Topic 7 - Organic Chemistry - Crude oils and fuels

Question 1

What are hydrocarbons?

Answer 1

any compound formed from carbon and hydrogen atoms only

Question 2

What are alkanes?

Answer 2

• simplest type of hydrocarbon you can get
• general formula CnH2n+2
• are a homologous series
• they are saturated compounds - each carbon atom forms 4 single covalent bonds - bonded to as many hydrogen atoms as possible

Question 3

What is a homologous series?

Answer 3

group of organic compounds that share a similar chemical structure - same functional group, and a general formula

Question 4

What is a saturated compound?

Answer 4

• an organic compound where all carbon-carbon bonds are single bonds
• contain as many hydrogen atoms as possible in each molecule

Question 5

What are the first 5 alkanes?

Answer 5

• methane
• ethane
• propane
• butane
• pentane

Question 6

Methane formula?

Answer 6

CH4

Question 7

Ethane formula?

Answer 7

C2H6

Question 8

Propane formula?

Answer 8

C3H8

Question 9

Butane formula?

Answer 9

C4H10

Question 10

Pentane formula?

Answer 10

C5H12

Question 11

Properties of hydrocarbons?

Answer 11

• shorter the carbon chain - less viscous it is
• shorter the carbon chain - more volatile it is
• shorter the carbon chain - more flammable it is
• shorter the carbon chain - lower its boiling point

Question 12

What happens during the complete combustion of a hydrocarbon?

Answer 12

• complete combustion of any hydrocarbon in oxygen releases lots of energy
• only waste products are CO2 and water vapour
• during combustion - both carbon and hydrogen from hydrocarbon are oxidised
• hydrocarbons are used as fuels due to the amount of energy released when they combust completely

Question 13

What happens during the incomplete combustion of hydrocarbons?

Answer 13

• carbon monoxide gas produced instead of CO2
• carbon monoxide is a toxic gas that is odourless and colourless

Question 14

How to test for complete combustion of hydrocarbons?

Answer 14

• carbon dioxide and water produced
• to test for CO2: bubble gas through limewater → if it turns cloudy CO₂ is present
• to test for H2O: add liquid to anhydrous copper sulfate → if turns from white to blue - water is present OR dip blue cobalt chloride paper into liquid - if turns from blue to pink water is present

Question 15

What is crude oil?

Answer 15

• crude oil is a fossil fuel - it is formed remains of plants and animals - mainly plankton - that were buried in mud
• over millions of yeasr - with high temperature and pressure - remains turn to crude oil wh ich can be drilled up from rocks where its found
• it is non-renewable

Question 16

What are fractions of crude oil?

Answer 16

• different groups of hydrocarbons separated from crude oil during fractional distillation
• groups of hydrocarbons with similar lengths of carbon chains and boiling points

Question 17

How is fractional distillation used to separate hydrocarbon fractions?

Answer 17

• oil is heated until most of it has turned into gas - gases enter fractionating column
• in fractionating column there is a temperature gradient - hotter at bottom and cooler as you go up
• longer hydrocarbons have high boiling points - they condense back into liquids and drain out of column early on when theyre near bottom
• shorter hydrocarbons have lower boiling points - they condense and drain out much later on - near top of column where its cooler
• you end up with crude oil mixture separated out into different fractions - each fraction contains a mixture of hydrocarbons with similar number of carbon atoms - so have similar boiling points

Question 18

Uses of hydrocarbon fractions?

Answer 18

• liquified petroleum gas (LPG) is used as a fuel inh range of applications
• kerosene varied uses e.g. fuel for oil lamps and cleaning agents

Question 19

How does the petrochemical industry use hydrocarbons?

Answer 19

• uses some hydrocarbons from crude oil as feed stock to make new compounds for use in things like polymers, solvents, lubricants and detergents

Question 20

What is cracking?

Answer 20

splitting up less useful long-chan hydrocarbons into more useful short-chain hydrocarbons

Question 21

Why are short chain hydrocarbons wanted more?

Answer 21

they are flammable so are in high demand for fuels

Question 22

What are the two types of cracking?

Answer 22

• catalyctic cracking
• steam cracking

Question 23

What type of reaction is cracking?

Answer 23

thermal decomposition reaction

Question 24

How does catalyctic cracking work?

Answer 24

• Long-chain hydrocarbons heated until they vaporise
• vapour passed over hot catalyst (zeolite) at around 500°C
• catalyst speeds up breaking of C–C bonds - splitting large alkanes into shorter alkanes + alkenes

Question 25

How is the structure of an alkane different from an alkene?

Answer 25

- alkanes are saturated hydrocarbons (only single C-C bonds) - alkenes are unsaturated hydrocarbons (contain 1 or more C=C double bonds)

Question 26

What chemical test is used to show presence of unsaturated hydrocarbons?

Answer 26

- decolouration of bromine water - alkenes react with bromine water - turn it from orange/brown to colourless

Question 27

General equation for cracking?

Answer 27

Long-chain hydrocarbon molecule → shorter alkane molecule + alkene