Ionisation Energies (1)
FIRST IONISATION ENERGY :- The energy required to removed one mole of electrons from one mole of gasesous atoms to form one mole of positive ions.
Na (g) --> Na+ (g) + e-
SECOND IONISATION ENERGY :- The energy required to removed one mole of electrons from one mole of positive gaseous ions to form one mole of dipositive ions.
Na+ (g) --> Na2+ + e-
- First Ionisation increases across a period
- First Ionisation decreases down a group
- Second Ionisation is greater than the first
- Atomic size increases across a period and down a group
NUCLEAR CHARGE - SHIELDING - DISTANCE - REPULSION
Ionisation Energies (2)
Li - Be - B - C - N - O - Fl - Ne
Drop between Beryllium and Boron? Boron has one extra electron than Be which goes into one of the 2p orbitals. This extra subshell adds shielding so there is less attraction to the nucleus which makes the electron easier to remove.
Drop between Nitrogen and Oxygen? Hun's rule - Nitrogen has an electron in each of the 2p orbitals. O has one more electron which pairs up one of the electrons already in 2p orbital. The repulsive force between the electrons means it is easier to removed.
Na - Mg - Al - Si - P - S - Cl - Ar
Drop between Magnisium and Aluminium? Mg loses a 3s electron, Al loses a 3p electron. The electron in the p sub shell is already in a higher energy level than the s-electron and so takes less energy to remove.
Drop between Phosphorus and Sulphur? Similar to drop between Nitrogen and Oxygen. Hun's rule; electron repulsion as 3p electron is paired.
Alkanes - Physical Properties and Fractional Disti
- Almost completely non-polar because the electronegativity for Carbon and Hydrogen is so similar.
- Only force between the molecules is Wan Der Waals forces, which increase with the size of the hydrocarbon
- Hence why the boiling point for longer chain alkanes is increased
- Alkanes with branched chains have lower melting points than straight chains with the same number of carbons because they cannot pack as closely together as unbranched so Van Der Waals aren’t as effective.
- Alkanes are insoluble in water because water molecules are held together by hydrogen bonding which is much stronger than the Van der Waals between the molecules in the alkanes. (Although alkanes do mix with other relatively non-polar liquids)
Relatively unreactive – strong C-C and C-H bonds
Crude oil is heated in a furnace, and passes through as a mixture of liquid and gas, that is cooler at the top than the bottom. The vapours pass up the column until they arrive tray which is sufficiently cool (lower than their BP) and they condense to liquid. They are tapped off from each of these trays. Shorter chains are closer to the top because they have a lower boiling point.
Alkanes - Cracking (Thermal and Catalytic)
We crack alkanes for two main reasons:-
- Shorter, more useful chains can be produced i.e. petrol
- Some of the products are alkenes, which are more reactive than alkanes.
Thermal cracking; High temperature (700-1200K) and pressure (up to 7000kPa.) C-C bonds break in such a way that one electron from the pair in the covalent bond goes to each carbon atom. So at first, two shorter chains are produced – the end is an unpaired electron called a free radical. Free radicals are highly reactive – there aren’t enough hydrogen atoms for two alkanes, so you get an alkane and an alkene. Can make chains of varying lengths. Hydrogen may be produced, and there is usually a high proportion of alkenes produced.
Catalytic cracking; Takes place at much lower temperatures (720K) and lower pressure (>atmospheric pressure.) Zeolite catalyst; silicon dioxide and aluminium oxide. Honeycomb structure with massive surface area (and are acidic). Mainly used to produce motor fuels; as the products are mainly branched alkanes, cycloalkanes and aromatic compounds. The products from cracking are separated by fractional distillation.
Test for C-C double bonds, showing an alkene is that the mixture decolourises bromine.
Alkanes - Combustion
- Relatively unreactive – strong C-C and C-H bonds
- Shorter chain alkanes burn completely in a plentiful supply of oxygen to give carbon dioxide and water.
- i.e. CH4(g) + 2O2(g) → CO2 (g) + 2H2O (g) or C2H6(g) + 3.5O2(g) → 2CO2 (g) + 3H2O (g)
- Give out a lot of heat (negative enthalpy of combustion) – more carbons = greater heat
- In a limited supply of oxygen. Produce the poisonous gas carbon monoxide.
- i.e. C3H8 (g) + 3.5O2 (g) → 3CO (g) + 4H2O
- With even less Oxygen, carbon (soot) is produced.
- Incomplete combustion often happens with longer chain hydrocarbons, which need more oxygen to burn than shorter chains.
Alkanes - Looking after the environment (1)
- Carbon monoxide, CO – poisonous gas from incomplete combustion
- Nitrogen oxides, NO, NO2, and N2O4 (NOx), produced when there’s enough energy for Nitrogen and Oxygen to combine
- N2 (g) + O2 (g) → 2NO (g) Happens in a petrol engine very often. They may react with water to form nitric acid; contributing to acid rain and photochemical smog.
- Sulphur dioxide, SO2 – acid rain. Oxide reacts with water vapour and oxygen to form sulphuric acid
- o SO2 + H2O → H2SO3 + 1/2O2 → H2SO4
- Carbon particles, particulates – exacerbate asthma and cause cancer
- Unburnt hydrocarbons may also enter the atmosphere – greenhouse gases and photochemical smog – cause a variety of health problems
- Carbon dioxide, CO2 – greenhouse gas. Always produce when hydrocarbons burnt.
- Water vapour is also a greenhouse gas.
Removing sulphur; Some chimneys (flues) now use Calcium Oxide (CaO) or limestone (CaCO3) to absorb sulphur dioxide, producing gypsum, CaSO4, which is used as plaster. This process is called flue gas desulphurisation.
Alkanes - Looking after the environment (2)
- Internal combustion produces most of these harmful gases (except sulphur which is now removed from petrol to reduce acid rain.) All new cars now have catalytic converters, to reduce carbon monoxide, nitrogen oxides and unburnt hydrocarbons in the exhaust gas mixture.
- It is a honeycomb made of ceramic material, coated with platinum and rodium metals. These are catalysts and it provides an enormous surface area for the harmful substances to react with.
2CO (g) +2NO (g) → N2 (g) + 2CO2 (g)
Hydrocarbons + nitrogen oxide → nitrogen + carbon dioxide + water
o C8H18 + 25NO → 12.5N2 + 8CO2 + 9H2O
Global warming and the greenhouse effect; visible energy rays pass into atmosphere, energy is absorbed by everything and re-radiated as infra-red energy, which has a longer wavelength and can’t pass out) and how it affects us, setting it in context (Earth has varied before etc)