Chemistry Of Hydrogen And Ionic Hydrides Flashcards
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
Why must ionic hydrides be stored in anhydrous conditions?
Because their reactions with water are extremely vigorous
What are the general properties of covalent hydrides?
Hydrides of of very electropositive elements react as H-
- powerful Bronsted base
Hydrides of very electronegative elements react as H+
- Bronsted acid
How do the bond enthalpies change in covalent hydrides?
Bond enthalpies decrease as you go down groups - disparity in orbital size results in poorer overlap
Bond enthalpies increase with electronegativity of E - increased ionic contribution
Hydrides of heavier elements often react by homolysis of E-H - giving off radicals
Give an example of an electron deficient hydride
Diborane B2H6
∆fH˚ = +36 KJ/mol
BP = -92.6˚C
Group 13 hydride
What’s special about the bonding in diborane?
Presents 3-centre-2-electron bonding
Tetrahedral B is sp3 hybridised
1s orbital on bridging H atom overlap with sp3 hybrid
Give examples of electron precise hydrides
CH4 and SiH4 (silane)
Group 14 hydrides
How does the reaction of methane and chlorine proceed by?
Reaction proceeds by free radical mechanism - homolytic cleavage of C-H from a photon
How does silane differ to methane?
Silane is spontaneously flammable in contact with oxygen - EXPLOSIVE
Si can also be hypervalent
Si-H bond less polar than C-H bond
Si-H bond is weaker than C-H - orbital disparity
Give an example of a electron rich hydride
Group 15 NH3 - Lewis basic
BP = -33.35˚C
∆fH˚ = -46.1 KJ/mol
What are important factors to note about ammonia?
Bond angles in NH3 consistent with sp3 hybridisation
Lone pair electrons in highly directional sp3 orbital - bonding
NH3 is a Bronsted & Lewis base
What does greater hydrogen bonding result in?
Results in intermolecular dipole-dipole interactions which leads to an increased BP
Explains NH3, H2O and HF anomalous BPs
Why is water’s BP higher than HF and NH3?
Because oxygen has 2 lone pairs and so can form a 3 dimensional network of H bonding
HF forms a 2D chain, same as NH3 as it only has 1 lone pair
What do carboxylic acids form due to H bonding?
Strong dimers are formed
How does methane hydrate form?
Forms at low temperatures when H2O crystallises around CH4
Under what conditions is methane hydrate stable under?
Stable at low temperatures and high pressures
What issues arise due to the crystallisation of methane hydrate?
Can cause blockages in gas pipelines in cold climates
Where does methane hydrate occur naturally?
Beneath ocean floors and beneath permafrost
Released into the atmosphere if it melts!
What’s the structure of methane hydrate?
20 x H2O molecules surround CH4 molecule in centre of pentagonal dodecahedron
What experimental evidence is there for H bonding?
Higher than expected BP/MPs
Higher than expected molecular weights - carboxylic acids forming dimers
Anomalous shifts in 1H NMR spectra
Broadening/lowering of frequency of O-H stretch in IR spectra
