Hydrogen

Question 1

How is hydrogen produced?

Answer 1

By heating natural gas or light crude oil fractions with steam at high temperatures over a nickel oxide catalyst (steam reforming).
CH4 + H2O –> CO + 3H2
CO can be oxidised to CO2 by reaction with additional steam to generate more hydrogen with an iron catalyst in the shift reaction.
CO + H2O –> CO2 + H2
Decomposition of water via electrolysis.
2H2O –> O2 + 2H2

Question 2

How is hydrogen used?

Answer 2

Haber Process to produce ammonia. 
N2 + 3H2 --> 2NH3        
Conversion of syngas into methanol 
Hydrogenation and desulfonisation reactions 
Extract metals from their ores

Question 3

Protic hydrides?

Answer 3

Hydrogen bonded with a more electronegative atom, hydrogen has an oxidation state of +1. Hydrides of group 15,16, and 17 are generally protic. Protic hydrides covalent rather than ionic since its difficult to produce a H+ ion.
Ions are only formed when dissolved in a solvent that is able to isolate protons, occurs when compound acts as an acid
HCl + H2O –> H3O+ + Cl-

Question 4

Covalent hydrides?

Answer 4

Electron precise where all valence electrons are involved in forming bonds eg group 14 hydrides
Electron deficient not possible to have 2 centre 2 electron bonds, 3 centre 2 electron bonds are present, BeH2 and group 13 hydrides are electron deficient.
Electron rich not all electron on central atom involved in bonding lone pairs are present, act as Lewis bases, hydrides of groups 15,16,17 are electron rich.

Question 5

Group 1 hydrides?

Answer 5

Formula MH, combination of elements at high temperature, largely ionic. Dissolve in molten alkali halides and undergo electrolysis. Strongly basic and react with water to form hydrogen and alkaline solution.
LiH + H2O –> LiOH + H2
Reactivity increases down the group - need to be used under an inert atmosphere

Question 6

Group 2 hydrides?

Answer 6

MH2, crystalline ionic solids, all react in water to give hydrogen and alkaline solution.

Question 7

Group 13 hydrides?

Answer 7

Decomposition temperatures of hydrides decreases as group is descended and compounds become more reactive. Decrease in bond enthalpies going down the group.

Question 8

Group 14 hydrides?

Answer 8

Form tetra hydrides, formed by direct combination of elements. Silane prepared by silicontetrachloride with a hydrogen source. Spontaneously combusts in air to form silicon dioxide and water. Silane, german and stagnant decompose on heating to their elements. Hydrides become less stable down the group.

Question 9

Group 15 hydrides?

Answer 9

XH3, ammonia produced in Haber process, acts as a Lewis base and in solution acts as Bronsted Lowry base, other group 15 hydrides poor lewis bases due to molecular strutters, the angle decreases down the group, angle in NH3 consistent with sp3 hybridisation orbital with lone pair is directional and can donate the lone pair. The other angles in other hydrides much smaller so hybridisation less important in these compounds, lone pair in s orbital. Nitrogen, phosphorus and arsenic also form compounds of X2H4

Question 10

Group 16 hydrides?

Answer 10

H2X, group 16 hydrides become less stable down the group. Acidity of the hydrides increases down the group. Oxygen can form water and hydrogen peroxide.

Question 11

Group 17 hydrides?

Answer 11

HX hydrogen halides, reaction with hydrogen and halogen. Enthalpy change of formation for HX becomes more position as X increases in size, main facet is H-X bond dissociation enthalpy decreases as group is descended. Polarity of H-X bond decrease down the group, acidity increases down the group, forward reaction is more favourable when group is descended, reverse reaction becomes less favourable as group is descended.

Question 12

Hydrides hydrides?

Answer 12

When hydrogen forms a binary compound with a more electropositive element described as hydridic and has an oxidation state of -1 normally formed by group 1 and 2 metals, form ionic structures

Question 13

Non polar hydrides?

Answer 13

When hydrogen is bonded to an element with a similar electronegativity the bonds are largely non polar but may have a small dipole in which the hydrogen atom may be partially positive or negative

Question 14

Nomenclature?

Answer 14

Hydrides compound simple hydrides eg lithium hydride
Hydrogen atom is protic of X-H bond is non polar compound ends in -ane eg SiH4 is silane
Group 17 and most group 16 hydrides have two word names with hydrogen following the other element with an -ide ending eg H2S is hydrogen sulfide

Question 15

Acidity?

Answer 15

Hydrides become more acidic going from left to right across a period, due to increasing polarisation of X-H bond.
More acidic going down a group, due to a decrease in bond dissociation enthalpy and the decreasing attraction between X- and H3O+
HX + H2O –> H3O+ + X-

Question 16

Bond strengths?

Answer 16

Bond dissociation enthalpies generally increase from left to right across a row due to increasing ionic contribution to the bond as electronegativity increases
Generally decrease going down a group the valence orbitals of the atoms get larger and more diffuse with increasing principal quantum number n this means that the interactions of these ns and np orbitals with the hydrogen 1s obtains are reduced going down a group

Question 17

Isotope effects of D2 and H2?

Answer 17

Deuterium is an isotope of hydrogen with twice the mass of a normal hydrogen atom, the bond dissociation enthalpy of any X-D bond is greater than that for the corresponding X-H bond so more energy is needed to break the X-D bond, this difference is because the zero point in energy of a D-D bond is lower than that for a H-H bond