Chemistry Of Hydrogen And Ionic Hydrides Flashcards
(43 cards)
Why is H- rare?
Because the energy to separate H2 into oppositely charged ions is very high
1675Jk/mol
What are the properties of H+?
Powerful Lewis acid - only exists in combination with a Lewis base
E.g. H3O+
Never affected by steric effects
Highly polarising
What are the properties of H-?
Bronsted base
Lewis base
Highly polarising
Why do the isotopes of H have very different physical properties?
Because D is 2x the mass, T is 3x the mass
(Nuclear spins of 1/2, 1 and 1/2 respectively (H, D, T))
What are the differences between water and ‘heavy’ water?
D2O:
MP is higher by 3.83K
BP is higher by 1.42K
The Kw of D2O is 2 x 10^-15
H2O symmetrical stretch is 3657cm-1 —> D2O is 2671cm-1
Bond enthalpy is 7.4 kJ/mol higher
What are the uses of deuterium?
D2O acts as moderator in nuclear reactors, slowing down neutrons to increase efficiency of reaction
Deuterated solvents used in 1H NMR
How is deuterium produced?
Electrolysis of water followed by fractional distillation of H2 / HD / D2
How is tritium produced?
Synthesised from LiD, LiF or Mg/Li enriched with 6Li which is then bombarded with neutrons
Producing He and Tritium
What are the uses of tritium?
Radioactive tracer
Weak ß-emitter - half-life of 12 years
- self powered lighting where ß emission interacts with phosphor to generate light
Nuclear weapons
- enhances efficiency of fission bombs
What are the 3 major global uses of hydrogen?
37% Hydro-desulfurization & cracking - strips sulfur from oil, benefits environment
22% Haber process - reduction of N
12% Catalyst - production of methanol, reducing CO
How can hydrogen be activated by a catalyst?
Pt allows for homolytic cleaveage which 2H atoms are added to Pt surface
ZnO allows for hetrolytic cleavage of H2 forming H+ and H- ions, H- binds to the Zn cation
- this occurs as oxide ion can act as a Bronsted base
Describe the radical chain reaction of H2 with halogens
Initiation
Propagation steps
Termination
Describe the reaction oh H2 with O2
H2(g) + 1/2O2(g) —> H2O(g)
Radical chain mechanism:
Why is H2 used as a rocket fuel?
Releases vast amounts of energy which is highly exothermic
What are the issues with using H2 as a clean fuel for buses?
Low energy density - giant tank is impractical, high pressure gas cylinders are unsafe and clumsy
Need a way to store H2 reversibly, keep H2 in a low pressure store
Where does H2 come from?
How do hydrogen fuel cells operate?
2 electrodes - anode and cathode
- these are separated by a proton exchange membrane, polymeric material
At anode H2 gas is oxidised to protons - e- is removed
At cathode O2 is reduced to hydroxide ions, using e- from the anode which are doing work
Protons then cross PEM and react with OH- at cathode to form water
What is the hydrogen economy?
Where water is split using renewable source
O2 is returned to atmosphere and H2 is used in fuel cells with O2
H2O produced from fuel cells is then returned to atmosphere
What are the current sources of hydrogen?
Steam reforming of methane
Water gas shift reaction
What are the pros/cons of the current sources of hydrogen?
Steam reforming of methane:
+ produces 3 moles of H2 gas
- very high temperatures needed
- very endothermic (unfavourable)
Water gas shift reaction:
+lower temperatures needed
+ exothermic reaction (favourable)
- only forms 1 mole of H2 gas
What are the drawbacks of releasing hydrogen through thermal decomposition of H2O?
Reaction takes place at 1000˚C
Only forms 1 mole of H2 gas
Initial reaction is very endothermic (unfavourable)
Describe the process of solar photochemical H2O splitting
Light sensitive material absorbs photon and electron promotes to a higher energy level
Higher energy level makes photo-exited catalyst a good reducing agent
Reacts with water to generate hydrogen, photocatalyst has list electron which it then gains back from H2O - oxidising oxygen
What are ionic hydrides only formed with? Give examples
Only formed with the most electropositive elements
Group 1 - Li to Cs
Group 2 - Mg to Ba
What structure does LiH form?
Rocksalt structure
How do we know ionic hydride actually contain hydrogen?
Because electrolysis of salt formed gives off H2 at the anode
