Hydrogen

Question 1

Why does hydrogen not fit the trends of the group 1 elements?

Answer 1

It is a gas at room temperature and pressure, the cation and anion are just as common and it forms covalent bonds.

Question 2

When was hydrogen first observed?

Answer 2

1671 by Robert Boyle.

Question 3

Who was credited for the discovery of hydrogen?

Answer 3

Henry Cavendish.

Question 4

What are the properties of hydrogen?

Answer 4

It is colourless and odourless, a melting point of 14K and a boiling point of 20K. It has the lowest density of all of the gases. It is generally unreactive at room temperature without a catalyst.

Question 5

Why is hydrogen unreactive at room temperature without a catalyst?

Answer 5

There is a high H-H bond dissociation enthalpy.

Question 6

How is most of the hydrogen in the world produced?

Answer 6

Heating natural gas or lighting crude oil fractions with steam at high temperatures over a nickel oxide catalyst.

Question 7

What is steam reforming?

Answer 7

Lighting crude oil fractions with steam at high temperatures over a nickel oxide catalyst.

Question 8

How can more hydrogen be generated in steam reforming and what is this process called?

Answer 8

The CO produced can be oxidised to CO2 with additional steam, usually with an iron catalyst. This is known as the shift reaction.

Question 9

What is the other method in which hydrogen can be produced?

Answer 9

The decomposition of water using electrolysis.

Question 10

What are the main uses of hydrogen?

Answer 10

Ammonia synthesis, petrochemical industry, extraction of metals from ores and the production of methanol.

Question 11

What are hydrides?

Answer 11

Binary compounds with other elements of the period table.

Question 12

What is hydrogens electronegativity?

Answer 12

2.20

Question 13

What is it called when hydrogen forms a binary compound with a more electropositive element?

Answer 13

The hydrogen is described as hydridic - it has an oxidation state of -1.

Question 14

What is it called when hydrogen forms a binary compound with a more electronegative element?

Answer 14

The hydrogen is described as protic - it has a oxidation state of +1.

Question 15

What groups form hydridic hydrides?

Answer 15

Group 1 and 2 - usually form ionic structures.

Question 16

What happens when group 1 hydrides are added to water?

Answer 16

They form hydrogen and an alkaline solution.

Question 17

Are protic hydrides covalent or ionic - and why?

Answer 17

Covalent - the ionisation enthalpy of hydrogen is very high and much larger than the electron gain enthalpy so it is difficult to product an H+ ion.

Question 18

How can ions be formed from protic hydrides?

Answer 18

Dissolving the compound in a solvent that is able to solvate the protons.

Question 19

What is the polarity of hydrogen in non-polar hydrides?

Answer 19

Despite the bonds being non-polar (similar electronegativity), there may be a small dipole in which the hydrogen atom can be delta positive or delta negative.

Question 20

How are hydridic compounds named?

Answer 20

They are called hydrides e.g. lithium hydride.

Question 21

How are protic hydrogen compounds named?

Answer 21

The systematic name ends in -ane e.g. SiH4 is Silane.

Question 22

How are group 17 hydrides (and most group 16) named?

Answer 22

They have two-word names with hydrogen followed by the other element with an -ide ending e.g. H2S is hydrogen sulfide.

Question 23

What are the three classes of covalent hydrides?

Answer 23

Electron-precise compounds, electron-deficient compounds and electron-rich compounds.

Question 24

What are the characteristics of electron-precise compounds?

Answer 24

All the valence electrons of the central atom are involved in forming bonds e.g. group 14 hydrides.

Question 25

What are the characteristics of electron-deficient compounds?

Answer 25

3-centre 2-electorn bonds are present e.g. BeH2 and the group 13 hydrides.

Question 26

What are the characteristics of the electron-rich compounds?

Answer 26

Not all of the electrons on the central atom are involved in bonding - there are lone pairs present and they can act as lewis bases e.g. group 15-17.

Question 27

What can a van Arkel-Ketelaar triangle show?

Answer 27

How the bonding character of the hydrides changes for the elements of the second period.

Question 28

What are the trends in acidity for the hydrides across a row?

Answer 28

They become more acidic going left to right across a row.

Question 29

Why is the trend observed in the acidity of the hydrides?

Answer 29

There is increasing polarisation of the H-X bond.

Question 30

What is the trend in acidity for the hydrides down a group and why?

Answer 30

They become more acidic down a group due to a decrease in bond dissociation enthalpy and the decreasing attraction between X- and H3O-.

Question 31

What happens to bond dissociation enthalpies across a row and why?

Answer 31

Bond dissociation energies increase due to the increasing ionic contribution to the bonding as the electronegativity increases.

Question 32

What happens to bond dissociation enthalpies down a group and why?

Answer 32

They decrease down a group as the valence orbitals of the atoms get larger and more diffuse with increasing PQN. The interactions with the H1s orbitals is reduced going down a group.

Question 33

What are the three isotopes of hydrogen?

Answer 33

Deuterium, tritium and 1H (protium).

Question 34

What is different about deuterium from H1?

Answer 34

2H, which contains a neutron as well as a proton in the nucleus.

Question 35

What is different about tritium from H1?

Answer 35

It is radioactive and contains a proton and two neutrons.

Question 36

Why are isotope effects much more significant in isotopes of hydrogen than in other elements?

Answer 36

The mass of deuterium is twice of that of hydrogen so the effects are much more significant.

Question 37

How does D2O compare to H2O?

Answer 37

It is denser - ice of D2O sinks in H2O.

Question 38

Why are the bond dissociation enthalpies for D-D different to H-H?

Answer 38

The zero point energy of a D-D bond is lower than a H-H bond. The BDE for any X-D bond is greater than any X-H bond, so more energy is needed to bread an X-D bond than a X-H bond.