4.1.2 Alkanes Flashcards
Definition of saturated
molecule containing single C-C bonds only
Type of bonding in alkanes
All sigma bonds formed by direct overlap of orbitals. The bonds have free rotation.
General formula
CnH2n+2 except cycloalkanes (CnH2n)
Shape and explanation
Tetrahedral with bond angle 109.5. Each C atom has 4 bonded pairs of electrons which repel equally.
Variations in boiling point
The longer the chain, the more electrons a molecule has so the stronger the intermolecular forces (London forces), meaning more energy to overcome and higher boiling points. Branched alkanes have a lower bp as they pack less closely so the intermolecular forces are weaker and require less energy to overcome.
Separation of alkanes
Fractional distillation. The mixture is vaporised and enters the fractionating column. As the vapours rise they cool and condense. The column is hotter at the bottom than top. The longer chains will cool and condense sooner. The different fractions can be collected at different points up the column.
Reactivity of alkanes
Low due to the high bond enthalpy and low polarity of the C-C bonds.
Combustion of alkanes
Alkanes are good fuels - they release a lot of energy when burnt.
Complete combustion in sufficient oxygen produces CO2 and H2O.
If insufficient O2 is present then incomplete combustion occurs producing CO or C particulates and water
Problem of CO
toxic gas. Colourless and odourless, it irreversibly combines with red blood cells causing suffocation.
Problems of alkane combustion.
carbon particulates cause global dimming and asthma.
CO is toxic.
CO2 contributes to global warming.
S impurities in the fuels form SO2, which reacts with water in clouds causing acid rain.
N2 reacts with O2 in the heat of car engines forming oxides of nitrogen, which also contribute to acid rain.
Solutions to the problems of combustion
Catalytic converters in cars - unburnt fuels, C, CO and NOx are converted to less harmful CO2, N2 and H2O
Sulfur is removed from car fuel in production. Chimneys of factories have flues fitted to remove SO2
How a catalytic converter works
Rh or Pd or Pt is coated on the converter in a honeycomb pattern. The coating is 1 atom thick to minimise waste and cost (the catalyst is v. expensive) and the structure has a very large SA.
Type of reaction with halogens
Free radical substitution
Definition of a free radical
Species with an unpaired electron. Very reaction.
Stages of free radical substitution
Initiation - halogen-halogen bond broken to form 2Xo radicals.
Propagation - a radical reacts with another molecule to form a new product and a new radical.
Termination - Two radicals join to form a stable product.
