Hydrogen Flashcards
boiling point
-253°C (20K)
melting point
-259°C (14K)
density
[v. low] - 0.08g dm-3
H-H bond length
0.74A (short, strong bond)
bond energy
436 kJ mol-1
electronegativity of hydrogen
2.20
H2+
only molecule with exact quantum mechanical solutions
H-H bond length = 1.06A
bond energy = 255 kJ mol-1
bond order = 1/2
uses of H2
Haber Bosch process - produce ammonia for fertilisers
N2 + 3H2 ⇌ 2NH3 (400°C; 200 atm; Fe catalyst)
Conversion of syn-gas (CO/H2) to methane
CO + 2H2 ⇌ CH3OH (300°C; 250 atm; Cu/Zn catalyst)
Hydrogenation of alkenes - e.g. margarine
Reduction of metals - e.g. copper from dissolved ores
compounds with strong δ- character
hydrides
compounds with strong δ+ character
hydrogen -ide
nearly non-polar compounds
-ane (or -ine)
group 1 hydrides
MH largely ionic + colourless
LiH = largest covalent character
2MH + H2O -> 2MOH + H2 (more violent down group)
moisture senstive - drying agent; storage under inert gas or oil
group 2 hydrides
M + H2 -> MH2 (except for Be)
largely ionic (except BeH2), colourless solids
MgH2 = large covalent character (similar to LiH)
diagonal relationship = top element in 1 group is related to 2nd element in next group
BeH2 = covalent polymeric structure
group 2 hydrides - reactivity
BeH2 = stable in water
Other MH2 react with water - MH2 + 2H2O -> M(OH2) + 2H2 (more violent down group)
CaH2 - can be stored in dry air (drying agent for organic solvents + source of H2 in remote locations)
group 13 hydrides
less ionic - more covalent
[Boron]
BH3 (dimerises to B2H6 and oligomers BnHm)
[Aluminium]
forms several forms of polymeric AlH3
most stable = α-AlH3 with AlH6 octahedral
[Gallium, indium + thalium]
no stable hydrides at rtp
Ga2H6 has been prepared at low temp. (decomposes at 10°C)
monomeric InH3 may exist in gas phase and has been detected in matrix isolation
NaBH4
synthesis - BX3 + 4NaH -> NaBH4 + 3NaX
[NaBH4]
-ionic solid
-inert in air
-dissolves in many organic solvents (even water via slow hydrolysis)
-reducing agent - aldehydes + ketones
LiAlH4
stronger reducing agent
reacts violently with water (efficient drying agent)
higher boranes
controlled pyrolysis of diborane (B2H6) gives BnHm - e.g. 5B2H6 -> B10H14 + 8H2 (decaborane)
more stable woth higher Mr
pyrolysis
produces hydrocarbon vapour whose constituents can be separated before being processed further
boron oxides
solid
not good for engines + rocket propellants
group 14 hydrides
carbon forms limitess no. of covalent and inert hydrocarbons - CnHm
silane, germane + stannane = gases that decompose to elements when heated
silane = 500°C (used for silicon purification)
germane = 300°C (used for Ge deposition in semiconductor films)
stannane = rtp
plumbane (PbH4) not well characterised - decomposes immediately
all flammable + combust to EO2 + H2O
preparation from chlorides - ECl4 + LiAlH4 -> EH4 + LiCl + AlCl4
higher oligermers are EnHm are known
group 14 hydrides - reactivity
electronegativity between group 14 elements and H = small
can act as source of H+ against strong bases
GeH4 + NH3 ⇌ NH4+ + GeH3- (in liquid ammonia)
CH3- // SH3- // SnH3- = strong bases
group 15 hydrides
form gaseous trihydrides (XH3) with pyramidal structure
all have free e- pair
only ammonia acts as a significant base in water - NH3 + H2O ⇌ NH4+ + OH-
acts as H+ source against strong bases to form: XH2- // NH2- // PH2-
in liquid ammonia, both processes lead to self-ionisation - 2NH3 ⇌ NH4+ + NH2-
liquid ammonia can be used as a solvent below -33°C
downside = have to use dry ice
can dissolve some compounds that won’t dissolve in water
higher group 14 hydrides
N2H4 (hydrazine), P2H4, As2H4
hydrazine produced by Raschig reaction with (aq) hypochloride
2NH3 + ClO- -> N2H4 + Cl- + H2O
ClO- obtained from chlorine and NaOH
group 15 hydrides - reaction with oxygen
ammonia needs a catalyst for combustion with air - 2NH3 + 3O2 -> N2 + 3H2O
hydrazine combustion is highly exothermic (rocket fuel)
phosphine ignites spontaneously due to P2H4 impurities
Group 16 hydrides - hydrogen peroxide
similar to water
m.p. = 0°C ; b.p. = 150°C
strong oxidant
thermodynamically unstable
2H2O2 -> 2H2O + O2
ΔH = -98kJ mol-1
catalysed by Pt, MnO2, catalase
[uses]
1. bleaching
2. “green” oxidant
3. propellant
Group 16 hydrides - sulfur
H2Sn
unstable
prepared by acidifying stable Sn2- anions (prepared by dissolution of sulfur in Na2S solution)
Group 15/16 hydrides - bond angles
[2nd period - N,O]
below 109°
expected for sp3 hybridisation
[heavier analogues]
closer to 90°
no hybridisation
[H-S bond formation]
interaction between S 3p orbitals and H 1s orbitals
consequences - less directional lone pairs; lower basicity than NH3 and H2O
group 17 hydrides - HX acids
form gaseous hydrides which act as acids (weakest = HF)
δ+ character of H decreases down group
bond strength makes dissociation more favourable
HF self-ionises (although weakest, fluoride can attack glass and bones)
consequences of H bonding
unusually high m.p. and b.p.
H bonding in ice leads to v. open network
structure with large holes (lowers density)
holes can be filled with greenhouse gases
