alkanes Flashcards
why are alkane molecules non-polar?
C and H have very similar electronegativities so form non-polar bonds
how are neighbouring alkane molecules attached to each other?
by Van Der Waals’ forces- weak intermolecular forces
describe alkane solubility in water?
hydrogen bonds between water molecules are much stronger than the Van Der Waals’ in alkanes so they are insoluble in water
the longer the carbon chain the…..
higher the bp
higher the viscosity
lower the flammability
why does the bp increase for longer chain alkanes?
-more SA (only talk about this for straight chains not branched)
-larger molecule
-greater number of electrons
SO more and stronger VDW forces between molecules = more energy to overcome
what is the general formula for alkanes?
Cn H2n+2
what is a hydrocarbon?
a compound containing hydrogen and carbon atoms only
are alkanes saturated or unsaturated, why?
saturated
only single bonds between carbon atoms
what is the general formula for cycloalkanes?
Cn H2n
what is crude oil/ petroleum?
a mixture consisting mainly of alkane hydrocarbons and it is our main source of fuels and petrochemicals
what is a mixture?
more than one different compounds or elements not chemically bonded together
what is fractional distillation?
the separation of the components of a liquid into fractions which differ in boiling points
what is a fraction?
a group of components that have similar boiling points and are removed at the same level in the fractionating column
explain why branched alkanes have a lower boiling point?
more branched means less SA which means there are less points of contact where VDW’s can form
less VDW formed and weaker VDW = less energy to overcome
when can we only use the assumption that branched alkanes have a lower boiling point?
when they have a similar Mr and so are similar in size
describe the process of fractional distillation?
-vaporise crude oil in the furnace so the vapor can rise up the column
-the top of the column is cooler than the bottom
-larger, longer hydrocarbons condense at the bottom of the column, shorter, smaller hydrocarbons condense at the top of the column
which two fractions don’t vaporise or condense?
bitumen has too high of a boiling point to vaporise so leaves at the bottom as a liquid
petroleum gas doesn’t condense so leaves the top of the column as a gas
what are the two types of cracking?
thermal and catalytic
how is cracking achieved?
by breaking C-C bonds, producing shorter chain hydrocarbons from longer chain hydrocarbons
why do we do cracking?
we have a lower supply and higher demand for shorter hydrocarbons
we have a higher supply and lower demand for longer hydrocarbons
what are the conditions for thermal cracking?
very high pressures and very high temperatures
what does thermal cracking produce?
alkanes and a high percentage of alkenes
how are mixtures of products produced in thermal cracking?
the C-C bonds can break at different positions in the carbon chain
what are the products of thermal cracking used to make?
polymers (mainly alkenes)
what conditions are needed for catalytic cracking?
high temperature, slight pressure and in the presence of a zeolite catalyst
what does catalytic cracking produce?
cycloalkanes and branched alkanes as well as aromatic hydrocarbons such as benzene
what are the products of catalytic cracking used as?
motor fuels
how is energy obtained from alkane fuels?
when they are combusted
products formed in the reaction depend on the supply of oxygen
what is the equation for complete combustion?
alkane + oxygen ——> carbon dioxide + water
when does incomplete combustion occur?
limited supply of oxygen
what is the equation for incomplete combustion?
alkane + oxygen ———> carbon monoxide + water
what is the equation for further incomplete combustion?
alkane + oxygen ——-> solid carbon (soot) + water
when does further incomplete combustion occur?
very limited supply of oxygen
name pollutants we need to know about?
unburnt hydrocarbons
carbon dioxide
carbon monoxide
carbon
nitrogen oxides
sulfur dioxide
how do unburnt hydrocarbons produce low level ozone?
reacts with NOx gases to form low level ozone
what are the effects of the pollutants unburned hydrocarbons?
leads to low level ozone which causes respiratory problems
global dimming
what is the effect of carbon dioxide and how is it produced?
global warming
combustion of fuels
what is the effect of carbon monoxide and how is it produced?
toxic gas
incomplete combustion of fuels in a limited supply of oxygen
what can carbon (soot) lead to, and how is it produced?
particles lead to asthma
incomplete combustion of fuels in a limited supply of oxygen
what is the effect of nitrogen oxides and how are they produced?
acid rain and photochemical smog
N2 and O2 from the air react at high temps in engines
what is the effect of sulfur dioxide and how is it produced?
acid rain
S from fuel impurities reacts with O2 in the air
what has been fitted in most cars to remove unburnt CO and NO hydrocarbons?
catalytic converters
write an equation for how a catalytic converter removes unburnt hydrocarbons?
2CO + 2NO ——-> 2CO2 + N2
what structure do catalytic converters contain?
honeycombed structure coated with a thin layer of Pt, Pd, Rh metals
why is a thin layer of metals used in catalytic converters?
to reduce the amount needed (reduce cost)
what does the honeycomb structure of a catalytic converter provide?
a large surface area
what reacts to produce less polluting products?
NO and CO
2NO + 2CO ——–> 2CO2 +N2
what are the two ways of removing unburnt hydrocarbons?
reaction with oxygen
reaction with NO
what is produced which requires flue gas desulfurisation?
power stations burn coal or natural gas to produce electricity. Sulfur dioxide is also produced and this needs to be removed
what are chimneys or flues coated with to react with sulfur dioxide produced?
calcium oxide or calcium carbonate which absorb and react with it
SO2 + CaO ——–> CaSO3
SO2 + CaCO3 ——–> CaSO3 + CO2
what is the test for the presence of an alkene?
bromine water turns from orange to colourless
