Tuesday 23 September 2014

Fossil fuels

Making crude oil useful
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Crude oil is a mixture of hydrocarbons. These are separated into useful products, such as fuels, using a process called fractional distillation.
The demand for short hydrocarbon molecules is greater than their supply in crude oil, so a reaction called cracking is used. Cracking converts long alkane molecules into shorter alkanes and alkenes, which are more useful. The exploitation of oil can damage the environment - for example, through oil spills.

Fossil fuels

Crude oil, coal and gas are fossil fuels. They were formed over millions of years, from the remains of dead organisms,
  • coal was formed from dead plant material
  • crude oil and gas were formed from dead marine organisms.
Fossil fuels are non-renewable. They took a very long time to form and we are using them up faster than they can be renewed. Fossil fuels are also finite resources. They are no longer being made or are being made extremely slowly. Once they have all been used up, they cannot be replaced.

How crude oil was formed (background information only)

Crude oil is found trapped in some of the sedimentary rocks of the Earth's crust.
Millions of years ago, huge numbers of microscopic animals and plants - plankton - died and fell to the bottom of the sea. Their remains were covered by mud.
As the mud sediment was buried by more sediment, it started to change into rock as the temperature and pressure increased. The plant and animal remains were ‘cooked’ by this process, and slowly changed into crude oil.

Oil is less dense than the water in the rocks and will rise as a result of pressure from below (as can be seen in the animation above). often the oil will escape altogether if the rocks are permeable (liquids can pass through them).
If some of the rocks above the oil are impermeable the oil cannot rise through them, so it gets trapped underneath.

Problems of exploiting oil

Geologists can often tell where oil is trapped by looking at the structure of the rocks. Oil tends to be trapped where rocks are domed upwards, or where permeable rocks are in contact with impermeable rocks at a fault line.

Drilling for oil

Oil companies can drill down through the impermeable rocks to get it out. They are then able to turn the oil into products that we can use.

Crude oil takes millions of years to form, and we are using it up more quickly than it is created. Present estimates suggest world supplies of crude oil will run out in about 30 years, unless we use it more efficiently. There are additional reserves of oil in rocks called oil shale. However, it is expensive to extract oil from oil shale because it needs to be heated to release it.

Environmental problems

Oil is carried from oil fields to refineries using ocean-going tankers. If it is spilled, it causes considerable damage to the environment:
  • oil slicks travel across the sea, far from the original spill
  • beaches and wildlife are harmed when they are coated with oil.
The oil damages feathers and birds may die. Detergents are often used to help clean up oil slicks, but these in turn may harm wildlife.


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Science
Making crude oil useful
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Distillation

Distillation is a process that can be used to separate a pure liquid from a mixture of liquids. It works when the liquids have different boiling points. Distillation is commonly used to separate ethanol (the alcohol in alcoholic drinks) from water.
Distillation process to separate ethanol from water
water and ethanol solution are heated in a flask over a bunsen burner, pure vapour is produced in the air above the solution within the flask.
Step 1 - water and ethanol solution are heated
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The mixture is heated in a flask. Ethanol has a lower boiling point than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid.
The thermometer shows the boiling point of the pure ethanol liquid. When all the ethanol has evaporated from the solution, the temperature rises and the water evaporates.
This is the sequence of events in distillation:
heating    →    evaporating    →    cooling    →    condensing


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Making crude oil useful
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Fractional distillation

Hydrocarbons have different boiling points. They can be solid, liquid or gas at room temperature,
  • small hydrocarbons with only a few carbon atoms have low boiling points and are gases
  • hydrocarbons with between five and 12 carbon atoms are usually liquids
  • large hydrocarbons with many carbon atoms have high boiling points and are solids.
Because they have different boiling points, the substances in crude oil can be separated using fractional distillation.

The fractionating column

Fractional distillation is different from distillation in that it separates a mixture into a number of different parts, called fractions. A tall column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top.
The crude oil is evaporated and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.

