7: Organic Chemistry Flashcards
what is a hydrocarbon?
compounds that only contain hydrogen and carbon atoms
what is a homologous series?
a series of organic compounds w similar features & chemical properties due to them having the same functional group
describe how crude oil is formed.
is formed over millions of years from the effects of high pressures and temperatures on the remains of plants and animals
what is an alkane and what is its formula?
a homologous series of hydrocarbon compounds with only single carbon bonds - making them saturated hydrocarbons; formula = CnH2n+2
what reactions can alkanes undergo?
combustion; can be cracked into smaller molecules; react with halogens in the presence of light
what are the first four alkanes and their structural formulas?
methane - CH4; ethane - CH3CH3; propane - CH3CH2CH3; butane - CH3CH2CH2CH3
what is the equation for combustion w methane?
CH4 + 2O2 → CO2 + 2H2O
describe how fractional distillation works to separate crude oil.
- a fractionating column is v hot at bottom & cool at top
- crude oil enters fractionating column & is heated -vapours rise
- vapours of hydrocarbons w v high boiling points will immediately condense into liquid at higher temp lower down & are tapped off at the bottom of the column.
- vapours of hydrocarbons w low boiling points will rise up column & condense at the top to be tapped off.
- different fractions condense at different heights according to boiling points & are tapped off as liquids.
- fractions containing smaller hydrocarbons are collected at top of fractionating column as gases; and vice versa
why do we need to separate crude oil?
as a mixture it isn’t very useful; but as fractions, they all have different uses
what can the fractions of crude oil be used for?
polymers, lubricants, solvents, detergents, adhesives - all products; fuels that are used in most modern methods of transport (cars, trains, airplanes etc.)
what are petrol, diesel oil, kerosene, bitumen, heavy fuel oil and liquefied petroleum gases used for?
(respectively)
fuel for cars (gasoline); diesel engines (gas oil); jet fuel (paraffin); surfacing roads and roofs; ships and power stations; domestic heating and cooking
why does boiling point increase as we go down the alkanes?
molecules get larger; intermolecular forces of attraction between molecules becomes greater - more surface area contact between them; more energy is needed to separate the molecules
what is viscosity and what are the trends in hydrocarbons?
refers to the ease of flow of a liquid - high viscosity liquids = thick & flow less easily; increases with increasing chain length
what are the trends in flammability in hydrocarbons?
smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules
what is produced in incomplete combustion?
insufficient oxygen = formation of carbon monoxide and water or carbon and water
why is carbon monoxide bad?
extremely poisonous gas - combines w haemoglobin in blood & prevents it from carrying oxygen; is colourless, odourless and tasteless, making it difficult to detect;
incomplete combustion occurs in some appliances e.g. boilers, stoves, and in internal combustion engines, where space is cramped
why is carbon that is produced from incomplete combustion bad?
causes respiratory problems; covers buildings and statues, making them look unclean; increases global dimming
what is an alkene? how is it different from an alkane, and why is it more desirable?
it has a carbon carbon double bond, making it unsaturated; more desirable as they are more reactive and can take part in reactions which alkanes cannot; can be used to make polymers, are the starting materials for the production of many other chemicals
what is cracking? what is produced?
process used to convert alkanes into short chain molecules which are more useful; alkenes and hydrogen are produced
what are the two methods for cracking? how do they work?
catalytic cracking and steam cracking
catalytic cracking: heat the hydrocarbon molecules to around 600 – 700°C to vaporise them; then pass the vapours over a hot powdered catalyst of aluminium oxide; this breaks covalent bonds in molecules as they come into contact with the surface of catalyst, causing thermal decomposition reactions; molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes; H and a higher proportion of alkenes are formed at temperatures of above 700ºC and higher pressure.
steam cracking: the same process is used; but the vaporised hydrocarbons are mixed with steam and heated to a high temperature which induces cracking.
what is the general formula for alkenes?
CnH2n
what is produced in the complete/incomplete combustion of alkenes?
complete: water & carbon dioxide;
incomplete: water & CO (always); CO2 or C
how can we test for alkenes?
add bromine water to the solution; it will go from orange to colourless
why are alkenes more reactive than alkanes?
double bond contains an area of high electron density; more bonds can be made w other atoms when double bond is opened
what is hydrogenation?
reaction w hydrogen to form an alkane; occur at 150ºC using a nickel catalyst; used to change vegetable oils into margarine to be sold in supermarkets
what is hydration?
reaction with steam to form an alcohol; used for production of alcohols; it occurs at temp. around 330ºC; pressure of 60 – 70 atm; needs a concentrated phosphoric acid catalyst; ethene is separated from ethanol and water easily - has a much lower boiling point; ethanol and water are separated afterwards by fractional distillation.
what is halogenation?
reactions with halogens; halogen atoms always add to the carbon atoms involved in the C=C double bond
