Section 3.1.6 Alkanes know that alkanes are saturated hydrocarbons know that petroleum is a mixture consisting mainly of alkane hydrocarbons understand that different components (fractions) of this mixture can be drawn off at different levels in a fractionating column because of the temperature gradient understand that cracking involves the breaking of C–C bonds in alkanes know that thermal cracking takes place at high pressure and high temperature and produces a high percentage of alkenes (mechanism not required) know that catalytic cracking takes place at a slight pressure, high temperature and in the presence of a zeolite catalyst and is used mainly to produce motor fuels and aromatic hydrocarbons (mechanism not required) understand the economic reasons for the cracking of alkanes (e.g. ethene used for poly(ethene); conversion of heavy fractions into higher value products) know that alkanes are used as fuels and understand that their combustion can be complete or incomplete and that the internal combustion engine produces a number of pollutants (e.g. NOx, CO and unburned hydrocarbons) know that these pollutants can be removed using catalytic converters know that combustion of hydrocarbons containing sulfur leads to sulfur dioxide that causes air pollution and understand how sulfur dioxide can be removed from flue gases using calcium oxide know that the combustion of fossil fuels (including alkanes) results in the release of carbon dioxide into the atmosphere know that carbon dioxide, methane and water vapour are referred to as greenhouse gases and that these gases may contribute to global warming

pentane: straight chain 2-methylbutane: branched chain Alkanes and isomersim Alkanes with four or more carbon atoms display structural isomerism because the carbon chain may be either straight or branched. pentane: straight chain 2-methylbutane: branched chain The naming of alkanes depends on whether they are straight or branched.

Trends in boiling points

Trends in boiling points The boiling point of straight-chain alkanes increases with chain length due to increasing van der Waals forces between molecules. As the length of the chain increases, so does its surface area, and so the van der Waals forces are stronger. Branched-chain alkanes have lower boiling points because the chains cannot pack as closely together. There are fewer points of contact between molecules so the van der Waals forces are weaker.

Crude oil and alkanes Crude oil is a mixture composed mainly of straight and branched chain alkanes. It also includes lesser amounts of cycloalkanes and arenes, both of which are hydrocarbons containing a ring of carbon atoms, as well as impurities such as sulphur compounds. The exact composition of crude oil depends on the conditions under which it formed, so crude oil extracted at different locations has different compositions.

Fractional distillation What is distillation? Distillation is a method of separating mixtures – usually liquids - based on differences in the boiling points of the components of the mixture. What is fractional distillation? The process in which the components of a mixture are separated into groups or fractions of different boiling points ranges.

Fractional distillation a fractionating column

Fractional distillation

Uses of the fractions

Uses of the fractions

Uses of the fractions

Uses of the fractions

Fractions and their uses

Basics of Organic Chemistry

C2.5.2 Targets H By the end of these two pages you should be able to: describe the process of cracking recall that when alkanes are cracked, mixtures of alkanes and alkenes are formed explain the differences between alkanes and alkenes describe how bromine water is used to show if something is an alkane or an alkene explain how ethene can be reacted with water to make ethanol. H

Cracking of Hydrocarbons Crude oil is separated into its parts by FRACTIONAL DISTILLATION Hydrocarbons, e.g. alkanes and alkenes, are extracted from.... CRUDE OIL

Cracking of Hydrocarbons TOO LITTLE! small molecules TOO MUCH! large molecules

Cracking of Hydrocarbons Heavy oil Petrol Produced Needed AMOUNT PETROL HEAVY FUEL PETROL HEAVY FUEL

Cracking of Hydrocarbons Ethene Decane Octane Smaller molecules C2H4 Catalyst bed aluminium oxide 500ºC Large molecules C8H18 C10H22

Cracking of Hydrocarbons decane C10H22 C H C H H C H C C H H C octane C8H18 ethene C2H4 SATURATED– contains only single bonds UNSATURATED– contains a double bond

Cracking of Hydrocarbons Cracking reaction – summary: C10H22(g)    C8H18(g)   +  C2H4(g) saturated saturated unsaturated *The (g) is called a ‘state symbol’ and stands for ‘gas’

Fractions and their uses

Supply and demand Demand for lower boiling point (short chain) fractions is greater than the proportion in crude oil. Crude oil contains more higher b.p. (longer chain) fractions, which are in lower demand and less economically valuable... So, there’s a shortage of shorter chain fractions and a surplus of longer chain ones.

What is cracking? Cracking is a process that splits long chain alkanes into shorter chain alkanes, alkenes and hydrogen. C10H22 → C7H16 + C3H6 Cracking has the following uses: it increases the amount of gasoline and other economically important fractions it increases branching in chains, an important factor for petrol it produces alkenes, an important feedstock for chemicals. There are two main types of cracking: thermal and catalytic.

