Hydrogen Properties PDF

 Electron structure and position Physical properties and isotopes Preparation and Reactions Hydrides Uses 1. Position in the Periodic Table: Hydrogen does not fit neatly into the periodic table  Sometimes placed at the head of the alkali metals in group 1  Less frequently, placed abov...

 Electron structure and position Physical properties and isotopes Preparation and Reactions Hydrides Uses 1. Position in the Periodic Table: Hydrogen does not fit neatly into the periodic table  Sometimes placed at the head of the alkali metals in group 1  Less frequently, placed above the halogens in group 17 Since its properties differ from those of both Group I and Group 17 elements. It could well be put in a group on its own.  Like group 1: it has only one valence electron. Unlike group 1: It is not a metal. It has a tendency to share the electron and form a covalent bond. Like the halogens: It requires one electron to complete its valence shell. It is one electron short of an inert gas structure. Unlike the halogen: It has an extremely small size Low electronegativity value.  1- Colorless: - can’t be seen Odorless: - has no smell Tasteless: - doesn’t have a taste Gas: - found in gas state, not liquid, or solid 2- Hydrogen is the lightest known gas. It has the lowest atomic weight and density of any known substance. 3- It has a very low melting and boiling point but a high dissociation energy. (D = 436 kJ mol-1) 4- High ionization energy (1310 KJ mol-1, 13.6 eV) and Low Pauling electronegativity (2.2) not very polar 5- Oxidation state: -1 with metals as in NaH and AlH3 +1 in nonmetals as in NH3 and HCl 5 of 32 © Boardworks Ltd 2005  Why is hydrogen unique? Hydrogen is knows as a very unique element as: - 1) It is a non-metal in group(I) - Usually elements in group(I) are metals 2) It tends to form more than one ion - H1+ is Known as a proton (stable) - H1- is known as a hydride (unstable) 6 of 32 © Boardworks Ltd 2005  Isotopes: The atoms of the same element with different mass numbers. This difference arise from the nucleus which contain different number of neutrons (have the same proton) 1 H, ordinary hydrogen most abundant (99.984%) 1 (1 electron, 1 proton, zero neutron) 2 H or D, heavy hydrogen abundance (0.016%) 1 (1 electron, 1 proton, 1 neutron) 3 H or T abundance 1 in 1021 hydrogen atoms 1 (1 electron, 1 proton, 2 neutron) 3 3 Half-life time of this decay H He +  = 12.4 years 1 2 7 of 32 © Boardworks Ltd 2005  These three isotopes have the same electronic configuration and have essentially the same , only differences in chemical properties are those that stem from mass, as the rates of bond-cleavage and equilibrium constant reactions. But they show greater difference in It is the effect which arises from the differences in mass number (differences in chemical or physical properties). 8 of 32 © Boardworks Ltd 2005 Stable form of elemental hydrogen under normal conditions  High bond enthalpy (436 KJ mol-1)  Short bond length (0.74 Å). In the Industry Commercially: Water-gas reaction from Coke Steam reforming from methane gas (hydrocarbons) In the laboratory Electrolysis of Water Acid + Metal Metal hydride + Water 9 of 32 © Boardworks Ltd 2005 1- From Coke By passing steam over red hot coke. 1000 oC C + H2O CO + H2 Water gas Very difficult to separate 1- H2O steam CO + H2 + CO2 2- Cool to 400 oC Passed over Fe2O3 Absorbed in a solution of K2CO3 K2CO3 + CO2 + H2O 2KHCO3 10 of 32 © Boardworks Ltd 2005 Hydrocarbons are produced at oil refineries. 800-900 oC CH4 + 2H2O CO2 + 4H2 Ni-catalyst Steam H2O H2 + ½ O2 Zn + H2SO4 ZnSO4 + H2 LiH + H2O LiOH + H2 11 of 32 © Boardworks Ltd 2005  H2O H2 bond is broken more readily than D2 bond. Thus by H2O D2O electrolysis, H2 is liberated faster than D2 and the DHO remaining water becomes enriched with heavy water D2O. 29000 L H2O 1L D2O 12 of 32 © Boardworks Ltd 2005  H2 Reacts slowly with most other elements because of its high bond enthalpy (436 KJmol-1) and hence high activation energy for reaction. Under special conditions, the reactions are rapid by: An example of this is chemisorption of hydrogen on Pt. This reaction accounts for the use of finely divided Pt metal to catalyze the hydrogenation of alkenes. Example of this is the reaction of H2 with a ZnO surface. H- H+ H2 + Zn O Zn O Zn O Zn O Cu/Zn CH3OH(g) CO(g) + 2H2(g) 13 of 32 © Boardworks Ltd 2005  This reaction is common in the reaction of hydrogen with halogens and oxygen. / h.. Br2 Br + Br.. Br + H2 HBr + H.. H + Br2 HBr + Br 14 of 32 © Boardworks Ltd 2005  The nuclear spins of the two atoms in the molecule are in the same direction At standard temperature and pressure, the gas H H contains about 75 % ortho hydrogen. The nuclear spins of the two atoms in the molecule are in opposite direction At zero K, o the gas contains 100 % para H H hydrogen. At standard Temp., the gas contains 25% para hydrogen. 