Hydrogen

-253°C (20K)

-259°C (14K)

[v. low] - 0.08g dm-3

0.74A (short, strong bond)

436 kJ mol-1

2.20

only molecule with exact quantum mechanical solutions H-H bond length = 1.06A bond energy = 255 kJ mol-1 bond order = 1/2

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

hydrides

hydrogen -ide

-ane (or -ine)

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

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

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)

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

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

stronger reducing agent reacts violently with water (efficient drying agent)

controlled pyrolysis of diborane (B2H6) gives BnHm - e.g. 5B2H6 -> B10H14 + 8H2 (decaborane) more stable woth higher Mr

produces hydrocarbon vapour whose constituents can be separated before being processed further

solid not good for engines + rocket propellants

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

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

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

N2H4 (hydrazine), P2H4, As2H4 hydrazine produced by Raschig reaction with (aq) hypochloride 2NH3 + ClO- -> N2H4 + Cl- + H2O ClO- obtained from chlorine and NaOH