CSEC Chemistry Organic Chemistry Notes - Syllabus Section B1

Global Chemistry Lessons

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L.P. #307 S.M.R. Cunupia

B 1.1 - Identify natural gas and petroleum as natural sources of hydrocarbons.

Petroleum and Natural Gas

Although how they were produced is not completely understood, petroleum and natural gas were most likely formed from the remains of marine organisms that lived approximately 500 million years ago. Petroleum is a thick, dark liquid composed mostly of compounds called hydrocarbons that contain carbon and hydrogen only. (Carbon is unique among elements in the extent to which it can bond to itself to form chains of various lengths.)

The table which follows gives the formulae and names for several common hydrocarbons. Natural gas, usually associated with petroleum deposits, consists mostly of methane, but it also contains significant amounts of ethane, propane, and butane. The composition of petroleum varies somewhat, but it consists mostly of hydrocarbons having chains that contain from 5 to more than 25 carbons. To be used efficiently, the petroleum must be separated into fractions by boiling. The lighter molecules (having the lowest boiling points) can be boiled off, leaving the heavier ones behind.

The commercial uses of various petroleum fractions are shown in the next table.

The petroleum era began when the demand for lamp oil during the Industrial Revolution outstripped the traditional sources: animal fats and whale oil. In response to this increased demand, Edwin Drake drilled the first oil well in 1859 at Titusville, Pennsylvania. The petroleum from this well was refined to produce kerosene (fraction C10–C18), which served as an excellent lamp oil. Gasoline (fraction C5–C10) had limited use and was often discarded.

However, this situation soon changed. The development of the electric light decreased the need for kerosene, and the advent of the “horseless carriage” with its gasoline-powered engine signaled the birth of the gasoline age. As gasoline became more important, new ways were sought to increase the yield of gasoline obtained from each barrel of petroleum. William Burton invented a process at Standard Oil of Indiana called pyrolytic (high-temperature) cracking. In this process, the heavier molecules of the kerosene fraction are heated to about 700 °C, causing them to break (crack) into the smaller molecules of hydrocarbons in the gasoline fraction.

As cars became larger, more efficient internal combustion engines were designed. Because of the uneven burning of the gasoline then available, these engines “knocked,” producing unwanted noise and even engine damage. Intensive research to find additives that would promote smoother burning produced tetraethyl lead, (C2H5)4Pb, a very effective “antiknock” agent.

The addition of tetraethyl lead to gasoline became a common practice, and by 1960, gasoline contained as much as 3 grams of lead per gallon. As we have discovered so often in recent years, technological advances can produce environmental problems. To prevent air pollution from automobile exhaust, catalytic converters have been added to car exhaust systems. The effectiveness of these converters, however, is destroyed by lead.

The use of leaded gasoline also greatly increased the amount of lead in the environment, where it can be ingested by animals and humans. For these reasons, the use of lead in gasoline has been phased out, requiring extensive (and expensive) modifications of engines and of the gasoline refining process.

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B 1.2 - List the main uses of at least three fractions obtained from the fractional distillation of petroleum.

Uses should include fuels, petrochemicals, lubricants. Refer to SO A 2.5.

Fractional Distillation of Crude Oil

The fundamental separation process in refining petroleum is fractional distillation (next diagram). Practically all crude petroleum that enters a refinery goes to distillation units, where it is heated to temperatures as high as 370 to 425°C and separated into fractions. Each fraction contains a mixture of hydrocarbons that boils within a particular range.

Fractional distillation of petroleum. The lighter, more volatile fractions are removed from higher up the column; the heavier, less volatile fractions are removed from lower down.

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B 1.3 - Describe cracking of petroleum fractions.

Thermal and catalytic cracking of alkanes.

Cracking

Excessive amounts of solid residue and other long chain hydrocarbons are usually produced during fractional distillation of crude oil. Cracking breaks them into more useful molecules with shorter chains and also produces ethene.

Thermal cracking uses high temperature and pressure to split long chain alkanes into short chain alkanes and alkenes. Hydrogen is a useful byproduct.

Catalytic cracking uses low pressure, high temperature and zeolite catalysts to split long chain alkanes in to fractions used to make petrol, together with arenes (aromatic hydrocarbons containing the benzene ring).

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Some Recent CSEC Past Paper Questions And Answers

Links to the past papers covered below were working as of September 2017 and are:

January 2016 P2: No online source known at this time.
June 2015 P2: No online source known at this time.
January 2015 P2: https://goo.gl/RuQdgm
June 2013 P2: No online source known at this time.


Questions which follow are based on theory required for SS B 1.1 -1.3.

January 2016 P2 Q5 (a) - (c)

5(a) Petroleum.

5(b) Fraction: Kerosene.
       Use: Jet fuel.

       Fraction: Lubricating oil.
       Use: Lubricants (for machines).

5(c)(i) The breakdown of long chain hydrocarbon molecules into shorter chain molecules.

5(c)(ii) Catalytic cracking uses a catalyst and relatively lower temperatures. Thermal cracking uses no catalyst and relatively higher temperatures.

CXC's comments on this question are now shown below.

June 2015 P2 Q1 (a)(i)

1(a)(i) Natural gas and petroleum.

CXC's comments on this question are now shown below.

January 2015 P2 Q3 (a), (b)

3(a) Natural gas and petroleum.

3(b)(i) Fraction 1: refinery gas.

3(b)(ii) Paving of roads.

CXC's comments on this question are now shown below.

June 2013 P2 Q3 (a)

3(a) The breakdown of long chain hydrocarbon molecules into shorter chain molecules.