Fractioning column
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Group 1 and Group 7 Chemistry.

Reactions of group 1 and 7 elements

The Group 1 elements
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The group 1 elements in the periodic table are known as the alkali metals. They include lithium, sodium and potassium, which all react vigorously with air and water.
The reactivity of the alkali metals increases down the group. Flame tests are used to identify alkali metal ions in compounds.

The alkali metals

The group 1 elements are placed in the vertical column on the left hand side of the periodic table.
Diagram showing group 1 of the periodic table
Lithium, sodium and potassium are the three group 1 elements you are likely to see at school. Like all the group 1 elements, they are very reactive. They must be stored under oil to keep air and water away from them. group 1 elements form alkaline solutions when they react with water, which is why they are called alkali metals.

Reactions with water

Potassium reacting with water
The reaction of potassium with water gives a lilac flame
Group 1 elements react vigorously with water to produce an alkaline metal hydroxide and hydrogen gas. In general:
Metal + water → metal hydroxide + hydrogen
For example, here are the equations for the reaction of sodium with water:
sodium + water → sodium hydroxide + hydrogen 2Na + 2H2O → 2NaOH + H2 (the 2s in front of Na, H2O and NaOH are for balancing)
The reactivity of the alkali metals increases down the group. Lithium is the least reactive and potassium is the most reactive of the three. The hydrogen ignites immediately during the reaction between potassium and water with the potassium producing a lilac coloured flame.




Making predictions

Because there are patterns in the way the elements are arranged in the periodic table, it can be used to predict their properties and interpret data.

Reactivity

The group 1 elements become more reactive as you go down the group. At the top, lithium is the least reactive and francium at the bottom is the most reactive. Francium is rare and radioactive, so it would be difficult to confirm predictions made about it. However, it is possible to predict the properties of rubidium and caesium and to see if the predictions were accurate.

 

Group 1 elementFeatures of reaction with water
LithiumFizzes steadily, gradually disappears
SodiumFizzes rapidly, melts into a ball and disappears quickly
PotassiumIgnites with sparks and a lilac flame, disappears very quickly
RubidiumExplodes with sparks
CaesiumViolent explosion due to rapid production of heat and hydrogen

Physical properties - Higher tier

The table shows the melting points of Group 1 elements, with one value missing.

 

Group 1 elementMelting point/ °C
Lithium181
Sodium98
Potassium63
Rubidium?
Caesium29
The melting points show a pattern, or trend, down the group. It is therefore possible to predict that the melting point of rubidium is between 29°C and 63°C (it is actually 39°C). The same can be done with other physical properties, such as the densities of rubidium and caesium, for example.

Science
The Group 1 elements
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Flame tests

It's possible to test a compound to detect the presence of an alkali metal ion.
Flame test loop shown being dipped in sodium solution in test tube, then shown in contact with blue flame of bunsen burner. The flame in contact with the test loop is orange.
A cleaned, moistened flame test wire is dipped into a solid sample of the compound and then put into a blue Bunsen flame. The flame colour indicates which alkali metal ion is present in the compound.

Flame colours and the alkali metal ion they represent

Flame colourIon present
RedLithium
OrangeSodium
LilacPotassium




Explaining trends

The Group 1 elements have similar properties because of the electronic structure of their atoms - they all have one electron in their outer shell.

 

Group 1 elementElectronic structureDiagram of atom
Lithium2.1Structure of a lithium atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger outer circle has one red dot on it, representing the second energy level with one electron
Sodium2.8.1Structure of a sodium atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger middle circle has eight red dots, representing the second energy level with eight electrons. A larger outer circle has one red dot on it, representing the third energy level with one electron
Potassium2.8.8.1Structure of a potassium atom.



The group 7 elements
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The group 7 elements are also known as the halogens. They include fluorine, chlorine, bromine and iodine, which all have seven electrons in their outer shell.
In a displacement reaction, a less reactive element is displaced by a more reactive element.

