Topic 8 - fuels and earth science

what are hydrocarbons

• hydrocarbons are compounds that contain carbon and hydrogen only

describe crude oil

• a complex mixture of hydrocarbons

• containing molecules in which carbon atoms are in chains or rings (names, formula and structures of specific ring molecules not required)

• an important source of useful substances (fuels and feedstock for the petrochemical industry)

• a finite resource

how is crude oil seperated into simpler, more useful mixtures

by the process of fractional distilation

• crude oil is a mixture of different hydrocarbons, different hydrocarbons have different boiling points (longer chain hydrocarbons have higher boiling points)

• the crude oil is heated in the fractionating column and the oil evaporates and condenses at a number of different temperatures

• the fractionating column works continuously, heated crude oil is piped in at the bottom. the vaporised oil rises up the column and the various fractions are constantly tapped off at the different levels where they condense

• the fractions can be processed to produce fuels and feedstock for the petrochemical industry

recall the names and uses of the fractions

• gases: domestic heating and cooking

• petrol: fuel for cars

• kerosene: fuel for aircraft

• diesel oil: fuel for some cars and trains

• fuel oil: fuel for large ships and in some power stations

• bitumen: surface roads and roofs

how do hydrocarbons in different fractions differ from each other in: the number of carbon and hydrogen atoms their molecules contain, boiling points, ease of ignition, and viscosity and are what homologous series they are

mostly members of the alkane homologous series

• some properties of hydrocarbons depend on the size of their molecules. these properties influence their use as fuels

• the shorter the molecules, the less viscous it is (more runny) and the longer the molecules, the more viscous it is

• the shorter the molecules, the lower the temperature at which that fraction evaporates or condenses - and the lower its boiling point

• the shorter the molecules - the more flammable it is, so the easier it is to ignite

explain what a homologous series is

• have the same general formula

• differ by CH₂ in molecular formulae from neighbouring compounds

• show a gradual variation in physical properties, as exemplified by their boiling points

• have similar chemical properties

describe the reaction of complete combustion of hydrocarbon fuels

• CO₂ and H₂O are produced

• energy is given out (exothermic)

why can the incomplete combustion of hydrocarbons produce carbon and carbon monoxide

• if there’s not enough oxygen, some of the fuel doesn’t burn - this is partial combustion. here, solid particels of soot (carbon) and unburnt fuel are released

• carbon monoxide (CO) is also released when there isn't enough oxygen to produce CO₂ instead

explain how carbon monoxide behaves as a toxic gas

• carbon monoxide is a colourless and odourless gas which, if breathed in, prevents red blood cells carrying oxygen around your body, which leads to death

describe the problems caused by incomplete combustion

produces carbon monoxide in appliances that use carbon compounds as fuels

• carbon monoxide causes health problems

• soot causes global dimming

explain how impurities in some hydrocarbon fuels result in the production of sulfur dioxide

• most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur

• when the fuels are burnt in oxygen, this sulfur can react to form sulfur dioxide

explain some problems associated with acid rain caused when sulfur dioxide dissolves in rain water

• damages buildings and statues (made of limestone)

• reduce the growth of or kill trees and crops

• lower pH of water in lakes, killing fish

explain why, when fuels burned in engines, oxygen and nitrogen can react together at high temperatures to produce oxides of nitrogen, which are pollutants

• nitrogen and oxygen from the air combine to produce nitrogen monoxide

• when this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide

• nitrogen monoxide and nitrogen dioxide are pollutants

advantages of using hydrogen, rather than petrol as a fuel in cars

• use of hydrogen - petrol is from crude oil, a finite resource

• only produces water - no CO₂ produced which contributes to global warming

disadvantages of using hydrogen, rather than petrol as a fuel in cars

• expensive

• difficult to transport and store hydrogen

• dangerous - hydrogen can be explosive

what are petrol, kerosene and diesel oil

• non-renewable fossil fuels obtained from crude oil

• and methane is a non-renewable fossil fuel found in natural gas

what does cracking involve

• hydrocarbons can be cracked to produce smaller, more useful molecules. this process involves heating the hydrocarbons to vaporise them

