Hydrocarbons

Question 1

What is a hydrocarbon?

Answer 1

a substance made from hydrogen and carbon atoms only

Question 2

List the first 5 hydrocarbons

Answer 2

-Methane
-Ethane
-Propane
-Butane
-Pentane

Question 3

What is the general formula for hydrocarbons that are alkanes?

Answer 3

CnH2n+2

Question 4

What does it mean that alkanes are saturated?

Answer 4

they have the maximum number of single bonds around each carbon atom.

Question 5

What is a homologous series?

Answer 5

a group of chemicals with the same chemical properties, but a change in physical properties. e.g. density and the size changes

Question 6

What is a crude oil?

Answer 6

a finite resource found in rocks and is a mixture of many different compounds

Question 7

How can you separate crude oil?

Answer 7

-fractional distillation because hydrocarbons have different boiling points (separated into fractions).

Question 8

Why is crude oil not useful as a product itself?

Answer 8

because there are too many substances in it with different boiling points; you cannot isolate the products you need.

Question 9

Method of fractional distillation

Answer 9

1. Crude oil heated to about 450•C and pumped into bottom of fractioning column as hot vapour.
2. Column kept very hot at bottom and cooler at the top.
3. As vaporised oil rises, it cools and condenses
4. Heavy fractions have high boiling points, so condense near bottom of column.
5. Lighter fractions have lower boiling points, so condense further up.
6. Heavy fractions have larger molecules; lighter have smaller
7. Different fractions collected as liquids at different levels.

Question 10

How does size of alkanes affect their properties?

Answer 10

Small alkanes: low boiling point, high flammability, clean flame, low viscosity/runny
Large alkanes: high boiling point, low flammability, smoky flame, high viscosity

Question 11

What is cracking?

Answer 11

-Larger, less useful hydrocarbon molecules being broken down into smaller, more useful ones.

Question 12

Why is cracking important?

Answer 12

-Heavier hydrocarbons in crude oils are made up of large molecules
->this means they have high boiling points, are difficult to vaporise and do not burn easily, so are poor fuels

Question 13

Methods of cracking

Answer 13

-A heavy fractions distilled from crude oil is heated to vaporise the hydrocarbons. The vapour is then either:
•passed over a hot catalyst
•mixed with steam and heated to high temperature

Question 14

What is an unsaturated hydrocarbon?

Answer 14

-produced by cracking
-an alkene
-contains at least one double bond between their carbon atoms.

Question 15

Positive test for unsaturated hydrocarbon

Answer 15

-turns orange bromine water colourless
-alkanes don’t react with bromine water