Hydrogen Flashcards
prep of hydrogen in the lab
1) acid + metal (Mg,Al,Zn,Fe)
H2SO4 + Mg —> MgSO4 + H2
2) metal + alkali
Al+ NaOH —> Na[Al(OH)4] +H2
Be+ NaOH —-> Na2[Be(OH)4] +H2
3)s block metals + water
4) ionic hydrides + water/alcohol
NaH+H2O—> NaOH +H2
(redox,acid/base,comproportionation)
mention 2 methods of industrial prep of hydrogen
1) steam reformation of natural gas
2) electrolysis of pure water
explain process of steam reformation
1)reforming: CH4 + H2O CO + 3H2
@ 700 dgerees, 20 atm w/ Ni catalyst
2) shift reaction : CO+ H2O CO2+ H2
@ 400 degrees w/ Fe catalyst
3) absorption of CO2 to reduce release in atmosphere
K2CO3 + CO2 + H2O 2KHCO3
explain the electrolysis of water
water is a poor conductor so H2SO4 is added.
at cathode (-ve)2H+ +2e- –> H2
at anode (+ve) 2OH- —> 1/2O2 + H2) + 2e-
overall H2O —> H2 + 1/2 O2
is hydrogen a reducing or oxidising agent? when does it show this characteristic?
- reducing
- with non metals (N, S and O)
- reduction of metal oxides eg. CuO
- reduce organic molecules eg alkene to alkane
mention the 4 types of hydrides
ionic, covalent, interstitial and complex
ionic hydrides
formed by hydrigen with group 1 elements and group 2 elemnts (Mg to Ba)
– react with water to produce H2
covalen hydrides: reactions of CH4 methane and phosphane PH3 with water
do not react nor dissolve
covalent hydride: reaction of SiH4 (silane) with water
reacts to form SiO2 (whit solid)
covalent hydride: reactions of H2S , HCl and HF
H2S + H2O HS- + H3O+
HS- + H2O H3O+ + S2- (slightly acididc solution)
HF+ H2O H3O+ + HF- (slightly acidic solution)
HCl+ H2O —> H3O+ + Cl- (very acidic soln)
reaction of NH3 with water
NH3+ H2O NH4+ + OH-
explain bonding in LiAlH4
made of Li+ AlH4-
in AlH4- 3covalent bonds and 1 co ordinate bond
preparation of LiAlH4
2Li(s) + H2(g) —> 2LiH (dry H2 gas over hot Li)
2Al (s) + 3Cl2(g) —> 2AlCl3
dry chlorine passed over hot Al and the vapor collected
4LiH(s) + AlCl3(s) —> LiAlH4 (ether) + 3LiCl (s)
they are mixed in dry ether solvent. LiCl is filtered off.
LiAlH4 as a reducing agent
reduces C=O and COOH but not C=C
interstitial hydride
compounds where small H atoms enter the spaces between metal atoms in a metallic structure (usually a transition metal). Thsi is done by applying pressure. they are non stiochiometric.
interstitial hydrides as catalysts in hydrogenation reactions
- the hydride form can be used instead of the metal eg. palladium hydride instead of palladium in hydrogenation of an alkene.
- added benefit of hydrogen in structure released during reaction—> increase in rate fo reaction
requirement for ionic compound to dissolve in water
entahlpy of hydration must be larger than the lattice dissociation enthalpy.
name these hydrogen isotopes: 1H, 2H, 3H
protium, deuterium, tritium
reaction to prepare ND3
3LiN3 + D2O—> ND3 + 3LiOD
reaction to prepare C2D2
D2O + CaC2 —> C2D2+ Ca(OD)2
reaction to prepare C6D6 (alkene to alkane reaction)
3C2D2 —> C6D6
heat in absence of oxygen (red hot metal tube)
explain the concept of nascent hydrogen
- old concept once used to explain reduction of organic compounds
- thought to be a reactive form of H formed in situ (eg zing amalgam + acid)
- nowadays: reduction takes place on metal surface
describe connection between bonding and physical state of NaH and MgH2
NaH and MgH2 are ionic solids. the strong electrostatic forces between the oppositely charged ions gives rise to structures which are solids at room temp
describe the connection between the physical state and bonidng of Si, P , S and Cl hydrides.
SiH4, PH3, H2S and HCl are all predominantly simple covalent structures. the presence of only weak van der waals forces makes the materials gaseous at room temp.
why is water liquid at room temp? unlike other hydrides of group 6 which are gases.
water has H bonding since O is very electronegative. In this bond the shared pair of electron is attracted to the electronegative atom, which makes the H readily avaliable for dipole dipole attractions. Permanent dipole dipole attractions form. h does not have inner electrons to set up forces of repulsiion with non bonding e- of the other atom, which makes it different from other permanent di-di attractions. H bonding leads to high mp and bp—> liquid