The halogens

The group 7 elements are placed in the vertical column second from right in the periodic table.
The Periodic Table, with group 7 highlighted
The periodic table
Chlorine, bromine and iodine are the three Group 7 elements you are likely to see at school. Fluorine is too reactive to keep or produce safely at school. Group 1 elements form salts when they react with metals, which is why they are called the halogens ('salt formers').

Properties and uses of the halogens

The table summarises some of the properties and uses of three halogens.

 

Group 7 elementPropertiesTypical use
ChlorineGreen gasSterilising water
BromineOrange liquidMaking pesticides and plastics
IodineGrey solidSterilising wounds
Iodine forms a purple vapour when it is warmed.

Reactions with group 1 elements

The group 7 elements react vigorously with group 1 elements such as sodium and potassium. In each case, a metal halide is formed (fluoride, chloride, bromide or iodide).
The table summarises the names and formulae for the metal halides formed by the reaction of group 1 elements with group 7 elements.

 

Group 1\7ChlorineBromineIodine
LithiumLithium chloride - LiClLithium bromide - LiBrLithium iodide - LiI
SodiumSodium chloride - NaClSodium bromide - NaBrSodium iodide - NaI
PotassiumPotassium chloride - KClPotassium bromide - KBrPotassium iodide - KI
For example, sodium reacts with chlorine to produce sodium chloride
Sodium + chlorine → sodium chloride
2Na + Cl2 → 2NaCl (the 2s in front of Na and NaCl are for balancing)
Notice that chlorine is Cl2 in the symbol equation, not Cl. The halogens all exist asdiatomic molecules, two halogen atoms joined together by a covalent bond:
  • Fluorine, F2
  • Chlorine, Cl2
  • Bromine, Br2
  • Iodine, I2

Displacement and reactivity

If a reactive element comes into contact with the compound of a less reactive element a chemical reaction may take place. The less reactive element isremoved from the compound and replaced by the more reactive element.
For example, if chlorine is added to a solution of potassium bromide, the bromine is replaced by the chlorine forming potassium chloride. Bromine is formed at the same time and can be detected by its colour.
Reactions of this type are called displacement reactions

Examples of displacement reactions

HalogenMetal halideThe most reactive halogenReaction
ChlorineSodium bromideChlorineChlorine + sodium bromide → sodium chloride + bromine
ChlorineSodium iodideChlorineChlorine + sodium iodide → sodium chloride + iodine
BromineSodium chlorideChlorineNo reaction
BromineSodium iodideBromineBromine + sodium iodide → sodium bromide + iodine
IodineSodium chlorideChlorineNo reaction
IodineSodium bromideBromineNo reaction



Explaining properties

The group 7 elements have have similar properties because of the electronic structure of their atoms - they all have seven electrons in their outer shell.

 

Group 1 elementElectronic structureDiagram of atom
Fluorine2.7Structure of a fluorine atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger outer circle has seven red dots on it, representing the second energy level with seven electrons
Chlorine2.8.7The structure of a chlorine atom
An extra outer electron is gained during chemical reactions.
Now try a Test Bite.

Higher tier

In a reaction, an atom of a group 7 element will form an ion with a single negative charge (a halide ion). For example, for a chlorine molecule forming two chloride ions
Cl2 + 2e– → 2Cl–
A change like this, where an electron is gained, is an example of reduction.
The ions formed have a stable electronic structure, like a noble gas from group 0.

 

Group 1 elementElectronic structure of ionDiagram of ion
Fluorine[2.8]-Fluoride ion configuration is [2.8]-
Chlorine[2.8.8]-Chloride ion configuration is [2.8.8]-
The reactivity of group 7 elements decreases down the group because, as you go down the group:
  • The atoms get larger
  • The outer shell gets further from the nucleus
  • The attraction between the nucleus and electrons gets weaker, so an electron is less easily gained
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