Unit 4 - Organic Chemistry: Chapter 23: Crude Oil

Crude Oil: The Heart of Modern Life

• The oil industry is central to modern society, providing fuels, plastics, and organic chemicals used in solvents, drugs, dyes, and explosives.

• This chapter explores the conversion of crude oil into useful products.

Learning Objectives

• Crude oil is a mixture of hydrocarbons.

• Fractional distillation separates crude oil into fractions.

• Main fractions: refinery gases, gasoline, kerosene, diesel, fuel oil, and bitumen.

• Trend in color, boiling point, and viscosity of main fractions.

• A fuel is a substance that releases heat energy when burned.

• Products of complete and incomplete combustion of hydrocarbons with oxygen.

• High temperatures in car engines cause nitrogen and oxygen to react, forming oxides of nitrogen.

• Combustion of impurities in hydrocarbon fuels results in sulfur dioxide formation.

• Sulfur dioxide and oxides of nitrogen contribute to acid rain.

• Long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking (silica or alumina catalyst, 600-700°C).

• Cracking is necessary to balance supply and demand for different fractions.

• Carbon monoxide is poisonous because it reduces blood's oxygen-carrying capacity.

Crude Oil Composition: Hydrocarbons

• Crude oil is a mixture of hydrocarbons (compounds containing only carbon and hydrogen).

• It contains hydrocarbons of various sizes, from a few atoms to over 100.
  Figure 23.2 This sticky black liquid is essential to modern life.

Physical Properties and Molecule Size

• As the number of carbon atoms increases, physical properties change due to increasing intermolecular attractions.

• Larger molecules have stronger intermolecular forces, making it harder to separate them.

• Boiling Point: Increases with molecule size due to stronger intermolecular forces. More energy is needed to break these forces to produce gas.

• Volatility: Decreases with molecule size. Larger molecules evaporate more slowly at room temperature due to stronger attractions.

• Viscosity: Increases with molecule size. Small hydrocarbon liquids are runny, while large ones flow less easily due to stronger attractions.

• Color: Liquids get darker with increasing hydrocarbon size.

• Combustibility: Larger hydrocarbons do not burn as easily, limiting their use as fuels.

Separating Crude Oil: Fractional Distillation

• Crude oil must be separated into fractions before use. Fractions are mixtures with a narrow range of hydrocarbon sizes and similar boiling points.

• Fractional distillation is carried out in an oil refinery using a fractionating column.

• Crude oil is heated until it boils, and the vapors enter the fractionating column (hot at the bottom, cooler at the top).

• Hydrocarbon movement in the column depends on its boiling point.

• Hydrocarbons with a boiling point of $120^\circ C$ remain gas until they reach that temperature in the column, then condense into liquid and are removed.

• Smaller molecules with lower boiling points rise higher before condensing. Longer chains condense lower in the column.

Figure 23.3 Fractional distillation of crude oil

Uses of Fractions

• All hydrocarbons burn in air (oxygen) to form carbon dioxide and water, releasing heat, which makes them useful as fuels.

• A fuel is a substance that releases heat energy when burned.

Refinery Gases

• Mixture of methane, ethane, propane, and butane, used as liquefied petroleum gas (LPG) for heating and cooking.

Gasoline (Petrol)

• A mixture of hydrocarbons with similar boiling points, used as fuel in cars.

Kerosene

• Used as fuel for jet aircraft, domestic heating oil, and 'paraffin' for small heaters and lamps.
  Figure 23.5 Kerosene is used as aviation fuel.

Diesel

• Used as fuel for buses, lorries, some cars, and some railway engines.

• Some is converted to more useful organic chemicals, including petrol, in a process called cracking.
  Figure 23.6 A train powered by diesel

Fuel Oil

• Used as fuel for ships and industrial heating.
  Figure 23.7 Ships' boilers burn fuel oil.

Bitumen

• A thick, black material, melted and mixed with small pieces of rock to make the top surface of roads.
  Figure 23.8 Bitumen is used in road construction.

Combustion of Hydrocarbons as Fuels

• Hydrocarbons burn in air (oxygen) to form carbon dioxide and water, releasing heat.

• Example: Burning methane (natural gas):
  $CH<em>4(g) + 2O</em>2(g) \rightarrow CO<em>2(g) + 2H</em>2O(l)$

• Burning octane (gasoline):
  $2C<em>8H</em>{18} + 25O<em>2(g) \rightarrow 16CO</em>2(g) + 18H_2O(l)$

Incomplete Combustion

• Occurs when there isn't enough air (oxygen), leading to the formation of carbon (soot) or carbon monoxide instead of carbon dioxide.

• Example: Methane burning in a poorly maintained appliance:

  $2CH<em>4(g) + 3O</em>2(g) \rightarrow 2CO(g) + 4H_2O(l)$

Figure 23.4 As well as all the other poisonous or cancer-causing compounds, cigarette smoke contains carbon monoxide due to incomplete combustion.

Carbon Monoxide Poisoning

• Carbon monoxide (CO) is colorless, odorless, and very poisonous.

• It reduces the ability of the blood to carry oxygen around the body, leading to illness or death.

• CO combines with hemoglobin more strongly than oxygen, preventing oxygen transport.

Environmental Problems of Burning Fossil Fuels

• Burning fossil fuels from crude oil causes major environmental problems.

• Carbon dioxide produced is a greenhouse gas, trapping heat and contributing to climate change (see Chapter 13).

Acid Rain: Sulfur Dioxide and Oxides of Nitrogen

• Rain is naturally slightly acidic ($pH = 5.6$) due to dissolved carbon dioxide.

