Hydrogen

Question 1

boiling point

Answer 1

-253°C (20K)

Question 2

melting point

Answer 2

-259°C (14K)

Question 3

density

Answer 3

[v. low] - 0.08g dm-3

Question 4

H-H bond length

Answer 4

0.74A (short, strong bond)

Question 5

bond energy

Answer 5

436 kJ mol-1

Question 6

electronegativity of hydrogen

Answer 6

2.20

Question 7

H2+

Answer 7

only molecule with exact quantum mechanical solutions

H-H bond length = 1.06A

bond energy = 255 kJ mol-1

bond order = 1/2

Question 8

uses of H2

Answer 8

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

Question 9

compounds with strong δ- character

Answer 9

hydrides

Question 10

compounds with strong δ+ character

Answer 10

hydrogen -ide

Question 11

nearly non-polar compounds

Answer 11

-ane (or -ine)

Question 12

group 1 hydrides

Answer 12

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

Question 13

group 2 hydrides

Answer 13

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

Question 14

group 2 hydrides - reactivity

Answer 14

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)

Question 15

group 13 hydrides

Answer 15

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

Question 16

NaBH4

Answer 16

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

Question 17

LiAlH4

Answer 17

stronger reducing agent

reacts violently with water (efficient drying agent)

Question 18

higher boranes

Answer 18

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

more stable woth higher Mr

Question 19

pyrolysis

Answer 19

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

Question 20

boron oxides

Answer 20

solid

not good for engines + rocket propellants

Question 21

group 14 hydrides

Answer 21

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

Question 22

group 14 hydrides - reactivity

Answer 22

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

Question 23

group 15 hydrides

Answer 23

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

Question 24

higher group 14 hydrides

Answer 24

N2H4 (hydrazine), P2H4, As2H4

hydrazine produced by Raschig reaction with (aq) hypochloride

2NH3 + ClO- -> N2H4 + Cl- + H2O

ClO- obtained from chlorine and NaOH

Question 25

group 15 hydrides - reaction with oxygen

Answer 25

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

Question 26

Group 16 hydrides - hydrogen peroxide

Answer 26

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

Question 27

Group 16 hydrides - sulfur

Answer 27

H2Sn

unstable

prepared by acidifying stable Sn2- anions (prepared by dissolution of sulfur in Na2S solution)

Question 28

Group 15/16 hydrides - bond angles

Answer 28

[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

Question 29

group 17 hydrides - HX acids

Answer 29

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)

Question 30

consequences of H bonding

Answer 30

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