C9

Question 1

What is crude oil?

Answer 1

Crude oil is a finite resource found in rocks.

It is a mixture of many different carbon compounds.

Nearly all of the compounds in crude oil are compounds containing only hydrogen and carbon atoms.

These compounds are called hydrocarbons.

Question 2

How was crude oil formed ?

Answer 2

It was formed over millions of years from the remains of tiny, ancient sea animals and plants, mainly plankton, that were buried in mud. Over time, layer upon layer of rock was laid down on top, creating the conditions (high pressure and temperature, in the absence of oxygen) to make crude oil.

Question 3

What is a mixture?

Answer 3

A mixture contains two or more elements or compounds that are not chemically combined together.

Question 4

What are fractions?

Answer 4

Before crude oil can be used, it must be separated into different substances with similar boiling points. These are known as fractions.

Question 5

How are the mixtures of substances in crude oil separated?

Answer 5

They are separated in the lab by distillation

Question 6

What are most of the hydrocarbons in in crude oil called?

Answer 6

Alkanes

Question 7

What are the first 4 alkanes and what are their carbon count?

Answer 7

Methane 1 carbon, ethane 2 carbon, propane 3 carbon, butane 4 carbon

Question 8

Why are alkanes called saturated hydrocarbons (what does it mean)?

Answer 8

You describe alkanes as saturated hydrocarbons. All the carbon-carbon bonds are single covalent bonds. This means that they contain as many hydrogen atoms as possible in each molecule. No more hydrogen atoms can be added.

Question 9

Whats the formulae for the first 4 alkane molecules

Answer 9

CH4 methane
C2H6 ethane
C3H8 propane
C4H10 butane

Question 10

Whats the general formula for alkane molecules?

Answer 10

CnH ( 2n+2 )
This means that for every n carbon atoms there are 2n+2 hydrogen atoms in an alkane molecule

Question 11

Why do the short chain molecules make up the hydrocarbons that tend to be more useful ?

Answer 11

These short-chain molecules make up the hydrocarbons that tend to be most useful.

These hydrocarbons make good fuels as they ignite easily and burn well, with less smoky flames than hydrocarbons made up of larger molecules. They are described as very flammable.

Other hydrocarbons have lots of carbon atoms in their long-chain molecules, and may have branches (side-chains) or form rings.

Question 12

What are the properties of short chain hydrocarbons ?

Answer 12

-Lower boiling point
-higher volatility ( the tendency to turn into a gas )
-very runny (low viscosity (how easily it flows))
-higher flammability

Question 13

What are the properties of long chain hydrocarbons ?

Answer 13

-higher boiling point
-lower volatility
-thick (high viscosity)
-lower flammibility

Question 14

What is fractional distillation ?

Answer 14

Crude oil is separated into hydrocarbons with similar boiling points, called fractions. This process is called fractional distillation

Question 15

What does each hydrocarbon fraction contain ?

Answer 15

Each hydrocarbon fraction contains molecules with similar numbers of carbon atoms. Each of these fractions boils at a different temperature range because of the different sizes of molecules in it

Question 16

Explain the process of fractional distillation

Answer 16

Crude oil is heated and fed in near the bottom of a tall tower (called a fractionating column) as hot vapour. The column is kept very hot at the bottom and much cooler at the top, so the temperature decreases going up the column. The gases move up the column and the hydrocarbons condense when they reach the temperature of their boiling points. The different fractions are collected as liquids at different levels. The fractions are collected from the column in a continuous process.

Hydrocarbons with the smallest molecules have the lowest boiling points.
They are piped out of the cooler top of the column as gases. At the bottom of the column, the fractions have high boiling points. They cool to form very thick liquids or solids at room
temperature.

Once collected, the fractions need more processing before they can be used.

Question 17

Why are the lighter fractions from crude oil very useful as fuels ?

Answer 17

Because when hydrocarbons burn in plenty of air they transfer lots of energy to the surroundings

Question 18

What are the products of complete combustion with a hydrocarbon ?

Answer 18

Carbon dioxide and water

Question 19

In complete combustion what gets oxidised ? And what the general equation for complete combustion ?

Answer 19

The carbon and hydrogen from the hydrocarbon gets oxidised
Hydrocarbon + oxygen —–> carbon dioxide + water

Question 20

How can you test for carbon dioxide and water in complete combustion of a hydrocarbon ?

Answer 20

The carbon dioxide turns limewater cloudy

The water turns blue colbalt chloride paper pink

Question 21

What is incomplete combustion ?

Answer 21

When there is not enough oxygen there is incomplete combustion
Instead of all the carbon changing into carbon dioxide, carbon monoxide is also formed (CO)
It is a toxic gas that is colourless and odourless

Question 22

What is cracking ?

Answer 22

It is the process where larger and less useful hydrocarbon molecules can be broken down into smaller, more usefull ones in a process we call cracking

Question 23

Exlain the process of cracking

Answer 23

The process takes place at an oil refinery in steel vessels called crackers. In the cracker, a heavy fraction distilled from crude oil is heated to vaporise the hydrocarbons. The vapour is then either:

-passed over a hot catalyst, or
-mixed with steam and heated to a very high temperature.

The hydrocarbons are cracked as thermal decomposition reactions take place. The large molecules split apart to form smaller, more useful ones.

Question 24

What is an alkene ?

Answer 24

It is a unsaturated hyrdrocarbon meaning that it has at least one double bond inbetween its carbon atoms

Question 25

Whats a positive test for an unsaturated hydrocarbon ?

Answer 25

It turns bromine water colourless