Organic Chemistry (Triple)

Question 1

What is the general formula for an alkane?

Answer 1

Question 2

What is the general formula for an alkene?

Answer 2

Question 3

By what method are the different parts of crude oil separated?

Answer 3

Fractional distillation

Question 4

Fractions of crude oil are separated in fractional distillation, based on what property?

Answer 4

boiling point

Question 5

Explain why longer-chain hydrocarbons have a higher boiling point.

Answer 5

Boiling point is dependent on the stength of intermolecular forces.

Larger molecules have stronger intermolecular forces.

This means that more energy is required to break them and so their boiling point is higher.

Question 6

Outline how crude oil is formed.

Answer 6

• Dead biomass falls to the ocean floor.
• Layers of sediment build up on top of the dead biomass.
• Over millions of years this creates huge pressure and heat.
• After 100s of millions of years, crude oil has formed.
• Crude oil can seep through porous rock and so rises.
• It is trapped under a non-porous cap rock.

Question 7

Why is crude oil described as a finite resource?

Answer 7

It forms much more slowly than it is being used up, and so it will run out.

Question 8

Why is crude oil described as a feedstock?

Answer 8

Because the fractions of crude oil are used as raw materials in a number of industrial processes.

Question 9

Define the term hydrocarbon

Answer 9

Molecules which are made up of hydrogen and carbon only.

Question 10

Describe the process of fractional distillation.

Answer 10

1. Crude oil is heated to 300-900 degrees.
2. It enters the fractionating column, which is hotter at the bottom and cooler at the top.
3. The very largest hydrocarbons with the highest boiling points may still be liquids at this temperature, and will be tapped out at the bottom of the column as bitumen.
4. Other vapourised fractions rise, and will condense at different points through the column based on their boiling points, e.g. fuel oil, diesel, kerosene, petrol.
5. At the top, some oil fractions may be small enough molecules to still be in gas form - these are tapped out at the top of the column as refinery gas.
6. From top to bottom, the size of the molecules condensing and being tapped out of the column increase in size and boiling point.

Question 11

Describe an alkane

Answer 11

Saturated hydrocarbon

General formula of C_nH_2n+2

Contains only single bonds

The first four are methane, ethane, propane, butane

Question 12

Describe an alkene

Answer 12

Unsaturated hydrocarbon

General formula C_nH_2n

Contains at least one double bond

First three are ethene, propene, butene

Tested for with bromine water, which turns orange to colourless.

Question 13

Describe the bromine water test for an alkene.

Answer 13

Bromine water (brown-orange) is added to the hydrocarbon.

An alkene has a double bond which allows it to react with the bromine.

The double bond breaks open, and two bromine atoms are joined to form a dibromo-alkane.

Question 14

Why are alkanes comparatively unreactive as compared with alkenes?

Answer 14

Because alkanes are saturated - they have no bonds which are available to join with other substances.

Alkenes have a double bond which can be broken open and used to form new compounds with other substances.

Question 15

Outline the hydrogenation of an alkene

Answer 15

This is the addition of hydrogen to an alkene.

Expt. conditions - 150 degrees and a nickel catalyst

Double bond breaks open and 2 hydrogens are added, forming an alkane.

Remember nickel is a catalyst, so at the end of the reaction you get your

Question 16

What is a catalyst?

Answer 16

Something that speeds up a reaction without taking part in or, or being chemically changed itself.

Question 17

What is cracking?

Answer 17

Cracking is the breaking up of large hydrocarbons in shorter chain hydrocarbons.

The process produces one alkene and one alkane, with the total number of carbons in these equalling the number of carbons in the original large hydrocarbon.

Question 18

Why are large molecules cracked?

Answer 18

Because the smaller molecules that cracking produces are often more useful - perhaps as fuels or feedstock for other industrial processes.

There is much lower demand for very large hydrocarbons.

Question 19

Describe the hydration of ethene.

Answer 19

Ethene is reacted with steam.

The double bon breaks open.

This allows one hydrogen and one OH group to join.

The OH group is the functional group of the alcohols, so ethanol is produced.

Question 20

Name the two ways of producing an alcohol.

Answer 20

Fermentation and hydration of alkenes

Question 21

Describe alcohols

Answer 21

Alcohols contain an -OH group

The first four are methanol, ethanol, propanol and butanol

Question 22

Describe the solubility of alcohols in water

Answer 22

The smaller alcohols are completely soluble in water, but the solubility decreases as the number of carbons increases.

After 4 carbons, the alcohols will often form a separate layer when mixed with water.

Question 23

What happens to the viscosity of hydrocarbons as the chain gets longer?

Answer 23

As the hydrocarbon chain gets longer, the viscosity increases.

Question 24

Name the two types of cracking

Answer 24

steam cracking and catalytic cracking.

Question 25

what do alkanes produce when burned in air?

Answer 25

carbon dioxide and water if combustion is complete

If combustion is incomplete, then carbon monoxide or carbons (soot) may also be produced.

Question 26

How do alkenes burn in air?

Answer 26

In the same way as alkanes, although they tend to burn with a more yellow smokey flame, due to incomplete combustion.

Question 27

What is a reversible reaction?

Answer 27

One which can happen in two directions; the products can form the reactants again.

Question 28

In a reversible reaction, which direction will a high pressure favour?

Answer 28

whichever side has the fewest number of moles on it.

Question 29

In a reversible reaction, which direction will a high temperature favour?

Answer 29

Whichever direction is endothermic.

Question 30

Why are biofuels better for the environment than fossil fuels?

Answer 30

biofuels are derived from plants, which absorb carbon dioxide as they grow. This is released when they have been fermented and the derived alcohol burned. However, the net release od carbon dioxide is negligible.