C9. Crude oil and fuels

Question 1

What is a crude oil?

Answer 1

Crude oil is a mixture of compounds; a fossil fuel consisting if the remains ancient biomass

Finite resource - cannot be replaced as it is used up

Question 2

What is a hydrocarbon?

Answer 2

A compound made up exclusively of hydrogen and carbon atoms

Question 3

What are alkanes?

Answer 3

Saturated hydrocarbons of a general formula CnH₂n+₂

Question 4

What is a homologous series?

Answer 4

Series of compounds with same general formula, same functional groups and similar chemical properties

Question 5

Describe the combustion of hydrocarbons

Answer 5

Exothermic reaction occurring when hydrocarbons are reacted with oxygen
Complete combustion produces carbon dioxide and water (carbon and hydrogen atoms are completely oxidised).
Incomplete combustion produces carbon or carbon monoxide and water

Question 6

Describe the physical properties of alkanes

Answer 6

First few in series are gases, then change to liquids, then to solids
In general, boiling points and viscosity increase as molecule gets bigger
Volatility and flammability decreases as molecules get bigger
Poor reactivity

Question 7

Explain how fractional distillation of crude oil takes place

Answer 7

Crude oil is heated and vaporised at a temperature of 450°C
Vapor rises up the fractionating column
The column is hotter at the bottom and cooler at the top
Hydrocarbons cool as they go up the column and condense at different heights, as they have different boiling points
Large molecules, high boiling points - collected at the bottom
Small molecules, low boiling points - collected at the top
This give fractions which can be used in various ways

Question 8

What is cracking?

Answer 8

When large hydrocarbons are thermally broken down into smaller and useful molecules

Question 9

What type of reaction is cracking?

Answer 9

Thermal decomposition

Question 10

What are the conditions for cracking?

Answer 10

Reactant heated to vapor, passed over a hot catalyst (catalytic cracking) or heated to vapor, mixed with steam and heated to high temperatures (steam cracking)

Question 11

How are the products of cracking used?

Answer 11

The products are alkanes and alkenes - used as polymers and starting materials for synthesis

Question 12

What is an alkene?

Answer 12

Unsaturated hydrocarbon. Contains a C=C bond

General formula for alkenes is CnH₂n

Question 13

What is the test for alkenes?

Answer 13

Add bromine water. Colour change occurs from orange to colourless

Question 14

Describe the combustion of alkenes

Answer 14

They burn with smoky flames due to incomplete combustion

Question 15

Describe addition reactions of alkenes?

Answer 15

Addition atoms across the carbon-carbon double bond so that the double bond becomes a single carbon-carbon bond.
a) With hydrogen - hydrogenation; requires a higher temperature and a nickel catalyst
b) With steam - hydration; requires high temperature, pressure and concentrated phosphoric acid (H₃PO₄) as a catalyst
c) With Br₂/Cl₂/I₂ - addition of halogens

Question 16

What are the properties if hydrocarbons?

Answer 16

Shorter the molecules, the less viscous it is
The longer the molecules, the more viscous it is
The shorter the molecules, the lower the temperature at which the fraction is vaporised or condensed - and the lower its boiling point
The shorter the molecules, the more flammable it is
Hydrocarbons are burnt so that they can be used as fuel, since the reaction produces energy.