C7 Organic chem (crude oil)

Question 1

describe how crude oil forms

Answer 1

• fossil fuel formed over millions of years
• biomass (mainly dead plankton) falls to sea bed, covered in layers of mud
• mud turns into rock and dead organisms decay forming oil

Question 2

describe the composition of crude oil

Answer 2

• a mixture of many different compounds
• mostly hydrocarbons- most found in crude oil are alkanes

Question 3

state the formula for alkanes

Answer 3

C(n) H(2n+2)

Question 4

state the first four alkanes

Answer 4

CH4 methane
C2H6 ethane
C3H8 propane
C4H10 butane

Question 5

describe the process of fractional distillation of crude oil

Answer 5

• crude oil vaporised (heated) so most alkane molecules turn into a gas
• gas put in the bottom of a fractionating column where it is hottest
• (as it is gas) they rise up the column and start to cool, once an alkane reaches its boiling point it condenses into a liquid and is collected
  -> crude oil molecules with extremely high boiling points will not vaporise, so are collected at the bottom of the column as a thick/sticky liquid
  -> long-chain hydrocarbons have higher boiling points so condense quicker+ will be collected near the bottom of the column

Question 6

describe the temperature gradient of a fractionating column

Answer 6

gets cooler as you move up the column
(oil begins as a gas and condenses as a liquid)

Question 7

describe the fractions produced from crude oil by fractional distillation (from long to short chain hydrocarbons / bottom to top of column)

Answer 7

• bicumin (unvaporised molecules remain as liquid, thick/sticky mixture) - tar for roads/roofs
• fuel/diesel oil - ships, power stations, cars
• kerosene - aircraft fuel
• petrol - cars
• refinery/petroleum gas (doesn’t condense) - liquified for fuel or burned for domestic heating/cooking (methane/ethane..)

Question 8

define volatility+ volatile

Answer 8

volatility- how easily a liquid vaporises
volatile- a chemical that vaporises easily (so also easily flammable)

Question 9

describe the boiling point of short vs long chain hydrocarbons

Answer 9

short - low as they are volatile (collected at top of column)
long - high (collected at bottom of column)

Question 10

describe the flammability of short vs long chain hydrocarbons

Answer 10

short - high (as they are volatile)
long - low

Question 11

define viscosity and viscous

Answer 11

viscosity - how easily a liquid flows (how thick it is)
viscous - a chemical that doesn’t flow easily, gloopy/thick

Question 12

describe the viscosity of short vs long chain hydrocarbons

Answer 12

short - low, runny
long - high (viscous), thick

Question 13

state the equation for the complete combustion of hydrocarbons

Answer 13

CnH2n+2 + O2 -> CO2 + H2O
hydrocarbon + oxygen -> carbon dioxide + water

Question 14

describe the main use of hydrocarbons

Answer 14

• used for fuels
• shorter-chain hydrocarbon molecules are best as they are more flammable so require less energy
• hydrocarbons are oxidised during combustion -> produces carbon dioxide + water + ENERGY released

Question 15

state the equation for the incomplete combustion of hydrocarbons

Answer 15

hydrocarbon + oxygen -> carbon monoxide OR carbon (soot) + water

Question 16

describe why cracking is used on hydrocarbons

Answer 16

• long-chain hydrocarbons are broken down into smaller-chain molecules
  -> smaller molecules are more useful for fuels as they are more flammable
• also produces alkenes which are used to make polymers (plastic)

Question 17

state the equation for cracking

Answer 17

long-chain alkane -> shorter-chain alkane + alkene

Question 18

are alkenes more/less reactive than alkanes

Answer 18

(also hydrocarbons)
more reactive than alkanes as they contain a double carbon bond (C=C)
-> used to make polymers (plastics)

Question 19

describe the test for alkenes

Answer 19

bromine water changes from orange-brown to colourless for a positive result
-> alkanes don’t react with bromine water

Question 20

describe catalytic cracking (4)

Answer 20

• used to crack long-chain alkanes
• produces moderate-chain alkanes used for fuels
• requires a lower temperature
• uses a catalyst (silica, SiO2) in powder form, increases SA to speed up the breakdown

Question 21

describe steam cracking (3)

Answer 21

• used to crack moderate-chain alkanes
• produces short-chain alkanes used for petrochemicals
• requires a higher temperature