Core Knowledge – Topic 8

41) What is a hydrocarbon?

Answer 41
A hydrocarbon is a compound that contains hydrogen and carbon ONLY.

42) What is crude oil?

Answer 42
Crude oil is a complex mixture of alkane hydrocarbons. Some of these hydrocarbons contain molecules in which carbon atoms are in chains and in some, they are in rings. Crude oil is an important source of useful substances and a finite resource.

43) With respect to crude oil, what is a “fraction”?

Answer 43
A fraction is a simpler, more useful mixture of hydrocarbons with a similar boiling point, e.g. petrol or bitumen.

44) What is the name of the process used to separate crude oil into its fractions?

Answer 44
Fractional distillation.

45) How does the fractional distillation of crude oil work?

Answer 45
The crude oil is heated and boiled. The vapour is then passed into a cooling tower. The hot vapours rise up the tower and cool as they do so. The first substance in crude oil to change back to a liquid is bitumen and this falls to the bottom of the tower and exits, the hot vapours rise through the tower and pass through one-way traps. This process continues until all the fractions have been separated and the gases at room temperature leave at the top of the tower.

46) The fractions come off the fractionating column in the following order (starting from the top of the column). Name the uses of each fraction:

  1. Refinery gases
  2. Petrol
  3. Kerosene
  4. Diesel oil
  5. Fuel oil
  6. Bitumen
Answer 46

  1. domestic heating and cooking
  2. fuel for cars
  3. fuel for aircraft
  4. fuel for some cars and trains
  5. fuel for large ships and in some power stations
  6. used to surface roads and roofs

47) Hydrocarbons in different fractions differ from each other in:

Number of carbon atoms in their molecules, boiling points, ease of ignition (flammability) and viscosity (stickiness).

a) which fraction has the most carbon atoms in its molecules (the longest carbon chain)? 

b) which fraction has the lowest boiling point?

c) which fraction is the hardest to ignite (least flammable)?

d) which fraction has the lowest viscosity? 

Answer 47

a) bitumen

b) refinery gases

c) bitumen

d) refinery gases

48) What is a homologous series?

Answer 48
A homologous series is a series of compounds that have similar properties and the same general formula. A compound will differ by CH2 in molecular formulae from neighbouring compounds. There will be a gradual change in physical properties as the carbon chain gets longer.

49) What are the reactants and products of the complete combustion of hydrocarbons?

Answer 49
Reactants – hydrocarbon and oxygen.

Products – carbon dioxide and water ONLY.

(Energy is released, but it is not a product, because it is not a chemical substance.)

50) What are the products of the incomplete combustion of hydrocarbons? Why are they different from the products of complete combustion? 

Answer 50
Products – carbon monoxide and/or carbon and water. Incomplete combustion produces a mixture of carbon compounds.)

Carbon monoxide (CO) and/or carbon (C) are produced because there is not enough oxygen available to form carbon dioxide (CO2). 

51) Why are we concerned about incomplete combustion?

Answer 51
Incomplete combustion can cause the release of carbon monoxide, which is toxic. The soot (carbon) produced can damage appliances.

52) What effect does carbon monoxide have on the body?

Answer 52
Carbon monoxide is toxic. It binds to heamoglobin and doesn’t let go. It therefore reduces the amount of oxygen that’s transported around the body by the blood depriving vital organs of oxygen. Unconsciousness and death follows.

53) What is “acid rain”, and how does it arise?

Answer 53
Acid rain is rain that is more acidic than normal. All fossil fuels (coal, gas and crude oil) contain impurities, particularly sulfur. When the fuel is burnt the sulfur combines with oxygen to produce sulfur dioxide gas. When water vapour in the atmosphere condenses the sulfur dioxide gas dissolves in it to form an acidic solution. This can then fall as rain and because it is more acidic than normal rainwater it is called “acid rain”.

54) What are the problems associated with acid rain?

Answer 54
Acid rain makes rivers, lakes and soils acidic, harming the organisms living there. 

Acid rain damages the leaves and roots of plants and trees.Acid rain can speed up the weathering of limestone (rocks or buildings) and marble.

55) How are nitrogen oxides produced?

Answer 55
Many hydrocarbons are burnt in engines. The high temperatures involved mean that the nitrogen and oxygen from the air combine to produce oxides of nitrogen.

56) What is a nonrenewable fuel?

Answer 56
A fuel that once it has been used cannot be used again. E.g. kerosene, diesel, petrol, methane (from natural gas).

