無(wú)機(jī)化學(xué)課件：17 5 Hydrogen

1、HydrogenThe Traditional View of Cosmology: The Big Bang!Origin of the UniverseHydrogenMost abundant element in the Universe; 3rd most abundant element in the Earths crust, found in minerals, oceans and all living things.Hydrogen is a unique element, does not belong to any groups in the periodic tabl

2、e.Hydrogen IsotopesDeuterium was discovered in 1932 by Harold C. Urey, deuterium is a stable isotope of the element hydrogen. An atom of deuterium consists of one proton, one neutron and one electron. About .015% of natural hydrogen is composed of deuterium.Tritium was discovered in 1934, and is an

3、unstable isotope of the element hydrogen. An atom of tritium consists of one proton, two neutrons and one electron. Tritium is radioactive and has a half-life of about 12.5 years.Isotopes of Hydrogen The three H isotopes have different nuclear spin which give rise to easily observed changes in IR, R

4、aman and NMR spectra of molecules containing these isotopes.Because the relative mass differences between Hs isotopes are so large, there is a significant dissimilarity in physical properties.Heavy water D2O: can be separated from H2O by distillation.IsotopesNo. ofNeutronsNuclearSpinMolarMass(g/mol)

5、BoilingPointKBondEnergy(kJ/mol)H01.01-D 112.02- -T 2 3.03- -H2 - - 2.02 20.6 436D2- - 4.03 23.9 443T2- - 6.03 25.2 447H2O- - 18.02 373.2 463.5D2O- 20.03 374.4 470.9Kinetic Isotope EffectBond dissociation energy of D2 is about 7 kJ/mol greater than H2. Therefore reaction rates are often measurably di

6、fferent for processes in which E-H and E-D bonds are broken, made or rearranged.For C-H vs C-D bonds, the difference in reaction rates kH/kD can be as high as about 7. This is usually referred as the kinetic isotope effect.The detection of this kinetic isotope effect can often help to support a prop

7、osed mechanism.ortho and para Hydrogen 正、仲氫These differ in the magnetic interactions of the protons due to the spinning motions of the protons. In ortho-hydrogen, the spins of both protons are aligned in the same directionthat is, they are parallel. In para-hydrogen, the spins are aligned in opposit

8、e directions and are therefore antiparallel. Physical Properties of ortho & para HydrogenNormal hydrogen at room temperature contains 25% of the para form and 75% of the ortho form. The ortho form cannot be prepared in the pure state. Since the two forms differ in energy, the physical properties als

9、o differ. The melting and boiling points of parahydrogen are about 0.1 deg C lower than those of normal hydrogen.Where do we get it from?In the laboratory, CaH2 + 2H2O Ca(OH)2 + 2H2This is efficient in that 50% of the hydrogen produced comes from water. Another approach Boyles early synthesis, the r

10、eaction of iron filings with dilute sulphuric acid.Fe + H2SO4 FeSO4 + H2Industrial methods for the production of hydrogen depend upon local factors such as the quantity required and available raw materials. Two processes in use involve heating coke with steam in the water gas shift reaction or hydro

11、carbons such as methane with steam.CH4 + H2O CO + 3H2 1100oCC(coke) + H2O CO + H2In both these cases, further hydrogen may be made by passing the CO and steam over hot (400C) iron oxide or cobalt oxide.CO + H2O CO2 + H2What about very pure H2?Cathode. 2H2O(l) + 2e- 2OH- + H2Anode. 2OH-(aq) H2O(l) +1

12、/2O2 +2e-This process is energy intensive, and requires electrolysis of basic solutions.Industrial Production of HydrogenWater gas reaction C + H2O CO + H2 (endothermic) Methane reforming CH4 + H2O CO + 3 H2 (endothermic)Thermal Decomposition Of WaterThermal decomposition, also called thermolysis 22

13、00 C about three percent of all H2O molecules are dissociated into various combinations of hydrogen and oxygen atoms, mostly H, H2, O, O2, and OH. Other reaction products like H2O2 or HO2 remain minorNuclear-thermalSolar-thermalWater Splitting1 Electrolysis 1.1 High pressure electrolysis1.2 High-tem

14、perature electrolysis2 Photoelectrochemical water splitting3 Photocatalytic water splitting4 Photobiological water splitting5 Thermal decomposition of water 5.1 Nuclear-thermal5.2 Solar-thermalProperties of DihydrogenDihydrogen is a colorless gas at room temperature.Boiling point -253 CMelting Point

