How chain length changes boiling point, viscosity and flammability, complete and incomplete combustion, and why long chains are cracked into shorter ones.
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1.How does boiling point change as hydrocarbon chains get longer?
It increases, because the intermolecular forces between larger molecules are stronger.
2.How does viscosity change as chains get longer?
It increases, so longer chain hydrocarbons are thicker and flow less easily.
3.How does flammability change as chains get longer?
It decreases, so longer chain hydrocarbons are harder to ignite.
4.Which fraction is most useful as a car fuel and why?
Petrol, because its short chains make it easy to ignite and it flows easily.
5.What is produced by the complete combustion of a hydrocarbon?
Carbon dioxide and water.
6.Write the word equation for complete combustion of methane.
methane + oxygen gives carbon dioxide + water
7.What is oxidised during combustion?
Both the carbon and the hydrogen in the fuel, since both gain oxygen.
8.When does incomplete combustion happen?
When there is not enough oxygen for the fuel to burn fully.
9.What are the products of incomplete combustion?
Carbon monoxide and carbon particles, called soot, as well as carbon dioxide and water.
10.Why is carbon monoxide dangerous?
It is a toxic gas that is colourless and odourless, so it cannot be detected by the senses. It reduces the ability of the blood to carry oxygen.
11.Why is soot a problem?
It causes breathing problems and blackens buildings.
12.What is cracking?
Breaking down long chain hydrocarbon molecules into shorter, more useful ones.
13.Why is cracking necessary?
The supply of long chain fractions is greater than the demand, while short chain fuels such as petrol are in high demand.
14.Name the two methods of cracking.
Catalytic cracking and steam cracking.
15.Describe catalytic cracking.
The long chain hydrocarbon is heated to vaporise it and passed over a hot catalyst.
16.Describe steam cracking.
The vaporised hydrocarbon is mixed with steam and heated to a very high temperature.
17.What is produced by cracking?
Shorter chain alkanes, which are useful as fuels, and alkenes.
18.Why are the alkenes produced useful?
They are used to make polymers and as starting materials for many other chemicals.
19.Is cracking a physical or chemical change?
A chemical change, because covalent bonds within the molecules are broken and new molecules are formed.
20.How does cracking differ from fractional distillation?
Fractional distillation separates molecules that are already there. Cracking breaks molecules apart to make new, different ones.
21.C10H22 cracks to give C8H18 and one other product. What is it?
C2H4, ethene. The atoms must balance on both sides.
22.How can you test whether a product of cracking is an alkene?
Add bromine water. It turns from orange to colourless with an alkene, but stays orange with an alkane.
23.Why is bromine water decolourised by an alkene?
The carbon to carbon double bond opens and the bromine adds across it, forming a colourless compound.
24.Which burns with a smokier flame, an alkane or an alkene?
An alkene, because it has a higher proportion of carbon so incomplete combustion is more likely.
25.Why does economic demand drive cracking?
Short chain products sell for more and are needed in greater quantity, so converting surplus long chains is profitable.
26.Which has a higher boiling point, C4H10 or C12H26?
C12H26, because it is the larger molecule with stronger intermolecular forces.
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