57) What is the cause of a sooty flame?

Answer 57
Incomplete combustion. (Not enough oxygen present to convert all the carbon in the hydrocarbon fuel to carbon dioxide, so carbon particles are one of the products of the reaction.)

58) Give an advantage and a disadvantage of combining hydrogen and oxygen in a fuel cell¸ rather than petrol, as a fuel for cars.

Answer 58
Advantage – hydrogen is a clean fuel. The only product of the combination of hydrogen and oxygen is water. Therefore no carbon dioxide, nitrogen oxide or acid rain would be produced.

Disadvantage – hydrogen can be explosive/hydrogen is not readily available in filling stations at present /the process needed to produce the hydrogen fuel results in the production of carbon dioxide. 

59) Are alkanes saturated or unsaturated?

Answer 59
Saturated. (They have no carbon-carbon double bonds that can open up to bond with any more hydrogen atoms – they are saturated with hydrogen.)

60) What is the formula for:

  1. methane
  2. ethane
  3. propane

Draw the structures of these molecules.

Answer 60

61) What is the formula for the alkenes:

  1. ethene
  2. propene
Answer 61

  1. C2H4
  2. C3H6

62) a) Explain what “cracking” is, and what products are made.

b) Why do oil companies bother to carry out this reaction?

Answer 62

a) Cracking is the splitting (using heat) of a long chain saturated hydrocarbon (an alkane) to form a shorter chained alkane and an alkene.

b) Shorter chained hydrocarbons make better fuels. Crude oil contains too many of the longer chained molecules, so oil companies crack them to i) make more of the useful fuels, and ii) make alkenes (which can be used to make polymers).

63) How was the earth’s first atmosphere formed?

Answer 63
From gases produced by volcanic activity.

64) What are thought to be the relative proportions of the gases that formed the early atmosphere?

Answer 64
Little or no oxygen, large amounts of carbon dioxide, large amounts of water vapour and small amounts of other gases.

65)Why can’t we be certain about how the earth’s atmosphere formed?

Answer 65
There is only limited evidence (e.g. from rocks and ice cores) about the earth’s early atmosphere.

66) How were the earth’s oceans formed?

Answer 66
Water vapour, released by volcanoes, condensed to form the oceans.

67) How did the amount of oxygen in the atmosphere gradually increase?

Answer 67
Green plants evolved. The growth of these primitive plants used carbon dioxide and released oxygen by photosynthesis.

68) What is a chemical test for oxygen?

Answer 68
Oxygen will relight a glowing splint.

69) Describe the processes, other than photosynthesis, that reduced the amount of carbon dioxide in the atmosphere.

Answer 69

  1. Carbon dioxide dissolved into the oceans.
  2. Dissolved carbon dioxide was incorporated into the shells of marine organisms. When marine organisms die their shells can eventually form carbonate rocks.

70) What is the greenhouse effect?

Answer 70
This is when various gases are added to the atmosphere, including carbon dioxide, methane and water vapour. These gases absorb heat radiated from the Earth and subsequently release the energy that keeps the Earth warm.

71) What evidence do we have for global warming and why can we not be absolutely certain about it?

Answer 71

Scientists have discovered a correlation between historical global temperature and carbon dioxide levels. They also know how much carbon dioxide we are presently adding to the atmosphere.

We cannot be certain about this because of historical accuracy of the temperature and carbon dioxide levels and also due to uncertainties caused by the location where measurements are taken.

72) List the percentages of the gases in our modern atmosphere.

Answer 72
Nitrogen 78%, oxygen 21%, 1% other gases (argon, carbon dioxide and water vapour).

73) What are the potential effects on the climate of increased levels of carbon dioxide and methane caused by human activity?

Answer 73
The climate will warm up although we cannot be certain by how much. It is also suspected we will have a long term change in weather (e.g. more/less rain) and more extreme weather events.

74) How might the greenhouse effect be mitigated?

Answer 74
We would need to reduce the consumption of fossil fuels by looking at alternative sources of energy e.g. nuclear or renewables. Also, a different fuel for transport e.g. electricity or fuel cells.

75) Why can we not just stop burning fossil fuels to generate electricity?

Answer 75
Nuclear power is not liked by all and the waste is a risk and can be a problem for the environment. Solar and wind don’t produce that much electricity so you would need thousands of solar and wind farms and this would take too much space and be extremely expensive. Generation from solar and wind is not always continuous.
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