Ionic Bonding and s-Block Chemistry- Hydrogen Compunds

Question 1

Describe why hydrogen doesn’t fit neatly into the Periodic table

Answer 1

1. Has 1S1 which fits with group 1 valence shell configuration
2. But in chemical terms it does not behave as a group 1 element
3. It is a gas at RT and pressure whereas group 1 elements are all reactive electropositive metals
4. Chemistry for group 1 is dominated by M+, but for hydrogen H+ and H- ions are both common as are covalent bonds

Question 2

Who first observed elemental hydrogen and when

Answer 2

1. Robert Boyle
2. 1671
3. Observed formation of a gas when iron filings were dissolved in HCl and demonstrated this gas was flammable

Question 3

Who is the discovery of hydorgen usually credited to

Answer 3

1. Henry Cavendish- 1766
2. Showed that the gas produced in this experiment combined with oxygen to form water, disproved the theory that water was an element

Question 4

Describe main properties of Hydrogen gas

Answer 4

1. Colourless and odourless
2. Melting point -259 degrees (14K)
3. Boiling point- -253 (20K)
4. Generally unreactive at room temperature in the absence of a catalyst
5. Lowest density of all the gases- as low molecular mass

Question 5

Why are the melting and boiling points of hydrogen so low

Answer 5

1. Because of the weak intermolecular forces between H2 molecules

Question 6

Why is H2 generally unreactive at room temperature

Answer 6

1. High H-H bond dissociation enthalpy so high activation energy

Question 7

What are exceptions to H2 being unreactive at rt

Answer 7

1. O2, F2, Cl2

2. Reacts explosively with all of these

Question 8

What happens to hydrogen under high pressure

Answer 8

1. H becomes a metallic fluid- on Jupiter

2. Causes a magnetic field

Question 9

What is the main way to produce hydrogen and give an example

Answer 9

1. Heating natural gas or light crude oil fractions with steam at high temperature over a nickel oxide catalyst
2. Called steam reforming
3. CH4(g) + H2O (g) CO (g) + 3H2(g)

Question 10

What is the shift reaction

Answer 10

1. CO produced from the steam reforming is oxidised to CO2 by reaction with additional steam to generate more hydrogen
2. Uses an iron catalyst
3. CO (g) + H2O (g) CO2 (g) + H2 (g)

Question 11

How else

Answer 11

1. Decomposition of water to hydrogen and oxygen on heating to 2000 degrees
2. But to high temperature to be commercially viable
3. But electrolysis can be used
4. More expensive way than steam reforming, only economically viable when electricity is cheap such as hydroelectric power plants
5. 2H2O (l) –> 2H2 (g) + O2 (g)

Question 12

What is the main use of hydrogen

Answer 12

1. Ammonia synthesis- Haber process
2. In the petrochemical industry (for hydrogenation and desulfurisation reactions)
3. Extraction of metals from ores
4. Production of methanol

Question 13

What are the binary compounds that hydrogen form called

Answer 13

1. Hydrides

Question 14

What is hydrogens electronegativity

Answer 14

1. 2.20

2. In the middle of the elements

Question 15

How is the hydrogen atom describe if it forms a binary compound with a more electropositive atom (Li)

Answer 15

1. Hydridic

2. Oxidation state of -1

Question 16

How is the hydrogen atom describe if it forms a binary compound with a more electronegative atom (Cl)

Answer 16

1. Protic
2. Oxidation state of +1
3. The bond is polarised with hydrogen atom delta+

Question 17

Describe hydridic hydrides

Answer 17

1. Formed by group 1 and 2 metals
2. Typically ionic structure
3. Group 1 hydrides are strongly basic and react with water forming hydrogen and an alkaline solution
4. The reactivity increases down the group and NaH can ignite in moist air
5. So need to be used under an inert atmosphere

Question 18

Show the equation for the reaction of LiH with water

Answer 18

1. LiH (s) + H2O (l) –> LiOH (aq) + H2 (g)

Question 19

Describe protic hydrides

Answer 19

1. Ionisation enthalpy of hydrogen is very high and much larger than the electron gain enthalpy.
2. Means it is difficult to produce an H+ ion so protic hydrides are covalent rather than ionic
3. Ions are only formed when the compound is dissolved in a solvent that is able to solvate the protons, and when this occurs the compounds act as acids
   e. g. HCl + H2O –> H3O+ + Cl-

Question 20

Describe non-polar hydrides

Answer 20

1. When hydrogen is bonded to an element with a similar electronegativity, the bonds are largely non-polar
2. May have a small dipole where hydrogen can be either delta+/-

Question 21

Describe the nomenclature for hydride compounds

Answer 21

1. Hydridic compounds are simply called hydrides
2. When H atom is protic or X-H bond is almost non-polar, the systematic name for the compound ends in -ane
3. Group 17 and most group 16 hydrides have 2-word names with ‘hydrogen’ followed by the other element with an -ide ending
4. For many compounds trivial names are almost always used.

Question 22

What are the classes of covalent hydrides

Answer 22

1. Electron-precise compound
2. Electron-deficient compound
3. Electron-rich compound

Question 23

Describe electron-precise hydrides

Answer 23

1. All the valence electrons of the central atom are involved in forming bonds
2. Group 14 hydrides are electron precise

Question 24

Describe electron-deficient compounds

Answer 24

1. 3-centre 2-electron bonds are present

2. BeH2 and group 13 hydrides are electron deficient

Question 25

Describe electron-rich compounds

Answer 25

1. Not all of the electrons are involved in bonding, so there are lone pairs present
2. This means that electron-rich hydrides can act as Lewis bases
3. Hydrides in Group 15-17 are electron rich

Question 26

Describe the trends in acidity for hydrides

Answer 26

1. Hydrides become more acidic going left to right across the row.
2. This is due to the increasing polarisation of the X-H bond
3. Hydrides become more acidic going up a group.
4. This is due to a decrease in bond dissociation enthalpy and decreasing attraction between X- and H3O+

Question 27

Describe the trends in bond strength of hydrides

Answer 27

1. Bond dissociation enthalpies generally increase from left to right across a row.
2. This is due to the increasing ionic contribution to the bonding as electronegativity of X increases
3. Bond dissociation enthalpies generally decrease going down a group.
4. The valence orbitals of the atoms getting 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 orbitals are reduced going down a group.

Question 28

Describe the isotopes of hydrogen

Answer 28

1. There are 3 isotopes
2. Vast majority is 1H (protium)
3. Small fraction is deuterium 2H or D contains a neutron as well as proton
4. Radioactive isotope tritium 3H or T, naturally occurs in miniscule amounts but can be prepared artificially

Question 29

Describe why there are differences in properties between the isotopes of hydrogen

Answer 29

1. Despite having same number of electrons they have different properties because of big difference in mass
2. Deuterium has twice the mass of a normal hydrogen, so isotope effects are far more important in hydrogen compounds than for other elements

Question 30

What are some of the different properties between hydrogen isotopes

Answer 30

1. D2O is denser than H2O
2. Bond dissociation enthalpy of D2 is slightly higher than for H2
3. They are different because the zero point of energy of a D-D bond is lower than that for H-H bond. The bond dissociation enthalpy for any X-D bond is greater than that for the corresponding X-H bond, so more energy is needed to break it