15、 -259 CBond Dissociation Energy 436 kJ/mol The H-H bond is stronger than the bonds hydrogen has with most other non-metals (e.g., H-N: 390 kJ/mol)Bond Length 0.74 Electronegativity 2.2 Similar to B, C, Si. Therefore E-H bonds involving these elements are not expected to be polar.Uses commercial fixa

16、tion of nitrogen from the air in the Haber ammonia process hydrogenation of fats and oils methanol production, in hydrodealkylation, hydrocracking, and hydrodesulphurization rocket fuel welding production of hydrochloric acid reduction of metallic ores for filling balloons (hydrogen gas much lighter

17、 than air; however it ignites easily) One of hydrogens isotopes, tritium (3H) is radioactive. Tritium is produced in nuclear reactors and is used in the production of the hydrogen bomb. It is also used as a radioactive agent in making luminous paints and as a tracer isotope Hydrogen as a modern fuel

18、.This car is fueled by hydrogen.Fuel tank has 70 layers of fiberglass and Al.What about the Challenger and Hindenburg?Compare and contrast the properties of H2 and gasoline.Chemical reactions Reaction of hydrogen with airHydrogen is a colorless gas, H2, that is lighter than air. Mixtures of hydrogen

19、 gas and air do not react unless ignited with a flame or spark, in which case the result is a fire or explosion with a characteristic reddish flame whose only products are water, H2O.2H2(g) + O2(g) 2H2O(l)Reaction of hydrogen with waterHydrogen does not react with water. It does, however, dissolve t

20、o the extent of about 0.00160 g kg-1 at 20C (297 K) and 1 atmosphere pressure.Reaction of hydrogen with the halogensHydrogen gas, H2, reacts with fluorine, F2, in the dark to form hydrogen(I) fluoride.H2(g) + F2(g) 2HF(g)Hydronium IonH (g) H+ (g) + e-I.E. = 1311 kJ/molLi (g) Li+ (g) + e- I.E. = 520

21、kJ/molNa (g) Na+ (g) + e-I.E. = 490 kJ/molI.E. is three times larger than the value of alkali metals. Therefore, H+ (g) does not exist under ordinary conditions.Ionic Radii: H+ 10-5 Li+ 0.9 Na+ 1.2 Due to its large charge/size ratio, H+ has a high polarizing power.In solutions, it is solvated by sol

22、vent molecules.In aqueous solution, exits as H(H2O)n+ (aq).In liquid ammonia, form NH4+Hydride Ion H2(g) + e- - H- (g) DH0 = +145 kJ/mol F2(g) + e- - F- (g) DH0 = -249 kJ/mol Cl2 (g) + e- - Cl- (g) DH0 = -228 J/molOnly most electropositive metals (e.g. Group 1 and 2 metals) can stabilize H- in an io

23、nic lattice.Ionic Radii:H- 1.30 (in LiH) 1.54 (in CsH)F- 1.33 Cl- 1.67 H-: low charge/size ratio; easily polarizable (i.e. it can easily distorted by any nearby cation).Strong basic character, reacts violently with H+ source.Hydrides.Hydrides are binary compounds of hydrogen and another element.Alth

24、ough all hydrogen compounds could be termed as hydrides, not all hydrogen containing compounds display hydridic character.Hydridic compounds are those that:React as H- donors or,Clearly contain anionic hydrogen.Hence NaH is hydridic, methane and HCl are not.This is parallel with the earlier statemen

25、tsThe loss of an electron to give H+.Acquisition of and electron to give H-.The formation of a single covalent bond. Classification of HydridesIt is useful to classify hydrogen compounds as :Ionic or covalentStoichiometric or nonstoichiometricBinary or complex.Be and Mg are extended and polymeric.Sa

26、lt-like = ionicIonic HydridesThe most electropositive elements react directly with H2 to form stoichiometric hydrides.These compounds are referred to as ionic or salt-like hydrides.When heavier metals are involved they are truly hydridic.Also, Be, Mg, and Li have slightly more covalent character and

27、 are better described as a polymer. Ionic hydride preparation and structure.Typically these compounds are prepared by direct interaction with the metals at 300-700oC.2 M(l) + H2(g) 2MH(s)M(l) + H2(g) MH2(s)The rates of these reactions are Li Cs K Na. All produce pure white solids that appear grey wh

28、en impure. Crystal Structures of Metal hydridesAlkali metal hydrides and LiH and CsH take on the NaCl Structure.What about others?MgH2 adopts the rutile金紅石 structure.CaH2SrH2BaH2All take on a PbCl2-likedistorted hcp六方緊密堆積 array.Reactions of Ionic metal hydrides1. All thermally decompose to give meta