Cracking

Cracking

Cracking

Cracking

Cracking

Cracking

Thermal v Catalytic cracking Catalytic cracking has several advantages over thermal cracking: it produces a higher proportion of branched alkanes, which burn more easily than straight-chain alkanes and are therefore an important component of petrol the use of a lower temperature and pressure mean it is cheaper it produces a higher proportion of arenes, which are valuable feedstock chemicals. However, unlike thermal cracking, catalytic cracking cannot be used on all fractions, such as bitumen, the supply of which outstrips its demand.

Other products from cracking Alkenes such as ethene are always produced in cracking. They are an important feedstock for use in the chemical industry, particularly in the production of polymers. Arenes such as benzene are also produced during catalytic cracking. Benzene is added in small quantities to petrol as a replacement for the lead compounds. It too is now the subject of health concerns, and its use is being reduced.

Complete combustion Complete combustion In excess oxygen, short chain alkanes can undergo complete combustion: alkane + oxygen → carbon dioxide + water For example: propane + oxygen → carbon dioxide + water C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) The combustion of alkanes is a highly exothermic process. This makes them good fuels because they release a relatively large amount of energy per gram of fuel.

Incomplete combustion If oxygen is limited then incomplete combustion will occur: alkane + oxygen → carbon monoxide + water alkane + oxygen → carbon + water For example: propane + oxygen → carbon monoxide + water C3H8(g) + 3½O2(g) → 3CO(g) + 4H2O(g) propane + oxygen → carbon + water C3H8(g) + 2O2(g) → 3C(s) + 4H2O(g)

The internal combustion engine: carbon Alkanes with chain lengths of 5–10 carbon atoms are used as fuels in internal combustion engines. This releases carbon dioxide into the atmosphere: nonane + oxygen → carbon dioxide + water C9H20(g) + 14O2(g) → 9CO2(g) + 10H2O(g) Although modern internal combustion engines are more efficient than in the past, incomplete combustion still occurs: nonane + oxygen → carbon monoxide + water 2C9H20(g) + 19O2(g) → 18CO(g) + 20H2O(g)

4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq) The internal combustion engine: nitrogen The temperature in an internal combustion engine can reach over 2000 °C. Here, nitrogen and oxygen, which at normal temperatures don’t react, combine to form nitrogen monoxide: N2(g) + O2(g) → 2NO(g) Nitrogen monoxide reacts further forming nitrogen dioxide: 2NO(g) + O2(g) → 2NO2(g) Nitrogen dioxide gas reacts with rain water and more oxygen to form nitric acid, which contributes to acid rain: 4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq)

The catalytic converter

The catalytic converter

The catalytic converter

The catalytic converter CO, NOx, HC CO2, N2, H2O

The catalytic converter

Sulphur contamination of fossil fuels Sulphur is found as an impurity in crude oil and other fossil fuels. It burns in oxygen to form sulphur dioxide: S(s) + O2(g) → SO2(g) Sulphur dioxide may be oxidized to sulphur trioxide: 2SO2(g) + O2(g) → 2SO3(g) Both of these oxides dissolve in water forming acidic solutions: SO2(g) + H2O(l) → H2SO3(aq) SO3(g) + H2O(l) → H2SO4(aq)

What is acid rain? Acid rain is caused by acidic non-metal oxides such as sulfur oxides and nitrogen oxides dissolving in rain water. Rain water is naturally acidic because carbon dioxide dissolves in it, forming weak carbonic acid. Sulphfur and nitrogen oxides form more acidic solutions, which can damage trees and affect aquatic life in lakes and rivers.

Removing sulfur dioxide pollution Sulphur dioxide emissions from vehicle fuels such as petrol and diesel are reduced by removing nearly all of the sulphur impurities from the fuel before it is burnt. Removing the sulphur from coal before it is burnt is not practical. Instead, the acidic sulphur oxides are removed from the waste gases using a base such as calcium oxide.

Carbon dioxide in the atmosphere Burning fossil fuels releases carbon dioxide into the atmosphere. Fossil fuels are being burned faster than they are being formed, which means that their combustion leads to a net increase in the amount of atmospheric carbon dioxide. It has been suggested that increases in the amount of carbon dioxide and other greenhouse gases may be responsible for apparent changes to the climate.

Greenhouse gases Carbon dioxide, water vapour and methane have been described as the main greenhouse gases. This is because these have been suggested as the gases responsible for the majority of the greenhouse effect. The greenhouse effect is a theory that has been suggested to explain apparent rises in the average temperature of the Earth. Increasing the amount of any of the greenhouse gases traps more heat energy from the Sun in the Earth’s atmosphere, raising the average temperature.

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