Hydrogen

Question 1

prep of hydrogen in the lab

Answer 1

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)

Question 2

mention 2 methods of industrial prep of hydrogen

Answer 2

1) steam reformation of natural gas

2) electrolysis of pure water

Question 3

explain process of steam reformation

Answer 3

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

Question 4

explain the electrolysis of water

Answer 4

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

Question 5

is hydrogen a reducing or oxidising agent? when does it show this characteristic?

Answer 5

• reducing
• with non metals (N, S and O)
• reduction of metal oxides eg. CuO
• reduce organic molecules eg alkene to alkane

Question 6

mention the 4 types of hydrides

Answer 6

ionic, covalent, interstitial and complex

Question 7

ionic hydrides

Answer 7

formed by hydrigen with group 1 elements and group 2 elemnts (Mg to Ba)
– react with water to produce H2

Question 8

covalen hydrides: reactions of CH4 methane and phosphane PH3 with water

Answer 8

do not react nor dissolve

Question 9

covalent hydride: reaction of SiH4 (silane) with water

Answer 9

reacts to form SiO2 (whit solid)

Question 10

covalent hydride: reactions of H2S , HCl and HF

Answer 10

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)

Question 11

reaction of NH3 with water

Answer 11

NH3+ H2O NH4+ + OH-

Question 12

explain bonding in LiAlH4

Answer 12

made of Li+ AlH4-

in AlH4- 3covalent bonds and 1 co ordinate bond

Question 13

preparation of LiAlH4

Answer 13

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.

Question 14

LiAlH4 as a reducing agent

Answer 14

reduces C=O and COOH but not C=C

Question 15

interstitial hydride

Answer 15

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.

Question 16

interstitial hydrides as catalysts in hydrogenation reactions

Answer 16

• 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

Question 17

requirement for ionic compound to dissolve in water

Answer 17

entahlpy of hydration must be larger than the lattice dissociation enthalpy.

Question 18

name these hydrogen isotopes: 1H, 2H, 3H

Answer 18

protium, deuterium, tritium

Question 19

reaction to prepare ND3

Answer 19

3LiN3 + D2O—> ND3 + 3LiOD

Question 20

reaction to prepare C2D2

Answer 20

D2O + CaC2 —> C2D2+ Ca(OD)2

Question 21

reaction to prepare C6D6 (alkene to alkane reaction)

Answer 21

3C2D2 —> C6D6

heat in absence of oxygen (red hot metal tube)

Question 22

explain the concept of nascent hydrogen

Answer 22

• 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

Question 23

describe connection between bonding and physical state of NaH and MgH2

Answer 23

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

Question 24

describe the connection between the physical state and bonidng of Si, P , S and Cl hydrides.

Answer 24

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.

Question 25

why is water liquid at room temp? unlike other hydrides of group 6 which are gases.

Answer 25

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