• the vapours are:

  • either passed over a hot catalyst

  • mixed with steam and heated to a very high temperature so that thermal decomposition reactions can occur

• the products of cracking includes alkanes and unsaturated hydrocarbons called alkenes

  • alkenes have the general formula C_nH_2n

  • the first 2 alkenes are ethene and propene

  • they are unsaturated because they have a double bond

why is cracking necessary

• demand for smaller chained alkanes is much greater than that for longer chained alkanes

• shorter chained hydrocarbons ignite more easily and so are more useful as fuels

what formed the Earth’s early atmosphere

• there was intense volcanic activity that released gases that formed the early atmosphere

  • at the start of this period, the atmosphere may have been like the atmosphere of Mars and Venus today, mainly CO₂ with little or no O₂ (g)

  • volcanoes also produced nitrogen which gradually built up in the atmosphere & there may have been small proportions of methane (CH₄) and NH₃

what was the Earth’s early atmosphere thought to contain

• little or no oxygen

• a large amount of CO₂

• water vapour

• small amounts of other gases

how did condensation of water vapour form oceans

• water vapour condensed to form the oceans - H₂O(g) → H₂O(l)

why did the amount of CO₂ in the atmosphere decrease when CO₂ dissolves as the oceans formed

• CO₂ dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of CO₂ in the atmosphere

explain how the growth of primitive plants used CO₂ and released O₂ by photosynthesis and consequently the amount of O₂ in the atmosphere gradually increased

• algae & plants produced the O₂ that is now in the atmosphere by photosynthesis

  • 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

  • carbon dioxide + water-(light)→ glucose + oxygen

• algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere

• over the next billion years plants evolved and the % oxygen gradually increased to a level that enabled animals to evolve

• algae and plants decreased the % of CO₂ in the atmosphere by photosynthesis

describe the chemical test for O₂

• use a glowing splint inserted into a test tube of the gas

  • splint relights in oxygen

describe how various gases in the atmosphere, including CO₂, CH₄ and water vapour, absorb heat radiated from the Earth, subsequently releasing energy which keeps the Earth warm (known as the greenhouse effect)

• electromagnetic radiation at most wavelengths from the sun passes through the Earth’s atmosphere

• the Earth absorbs some radiation and thus warms up (essential for life on Earth). but some heat is radiated from the Earth as infrared radiation

• some of this IR radiation is absorbed by greenhouse gases in the atmosphere

• atmosphere warms up leading to the greenhouse effect and global warming

examples of human activity causing climate change

• the correlation between the change in atmospheric carbon dioxide concentration

• the consumption of fossil fuels

• temperature change

• the uncertainties caused by the location where these measurements are taken

• historical accurary

describe the potential effects on the climate of increased levels of CO₂ and CH₄ generated by human activity

• activities increase levels of CO₂ & CH₄

• examples of human activity include:

  • driving (CO₂)

  • consuming electricity (CO₂)

  • raising livestock (cows - CH₄)

  • decay of organic waste in landfill sites (CH₄)

• based on peer reviewed evidence, many scientists believe that human activities will cause the temperature of the Earth’s atmosphere to increase at the surface and that this will result in global climate change

  • but, it is difficult to model such complex systems as global climate change

  • this leads to simplified models, speculation and opinions presented in the media that may be based on only parts of the evidence which may be biased

what is the current composition of the Earth’s atmosphere

• nitrogen - 78%

• oxygen - 21%

• argon - 0.93%

• carbon - 0.04%

what are the effects of global warming

• melting of polar ice caps

• difficulties acquiring drinking water

• flooding

• forest fires

• destruction of ecosystems

how can the effects of global warming be mitigated

• construct flood defences in areas of low lying land

• use irrigation systems to provide water in drought

• produce alternative crops which are better adapted to the new enviroment