18 — fuels and crude oil

Question 1

Fossil fuels

Answer 1

Are formed from decayed plants and animals from millions of years ago they r mainly organic compounds that contain only carbon and oxygen,known as hydrocarbons. When burnt in excess, main products formed r CO2 and H2O.

Question 2

Natural gas

Answer 2

A gaseous fossil fuel consisting mainly of methane. Natural gas is colourless and odourless.

Question 3

Crude oil

Answer 3

Crude oil, or petroleum, contains a mixture of hydrocarbons. It usually appears as a dark, sticky liquid.

Question 4

Explain why larger hydrocarbons have higher boiling points

Answer 4

Hydrocarbons that contain more carbon atoms have higher boiling points as the molecule size is bigger so they have stronger intermolecular forces of attraction, and hence more energy is required to overcome the stronger intermolecular forces of attraction. Hydrocarbons w lower boiling points r less viscous and burn more easily.

Question 5

Describe the process of fractional distillation of crude oil

Answer 5

Crude oil is heated and completely vaporised in the furnace. The column is cooler at the top than at bottom. As the vapour rises up the column, it cools and condenses. Each fraction condenses at a different temperature. The fractions w the lowest boiling point condense and r removed at the top and those w highest boiling point condense at the bottom.

Question 6

Fractions of crude oil

Answer 6

1. Petroleum gas | fuel for cooking + heating
2. Petrol (gasoline) | fuel for car engines in vehicles
3. Naphtha | feedstock (raw material) for the petrochemical industry
4. Paraffin (kerosene) | fuel for aircraft engines, cooking n heating
5. Diesel oil | fuel for diesel engines in buses, lorries and trains
6. Lubricating oil | for lubricating machines, making waxes and polishes
7. Bitumen | for paving road surfaces; for roofing

Question 7

Issues due to competing uses of crude oil

Answer 7

• supply of crude oil is limited
• crude oil fractions r impt as a chemical feedstock for the manufacture of daily products thus is a waste to burn crude oil away
• conservation of crude oil to ensure continuous supply of crude oil

Question 8

Ways to conserve crude oil

Answer 8

1. Reduce the use of crude oil
2. Use alternative energy sources (biofuels, solar and nuclear energy)
3. Improve the design of power stations and vehicles to use crude oil more efficiently

Question 9

Biofuels

Answer 9

Eg ethanol obtained from fermentation of sugar in sugar cane plants
Ethanol is burnt in excess oxygen, CO2 and H2O formed.
C2H5OH + 3O2 -> 2CO2 + 3H2O

— Sugarcane plant takes in CO2 during photosynthesis as it grows, which offsets the CO2 produced when bio ethanol is burnt during combustion. Hence, there is no net change in the amount of CO2 in the atmosphere.

Other examples:
1. Biodiesel from vegetable oils and liquid animal fats
2. Green diesel from algae and other plant sources
3. Biogas from methane in decomposed organic waste

Question 10

Explain why cracking of naptha is an important process in an oil refinery. [2]

Answer 10

Cracking breaks down larger alkanes in crude oil into short-chain alkenes and alkanes that are more useful and in higher demand.
Alkenes such as ethene are used to synthesise ethanol and plastics which are in high demand.
Short-chain alkanes such as petrol is used as fuels in motor vehicles.
Hydrogen is produced as a reactant in the manufacture of ammonia in Haber Process.

Question 11

Describe and explain how the process of fractional distillation separates crude oil into fractions [5]

Answer 11

Crude oil is a mixture of hydrocarbons with different boiling points.
Crude oil is strongly heated in a furnace to more than 350dgC to vaporise the hydrocarbons before passing into the fractionating column.
As the hydrocarbon vapours move up the fractionating column, the vapours cool and condense at different temperatures.
Shorter hydrocarbons with lower boiling points condense and exit at higher sections of the fractionating column
Longer hydrocarbons with higher boiling points condense at lower sections.
Each fraction consists of a group of hydrocarbons with similar molecular mass and similar boiling points.

Question 12

Magnesium was added to a solution of 2 carboxylic acids respectively. State and explain expected observations. Include reference to rate of reaction in your answer. [2]

Answer 12

Effervescence of colourless and odourless gas is produced. Gas extinguishes a lighted splint with a “pop” sound. Rate of reaction for both carboxylic acids will be similar since they are monobasic weak acids.