• Acid rain has a lower pH (pH < 5.6) due to pollutants. Often the pH of acid rain is about 4.

Figure 23.9 Use of very low-sulfur fuels limits the production of sulfur dioxide, but
the spark in a petrol engine causes oxygen and nitrogen from the air to combine to make
oxides of nitrogen, $NO_x$

Figure 23.10 Trees dying from the effects of
acid rain.

Formation of Acid Rain

• Water and oxygen in the atmosphere react with sulfur dioxide to produce sulfuric acid ($H<em>2SO</em>4$), or with oxides of nitrogen ($NO<em>x$) to produce nitric acid ($HNO</em>3$).

• $SO<em>2$ and $NO</em>x$ come mainly from power stations, factories burning fossil fuels, or motor vehicles.

Sulfur Dioxide

• Fossil fuels contain sulfur. Burning the fuel produces sulfur dioxide:
  $S(s) + O<em>2(g) \rightarrow SO</em>2(g)$

• Reactions in the atmosphere convert it to sulfuric acid:
  $2SO<em>2(g) + 2H</em>2O(l) + O<em>2(g) \rightarrow 2H</em>2SO_4(aq)$

• Sulfur dioxide can also react with water to form sulfurous acid:

  $SO<em>2(g) + H</em>2O(l) \rightarrow H<em>2SO</em>3(aq)$

Oxides of Nitrogen

• In petrol engines, high temperatures cause nitrogen and oxygen to combine:

  $N<em>2(g) + O</em>2(g) \rightarrow 2NO(g)$

• These nitrogen oxides can be converted to nitric acid ($HNO_3$) in the atmosphere.

Effects of Acid Rain

• Acid rain damages trees and kills fish in lakes. Some lakes become too acidic to support life.

• Limestone buildings and marble statues (calcium carbonate) and some metals are also attacked.

• Reaction between limestone and sulfuric acid:

  $CaCO<em>3(s) + H</em>2SO<em>4(aq) \rightarrow CaSO</em>4(s) + H<em>2O(l) + CO</em>2(g)$

Solutions to Acid Rain

• Removing sulfur from fuels, 'scrubbing' gases from power stations and factories to remove $SO<em>2$ and $NO</em>x$, and using catalytic converters in cars.

Cracking: Balancing Supply and Demand

• Some fractions from crude oil distillation are more useful and profitable than others.

• The proportions of fractions obtained depend on the crude oil composition, not market demand.

• There are too many long-chain hydrocarbons (lower demand) and not enough short-chain hydrocarbons (high demand for fuel).

• Cracking converts long-chain alkanes to alkenes and shorter-chain alkanes, breaking down big molecules into smaller ones needed for petrol.

• A very simple equation would be:

  $C<em>{14}H</em>{30} \rightarrow C<em>{10}H</em>{22} + C<em>4H</em>8$

How Catalytic Cracking Works

• The fuel oil fraction is heated to a gas and passed over a catalyst of silicon dioxide (silica) and aluminum oxide (alumina) at about $600-700^\circ C$.

• Cracking can also be done at higher temperatures without a catalyst (thermal cracking).

• Cracking is thermal decomposition: a big molecule splitting into smaller ones on heating.

• C-C single bonds are broken, and new C=C double bonds are formed.
  Figure 23.11 How cracking works.

• As an equation, this would read:

  $C<em>{10}H</em>{22} \rightarrow C<em>2H</em>4(g) + C<em>3H</em>6(g) + C<em>5H</em>{12} (l)$

• Cracking produces a mixture of alkanes and alkenes. In this case, ethene and propene are produced. Octane is also formed.

• Cracking produces useful molecules such as Ethene and propene are both used to make important polymers, as you will find in Chapter 29. Octane, which is a component of petrol (gasoline).

Possible Reactions:

• Other possibilities for cracking $C<em>{13}H</em>{28}$:

  • $C<em>{13}H</em>{28} \rightarrow 2C<em>2H</em>4(g) + C<em>9H</em>{20}(l)$

  • $C<em>{13}H</em>{28} \rightarrow 2C<em>2H</em>4(g) + C<em>3H</em>6(g) + C<em>6H</em>{14}(l)$

  • $C<em>{13}H</em>{28} \rightarrow 2C<em>2H</em>4(g) + C<em>3H</em>6(g) + C<em>6H</em>{12}(l) + H_2(g)$

• The fraction being cracked contains a complex mixture of hydrocarbons, not just one.

• You will have an equally complex mixture of smaller hydrocarbons, both alkanes and alkenes.

• This mixture will have to go through a lot of further processing (including further fractional distillation) to separate everything out into pure compounds.

Reasons for Cracking

• To produce more petrol.

• To produce more alkenes that can be used for making polymers (plastics).

Extension notes on Industrial Catalytic Cracking

• In industry, catalytic cracking is done by passing hydrocarbons through a bed of zeolite (aluminosilicate) catalyst at about 500°C and moderate pressures.

• Apart from breaking down large straight-chain molecules into smaller ones, isomerisation can occur during the process to produce branched-chain alkanes and alkanes with ring structures.

• The presence of these isomers in a fuel helps to increase the octane
  number of a fuel, which is a measure of the tendency of a fuel to not undergo auto-
  ignition in an engine. Auto-ignition is when fuel burns spontaneously out of control. It
  leads to wear in the engine and wastage of petrol. The higher the octane number, the
  lower the tendency for a fuel to undergo auto-ignition.