29、l and hydrogen.Only LiH is stable to its melting point of 688oC.Note that LiH is unreactive at moderate temperatures toward oxygen and chlorine. Generally ionic hydrides are highly reactive toward air and water.MH(s) + H2O H2(g) + MOH(s)MH2(s) + H2O H2(g) + MOH2(s)Ionic hydrides are powerful reducin

30、g agents and good hydrogen-transfer agents.NaH + B(OCH3)3 NaHB(OCH3)3NaH + TiCl4 Ti0 + 4NaCl +2H2Covalent hydridesCovalent hydrides include:Neutral binary XH4 compounds of Group 14, like methane.Slightly basic binary XH3 compounds of Group 15, NH3 and PH3.Weakly acidic or amphoteric, binary XH2 of G

31、roup 16, H2O and H2S.Strongly acidic binary HX compounds of Group 17, HCl and HI.Covalent hydrides of boron.Hydridic, complex compounds of hydrogen. Examples include LiAlH4 and NaBH4. These are powerful reducing agents despite the covalent nature of the Al-H and B-H bonds.Some interesting notes abou

32、t LiAlH4 and NaBH4.These two compounds are ionic in nature BUT they possess tetrahedral anions containing covalent bonds to HHow are LiAlH4 and NaBH4 prepared?Both these reactions are carried out in ether.8LiH + Al2Cl6 2LiAlH4 + 6 LiCl2 NaH + B2H6 2NaBH4The anions are powerful hydrogen transfer agen

33、ts.2LiAlH4 + 2 SiCl4 2SiH4 + 2 LiCl + Al2Cl6I2 + 2 NaBH4 B2H6 + 2NaI + H2 Gas phase BH3B2H6Some more details about covalent hydrides.Covalent hydrides can be divided into three subcategories which are reliant on the nature of the H atom.The H-atom is neutral.The H-atom is positive.The H-atom is nega

34、tive. These are a generalization of the statements that we made before.By far the majority of covalent hydrides fall into the first category.Given their low polarity these compounds are only held together byweak intermolecular forces termed dispersion forces.This results in low boiling points.SnH4 -

35、52oC , PH3 -90oC Carbon-based systems comprise the largest set of hydrides.Hydrides of the Alkali and Alkaline Earth MetalsThe alkali and alkaline earth metals react exothermically with hydrogen at elevated temperature to form ionic hydrides:2 Na + H2 2 Na+HCa + H2 Ca2+H2The chemical properties of t

36、he hydrides are determined by their high basicity and reducing power. The hydride ion reacts irreversibly with compounds containing acidic protons, liberating hydrogen:M+H + Hd+Xd M+X + H2X = halogen, OH, OR, HNR, SRThree Elements dominate hydride chemistry.C, Si, and BIn summary. Si and C: X-H bond

37、s are “normal in that they are covalent in the traditional sense of e- sharing.B: bonds in these systems are unusual because H can bridge B atoms.I2 + 2 NaBH4 B2H6 + 2NaI + H2 1976 Nobel Prize for ChemistryProfessor William N. Lipscomb, Harvard University, USA, for his studies on the structure of bo

38、ranes illuminating problems of chemical bondingThe electronic structure of diborane.The electron configuration of B is 1s2 2s2 2p1It has three valence electrons and is expected to form BH3.BUT IN THIS SITUATION THE OCTET RULE IS NOT FULFILLEDTo achieve a pseudo-full octet boron is hybridized to an s

39、p3 orbital organization. It forms two 2-center-2-electron bonds (B-H) with two hydrogen atoms as well as two 3-center-2-electron bonds (B-H-B).Transition Metal HydridesTransition metal hydrides are diverse in structure and properties.Many contain Metal-hydrogen bonds including:1. Stoichiometric Bina

40、ry anions: ReH92- and FeH64-2. Complex Stoichiometric compounds with covalent bonds to hydrogen. like H(Mn(CO)6) and Re2H8(PR3)4.3. Non-stoichiometric compounds formed by exposure of metals to H2.Reaction of H2 with transition metals yields complex substances.Typically these compounds are black or g

41、rey and have formulas like LaH2.87, YbH2.55, TiH1.7, ZrH1.9. Under extreme circumstances limited compositions can be attained.Right now we do not understand the dynamics of these systems.H2 as a ligandH2 can behave as a ligand and occupy a coordination space around a metal center. This occurs for metals in low oxidation states.Hydrogen can take