3.3.2 Alkanes

how do we get alkanes to react

free radical substitution

what does free radical substitution turn alkanes into

halogenoalkanes

define a mechanism

a reaction which takes place over several steps

what is stage one of free radical substitution

initiation

what does initiation of free radical substitution require

UV light

what is the role of UV light in chlorification

excites the electron, breaking the covalent bond

define radical

excited, unpaired electron

give the equation for the initial stage of free radical substitution

Cl2 —> 2Cl.

what is stage two of free radical substitution

propagation

give the two equations for propagation in chlorification of methane

Cl. + CH4 —> HCl + .CH3, .CH3 + Cl2 —> Ch3Cl + Cl.

define a chain reaction

where the product of one reaction is the reactant of another

What is the name for stage three of free radical substitution

termination

Give 3 equations as an example of termination in chlorification of methane

Cl. + Cl. —> Cl2, .CH3 + Cl. —> CH3Cl, .CH3 + .CH3 —> C2H6

How does the reaction end

randomly, cannot be forced

What must the products of free radical substitution not be

radicals

define complete combustion

sufficient amounts of oxygen to fully react

what is the formula for complete combustion

alkane + oxygen —> carbon dioxide + water

define incomplete combustion

insufficient oxygen to fully react

what is the formula for incomplete combustion with limited oxygen

alkane + limited oxygen —> carbon monoxide + water

what is the formula for incomplete combustion with very limited oxygen

alkane + very limited oxygen —> carbon + water

what is carbon monoxide

a toxic, orderless and colourless gas

what is carbon

a black solid

what is a use of alkanes

for fuel

name some pollutants

CO, C, NO

why are alkanes generally used as fuel

that are generally unreactive

define global warming

increase in average temperature of the earth’s surface

what are causes of global warming

CO2, CH4, water vapour

define acid rain

acidification of water

what pH is acid rain

5-6

what is the cause of acid rain

sulfur dioxide (SO2)

what are the two formulas for sulphuric acid

H2SO3, H2SO4

define global dimming/smog

harmful and toxic chemicals in the atmosphere

what are causes of global dimming and smog

NOx, carbon particulates and unburnt hydrocarbons

how could we prevent acid rain

reacting a base with sulfur dioxide

how do we prevent acid rain (IRL)

filter sulfur dioxide through calcium oxide

write an equation for prevention of acid rain

CaO + SO2 —> CaSO3 + CaSO4

what is calcium oxide

lime

how can we reduce CO and NO emissions

catalytic converters

what elements do catalytic converters usually contain

metals such as Pt or Rh

how do we maximise rate at a low cost with catalytic converters

small amount of catalyst over a large surface area

write the equation for a catalytic converter

NO + CO —> CO2 + ½ N2

what are alkanes

saturated hydrocarbons

what does saturated mean

single carbon-carbon bonds

what are hydrocarbons

something containing hydrogens and carbons

what is petroleum

crude oil

what is crude oil

a mixture of mainly alkanes

how can you seperate crude oil into fractions

fractional distillation

define fractional distillation

continual evaporation and condensation of a mixture, causing separation of components due to different boiling points

what is the temperature gradient of a fractionating column

top is cooler than the bottom

what is the correlation between size and boiling point

smaller hydrocarbons have lower boiling points

what boiling points do molecules at the top have

lower boiling points

what is the formula for an alkane

CnH2n+2

what functional group do alkanes have

C-C single bonds

What boiling point do alkanes have and why

low due to weak Van Der Walls

what is the trend in alkane’s boiling points

increase with chain length

why do boiling points increase with chain length

larger Mr means more Van Der Walls

what type of boiling points do straight chain alkanes have and why

higher boiling points as they can stack closer increasing Van Der Walls

what does cracking do

breaks longer hydrocarbons into shorter alkanes and alkenes

how does cracking separate longer hydrocarbons

breaking C-C bonds

what two types of cracking are there

thermal and catalyst

why do we do cracking

short-chained molecules are typically more useful and more in demand

what are two requirements for thermal cracking

very high temperature, high pressure

what is the product of thermal cracking

short alkanes and alkenes

what are the two requirements for catalytic cracking

high temperature, zeolite crystal catalyst

what does catalytic cracking produce

alkanes and aromatic compounds

why may catalyst cracking be more effective

it cuts costs as the reaction can be done at a lower temperature and pressure and is quicker