15 of 32 © Boardworks Ltd 2005  The primary source of nitrogen- containing chemicals, plastics, and fertilizers. by catalytic hydrogenation of carbon monoxide to methanol. because of its high specific enthalpy (Specific enthalpy is the enthalpy of combustion of a sample divided by the mass of the sample; the specific enthalpy of hydrogen is 142 KJ g-1; that of a typical hydrocarbon is 50 KJ g-1). 16 of 32 © Boardworks Ltd 2005  Binary compounds of ype of hydrides depends on  Electronegativity of the element (Type of bond) 17 of 32 © Boardworks Ltd 2005  They formed when only elements with low electronegativity values can transfer electrons to the hydrogen atom. These include the and and 18 of 32 © Boardworks Ltd 2005 1- Solids have for for. 2- High melting points (LiH m.p = 691oC) density of Saline hydride that of the metal 3- Conduct electricity. 4- High heats of formation, and are always stoichiometric. 5- It Can ignite (NaH) if it left exposed to humid air. Such fires are difficult to extinguish Water more flammable hydrogen blanketed with an inert solid, such as silica sand 19 of 32 © Boardworks Ltd 2005 1- They react with and. This reaction is dangerously vigorous: NaH(s) + H2O(l)  NaOH (aq) + H2 (g) Reaction between MH and H2O is to. in this way because the strongly exothermic reaction evolves flammable hydrogen. because It is the cheapest of the saline hydrides Available in granular form, which is easy to handle. 20 of 32 © Boardworks Ltd 2005  Common for the electronegative elements Formed from group 13/III through 17/VII and hydrogen, { small difference in electronegativity} Since small difference in electronegativity between atoms favors a sharing of electrons and hence covalent bonding 21 of 32 © Boardworks Ltd 2005 1- Have molecular lattice made up of molecules with only forces or holding the molecules together. 2- Low melting and boiling points 3- Usually soft and volatile. 4- Lack of conductivity. 5- The general formula of these hydrides, Or n = the group no. which x belongs. e.g. , Hydrides of group III are , due to Note electron deficiency (have polymeric structure) 22 of 32 © Boardworks Ltd 2005  Systematic names (IUPAC) of the molecular hydrides: Nonsystematic names are used for some compounds like Group Formula Traditional name IUPAC name 13/III B2H6 Diborane Diborane 14/IV CH4 Methane Methane SiH4 Silane Silane GeH4 Germane Germane SnH4 Stannane Stannane 15/V NH3 Ammonia Azane PH3 Phosphine Phosphane AsH3 Arsine Arsane SbH3 Stilbine Stibane 16/VI H2O Water Oxidane H2S Hydrogen sulfide Sulfane 17/VII HF Hydrogen fluoride Hydrogen fluoride 23 of 32 © Boardworks Ltd 2005  Formed by elements of group 14/IV where all valence es of the central atom are engaged in bonds. e.g. Methane CH4, Ethane C2H6, Silane SiH4, Germane GeH4 Characterized by absence of lone pairs on central atom Common for boron and aluminum. e.g. Diborane, B2H6 Explanation of its structure is presence of B-H-B three center, 2 es bonds (3c,2e) acting as bridges between two B atoms, so two es can help to bind three atoms. Formed by the elements in groups 15/V through 17/VII e.g. Ammonia, NH3, with one lone pair on nitrogen Water, H2O, with two lone pairs on oxygen HF, HCl, HBr, and HI 24 of 32 © Boardworks Ltd 2005  This type is formed with the and. 25 of 32 © Boardworks Ltd 2005  Heat Metal + Hydrogen Metallic Hydrides Solid High Pressure Finely powdered Heat Metallic Hydrides Very finely divided Metal + H2 Decomposition  Metallic hydrides have high mobility of H2 at elevated temps. This property make  Hard.  Have metallic luster.  Have magnetic properties.  Conduct electricity.  In many cases the compounds are non stoichiometric (LaH2.87, TiH2.55, ZrH1.9, VH1.6)  Less dense than the parent metal. 26 of 32 © Boardworks Ltd 2005 27 of 32 © Boardworks Ltd 2005 28 of 32 © Boardworks Ltd 2005  Ionic Hydride Covalent Hydride Metallic Hydride Intermediate Hydride Hydrogen Gap 29 of 32 © Boardworks Ltd 2005 30 of 32 © Boardworks Ltd 2005  a. Classify the compounds CsH, PH3, B2H6, HCl and discuss their probable physical properties?. b. In one or two sentences define: ortho hydrogen, para hydrogen, molecular hydrogen ion. c. Comment on the use of hydrogen as a fuel for the large rockets and as a fuel in the future? d. Why does hydrogen react slowly and how can we activated? e. Comment on the use of saline hydrides and specifically calcium hydride in purification of nonaqueous solvents.? 31 of 32 © Boardworks Ltd 2005