Hydrogen Flashcards
Why does hydrogen not fit the trends of the group 1 elements?
It is a gas at room temperature and pressure, the cation and anion are just as common and it forms covalent bonds.
When was hydrogen first observed?
1671 by Robert Boyle.
Who was credited for the discovery of hydrogen?
Henry Cavendish.
What are the properties of hydrogen?
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.
Why is hydrogen unreactive at room temperature without a catalyst?
There is a high H-H bond dissociation enthalpy.
How is most of the hydrogen in the world produced?
Heating natural gas or lighting crude oil fractions with steam at high temperatures over a nickel oxide catalyst.
What is steam reforming?
Lighting crude oil fractions with steam at high temperatures over a nickel oxide catalyst.
How can more hydrogen be generated in steam reforming and what is this process called?
The CO produced can be oxidised to CO2 with additional steam, usually with an iron catalyst. This is known as the shift reaction.
What is the other method in which hydrogen can be produced?
The decomposition of water using electrolysis.
What are the main uses of hydrogen?
Ammonia synthesis, petrochemical industry, extraction of metals from ores and the production of methanol.
What are hydrides?
Binary compounds with other elements of the period table.
What is hydrogens electronegativity?
2.20
What is it called when hydrogen forms a binary compound with a more electropositive element?
The hydrogen is described as hydridic - it has an oxidation state of -1.
What is it called when hydrogen forms a binary compound with a more electronegative element?
The hydrogen is described as protic - it has a oxidation state of +1.
What groups form hydridic hydrides?
Group 1 and 2 - usually form ionic structures.
What happens when group 1 hydrides are added to water?
They form hydrogen and an alkaline solution.
Are protic hydrides covalent or ionic - and why?
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.
How can ions be formed from protic hydrides?
Dissolving the compound in a solvent that is able to solvate the protons.
What is the polarity of hydrogen in non-polar hydrides?
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.
How are hydridic compounds named?
They are called hydrides e.g. lithium hydride.
How are protic hydrogen compounds named?
The systematic name ends in -ane e.g. SiH4 is Silane.
How are group 17 hydrides (and most group 16) named?
They have two-word names with hydrogen followed by the other element with an -ide ending e.g. H2S is hydrogen sulfide.
What are the three classes of covalent hydrides?
Electron-precise compounds, electron-deficient compounds and electron-rich compounds.
What are the characteristics of electron-precise compounds?
All the valence electrons of the central atom are involved in forming bonds e.g. group 14 hydrides.
What are the characteristics of electron-deficient compounds?
3-centre 2-electorn bonds are present e.g. BeH2 and the group 13 hydrides.
What are the characteristics of the electron-rich compounds?
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.
What can a van Arkel-Ketelaar triangle show?
How the bonding character of the hydrides changes for the elements of the second period.
What are the trends in acidity for the hydrides across a row?
They become more acidic going left to right across a row.
Why is the trend observed in the acidity of the hydrides?
There is increasing polarisation of the H-X bond.
What is the trend in acidity for the hydrides down a group and why?
They become more acidic down a group due to a decrease in bond dissociation enthalpy and the decreasing attraction between X- and H3O-.
What happens to bond dissociation enthalpies across a row and why?
Bond dissociation energies increase due to the increasing ionic contribution to the bonding as the electronegativity increases.
What happens to bond dissociation enthalpies down a group and why?
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.
What are the three isotopes of hydrogen?
Deuterium, tritium and 1H (protium).
What is different about deuterium from H1?
2H, which contains a neutron as well as a proton in the nucleus.
What is different about tritium from H1?
It is radioactive and contains a proton and two neutrons.
Why are isotope effects much more significant in isotopes of hydrogen than in other elements?
The mass of deuterium is twice of that of hydrogen so the effects are much more significant.
How does D2O compare to H2O?
It is denser - ice of D2O sinks in H2O.
Why are the bond dissociation enthalpies for D-D different to H-H?
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.
