Alkanes

define petroleum fraction

mixture of hydrocarbon with a similar chain length and boiling point range

state the stages of how fractional distillation works (9)

• oil is pre heated

• then passed into column

• the fractions condense at different heights

• the temperature of the column decreases upwards

• the separation depends on boiling points

• boiling point depends on size of molecules

• similar molecules condense together

• small molecules condense at the top at lower temps

• and big molecules condense at the bottom

state the process of vacuum distillation unit (2)

• heavy residue from the fractionating column are distilled again under a vacuum

• lowering the pressure over a liquid will lower its boiling point

what is vacuum distillation used for (2)

• allows heavier fractions to be further separated

• without high temperatures which could break them down

define cracking (1)

conversion of large hydrocarbons to smaller hydrocarbon molecules by breaking the C-C bonds

what are the economic reasons for cracking (3)

• the petroleum fractions with shorter C chains are more in demand than larger fractions

• to make use of excess larger hydrocarbons and to supply shorter ones

• the products of cracking are more valuable than the starting materials

what are the conditions for thermal cracking (2)

• high pressure = 7000 kPa

• high temp = 400^oC to 900^oC

what are the conditions for catalytic cracking (3)

• slight pressure

• high temperature = 450^oC

• zeolite catalyst

what are the products of thermal cracking (2)

• alkenes

• sometimes hydrogen

what are the products of catalytic cracking (2)

• branched and cyclic alkanes

• aromatic hydrocarbons

write the two equations for the production of SO₂ (3)

• S + O₂ = SO₂

• CH₃SH+ 3O₂ = SO₂ + CO₂ + 2H₂O

explain how sulfur can be removed from the waste gases from furnaces (3)

• the gases pass through a scrubber containing basic calcium oxide

• which reacts with the acidic sulfur dioxide

• in a neutralisation reaction

write the equation for neitralising sulfur dioxide (2)

• SO₂ + CaO = CaSO₃

what is the product from neutralising sulfur dioxide (1)

calcium sulfite

write the equations for production of nitrogen oxide (2)

• N₂ + O₂ = 2NO

• N₂ + 2O₂ = 2NO₂

how is nitrogen oxide released from car engines (1)

the high temperature and spark in the engine provides sufficient energy to break the N₂ bonds

what is the environmental consequence of nitrogen oxide (1)

is toxic and cam form acidic gas NO₂ which can form acid rain

what is the environmental consequence of carbon monoxide (1)

toxic

what is the environmental consequence of carbon dioxide (1)

contributes towards global warming

what is the environmental consequence of unburnt hydrocarbons (1)

contributes towards formation of smog

what is the environmental consequence of soot(1)

global dimming

what are catalytic converters (1)

remove CO and NO and unburned hydrocarbons from exhaust gases to turn them into harmless products

write the equation for the conversion of CO and NO into harmless products (1)

2 CO + 2 NO = 2 CO₂ + N₂

write the equation for the conversion of hydrocarbons and NO into harmless products (2)

C₈H₁₈ + 25 NO = 8 CO₂ + 12½ N₂ + 9 H₂O

explain the mechanism of greenhouse effect (5)

• UV wavelength radiation passes through the atmosphere to the earths surface and heats it up

• earth radiates out infrared long wavelength radiation

• the C=O bonds in CO₂ absorb infrared radiation

• so the IR radiation does not escape from the atmosphere

• this energy is transferred to other molecules in the atmosphere via collision so the atmosphere is warmed

write the equation for the reaction of alkanes with chlorine (1)

CH₄ + Cl₂ = CH₃Cl + HCl

what is the mechanism used to react alkanes with halogens (1)

free radical substitution

write the initiation step for the reaction of alkanes with chlorine (1)

Cl₂ = 2Cl.

what is used to make initiation happen (1)

UV light

write the propagation steps for the reaction of alkanes with chlorine (2)

CH₄ + Cl. = HCl + .CH₃

.CH₃ + Cl₂ = CH₃Cl + Cl.

write the termination steps for the reaction of alkanes with chlorine (2)

.CH₃ + Cl . = CH₃Cl

.CH₃ + .CH₃ = CH₃CH₃

give the equation for a reaction that removes nitrogen monoxide in a catalytic converter

2 NO + 2 CO → N₂ + 2 CO₂

suggest reasons why enthalpy of combustion of a liquid alkane is different from the value obtained using the expression

• use of mean bond enthalpies

• alkanes may not be gaseous