative Analysis - Lecture Handout and more Exercises Chemistry in PDF only on Docsity!

Chapter 18, Hydrogen, Oxygen, & H 2 O

The previous chapters have significant theoretical components and were oriented toward quantitative application. This chapter is somewhat more descriptive in nature. We will review:

  1. Electronic & bonding properties of H and O.
  2. Properties of common compounds formed by H & O.
  3. A little bit about H 2 O. (Check out “photo,” p. 730.)

I. Hydrogen (“water former”)

A. Hydrogen is a diatomic gas (H 2 ) under normal T & P.

  1. The H!H covalent bond is very strong. a) Bond dissociation energy = 436 kJ/mol b) Even at 2000 K, only about 0.04% dissociates.
  2. Does this strong bond seem inconsistent with the idea that H 2 is quite reactive?

B. Where is hydrogen found?

  1. In stars (like our sun) and nebulae.
  2. In the ocean and Lake Bowen. (9 th^ most prevalent by mass, 3 rd^ most prevalent by number of atoms)
  3. In many organic polymers. (95% of industrial H 2 is used for production of other chemicals)
  4. In your body. (DNA, RNA, proteins, fats, etc. )

II. Isotopes of Hydrogen

A. Recall that isotopes are atoms that have the same number of protons, but different numbers of neutrons.

  1. Therefore, all isotopes of H have one p +^.
  2. 3 major isotopes of H have 0, 1, and 2 neutrons. Names: protium, deuterium, tritium, respectively.
  3. See Table 18.1 for a summary of isotope properties.

B. Isotope effects

  1. Have you seen the Diatomic computer program? a) You can view two covalently bonded atoms like two spheres attached by a spring (the bond). (Sounds simple, but has predictive value.) b) Can you see why a bond between 1 H & an atom would behave differently than an equivalent bond between 2 H & the same atom? c) H isotope effects tend to be large. ( 1 H mass is half that of 2 H. Relative to 12 C & 14 C?)
  2. This difference in mass causes differences in the rates of reactions involving H. Isotope effects can give mechanistic information about chemical rxns.
  3. Example:

H 2 O( l ) W H+^ ( aq ) + OH!( aq ) K = 1.01 x 10!^14 D 2 O( l ) W D+^ ( aq ) + OD!( aq ) K = 0.195 x 10!^14

Do you think k (^) f is different, k (^) r is different, (both)?

  1. Compare w/ other elements (Fig. 4.6, 4.7, pp. 106-7). Contrast with other alkali metals.
  2. Because of this high affinity, H tends to share e!, not give them. (See electronegativity also.)
  3. Free H+ (g) does exist, however, H + (aq) combines w/ one (or more) water molecules to form H 3 O+^ , etc.

B. Hydrogen as a halogen:

H( g ) + e!^ ÿ H!( g ) Eea = !73 kJ/mol

  1. This electron affinity value is considerably lower than that for halogens. (See Fig. 4.8, p. 110.)
  2. However, H will accept e!^ from very active metals.
  3. H 2 is relatively unreactive in a kinetic sense. However, many of its reactions are strongly favored in terms of equilibrium. Example:

2 H 2 ( g ) + O 2 ( g ) ÿ 2 H 2 O( l ) )H° = !572 kJ

Traveling by dirigible? (photo, p. 736)

V. Binary Hydrides (What does binary mean?)

A. Ionic Hydrides

  1. Melting point is a good indicator (see Fig. 18.2)

2 Na ( l ) + H 2 ( g ) ÿ 2 NaH( s ) m.p. (d?) > 800°C Ca ( s ) + H 2 ( g ) ÿ CaH 2 ( s ) m.p. = 816°C

Are the electronegativity differences indicative of bond character?

  1. Ionic hydrides are quite reactive (example: KH).

B. Covalent Hydrides

  1. Groups 3A !7A hydrides are generally covalent.
  2. The Haber process, an important industrial example: In the USA: 3 H 2 ( g ) + N 2 ( g ) ÿ 2 NH 3 ( g ) 20 x 10^6 tons NH 3 /year
  3. Other important examples: H 2 O, CH 4 , HCl

C. Metallic Hydrides

  1. Actinides, lanthanides, and some d -block elements form these. Examples: TiH (^) 1.7, TiH 2 , ZrH1.9, UH 3.
  2. These interstitial hydrides (Fig. 18.3) contain hydrogen in some single atom based form (not H 2 )
  3. Note that some of these hydrides are nonstoichiometric , the subscript is non-integer.
  4. These hydrides might be useful as a H 2 storage medium, as the H 2 “binding” is heat dependent. Try Prob.18.6-18.8, pp. 739-741 on your own.
  1. Was O 2 production important in an airplane disaster in the US in the 1990's? (Too long ago?)

B. O 2 is produced (actually purified) on a large scale by distilling liquid air. b.p.’s: N 2 = !196°C Ar = !186°C O 2 = !183°C

  1. 30 x 10 6 tons/year
  2. Check out the large white vertical tanks by the hospital.
  3. 2 C 2 H 2 + 5 O 2 ÿ 4 CO 2 + 2 H 2 O )H°=!2511 kJ

VIII. Properties of Oxygen

A. O 2

  1. is paramagnetic as in all three states.
  2. contains a double bond (compare length & strength).

B. Oxygen forms

  1. ionic oxides with metals.
  2. covalent oxides with nonmetals.
  3. strong double bonds with Period 2 elements N & C.

IX. Oxides (see Fig. 18.6, p. 745)

A. The oxygen is in the !2 oxidation state in oxides.

  1. Metallic oxides are basic: MgO( s ) + H 2 O( l ) ÿ Mg 2+^ ( aq ) + 2 OH!( aq )
  2. Covalent oxides are acidic: N 2 O 5 ( s ) + H 2 O( l ) ÿ 2 H+^ ( aq ) + 2 NO 3 !( aq )
  3. Amphoteric oxides are switch hitters: Al 2 O 3 ( s ) + 6 H+^ ( aq ) ÿ 2 Al 3+^ ( aq ) + 3 H 2 O( l ) Al 2 O 3 ( s ) + 2 OH!( aq ) + 3 H 2 O( l ) ÿ 2 Al(OH) 4 !( aq ) Al 2 O 3 can neutralize acid or base.

B. The ionic character of the oxides across a period (as judged by m.p.) varies more or less as you would predict based on electronegativity. (Fig. 18.7, p. 746)

XI. Hydrogen Peroxide

A. H 2 O 2 has numerous industrial uses.

B. H 2 O 2 can act both as an oxidant and a reductant, both with other chemicals & with itself (disproportionation):

2 H 2 O 2 ( l ) ÿ 2 H 2 O( l ) + O 2 ( g ) )H°=!196 kJ + boom?

C. Do we have any water pollution problems in our area?