Topic 6B - Alkanes Flashcards Preview

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Flashcards in Topic 6B - Alkanes Deck (28):
1

6B.8
What is the general formula for alkanes?

C(n)H(2n+2)

2

6B.9
What are the saturated hydrocarbons?

Alkanes and cycloalkanes.

3

6B.10
What is reforming?

The processing of straight-chain hydrocarbons into branched-chain alkanes and cyclic hydrocarbons for efficient combustion.

4

6B.10
How are alkane fuels obtained?

From the fractional distillation, cracking and reforming of crude oil.

5

6B.10
At what temperature does crude oil enter the fractional distillation tower?

400 degrees centigrade.

6

6B.10
What conditions are used for reforming?

High pressure, 500 degrees centigrade with a platinum catalyst.

7

6B.10
What is a valuable by-product of reforming?

Hydrogen.

8

6B.10
When a component of crude oil has a boiling temperature that is too high what happens to it?

It undergoes vacuum distillation.

9

6B.11
Which pollutants are formed during the combustion of alkane fuels?

Carbon monoxide, nitrous oxides, sulfur oxides, carbon particulates and unburned hydrocarbons.

10

6B.11
What is always a product of combustion even if it is incomplete?

Water is always a product.
[This is because oxygen always combines with hydrogen in preference to carbon].

11

6B.12
What are the dangers of carbon monoxide being produced during the combustion of alkane fuels?

Carbon monoxide is a toxic gas that combines strongly with haemoglobin so blood can carry less oxygen.

12

6B.12
What are the dangers of nitrous oxides being produced during the combustion of alkane fuels?

Nitrous oxides can react with water and more oxygen to form nitric acid. This then goes on to form acid rain.
[In bright sunshine, nitrogen dioxide molecules break down into nitrogen monoxide and oxygen free radicals. The oxygen free radicals can react with oxygen to produce ozone, a serious pollutant. But this ozone can react with unburned hydrocarbons to produce irritant chemicals that can build up into photochemical smog in the absence of wind.]

13

6B.13
How do catalytic converters improve air quality?

By removing the pollutants that would otherwise be released from car exhausts.

14

6B.13
In a catalytic converter, carbon monoxide and nitrogen monoxide react in a redox reaction to produce what safer products?

Carbon dioxide and nitrogen.

15

6B.13
Although catalytic converters remove harmful pollutants, suggest a reason for why they are not ideal.

Carbon dioxide is still produced and it is a greenhouse gas.

16

6B.14
What does the term carbon neutral mean?

When the carbon dioxide released is balanced by actions which remove an equivalent amount of carbon dioxide from the atmosphere.

17

6B.14
Give an example of a source of biodiesel.

Soya beans (or maize)

18

6B.14
In terms of biofuels, why are soya beans a better source than maize?

Energy is not needed for distillation in the production of the fuel.

19

6B.14
How can ethanol be produced as a biofuel?

Fermentation of starch produces glucose that is further fermented to ethanol. In Brazil sugars from sugar cane are used in fermentation.

20

6B.14
How is biodiesel produced?

By extracting and processing oils from crops such as rapeseed.

21

6B.15
What is a radical?

A species with an unpaired electron. [Denoted by a single dot in equations.]

22

6B.15
How is a radical formed?

In the homolytic fission of a covalent bond. [UV light is needed for halogens]

23

6B.16
When alkanes react with oxygen during combustion what two products are formed?

Water and carbon dioxide.

24

6B.16
What mechanism is used in the substitution of a hydrogen for a halogen in a compound?

Free radical substitution.

25

6B.16
What is the initiation step of free radical substitution?

Cl2 --> 2Cl(fr)
where (fr) is a free radical.

26

6B.16
What are the two propagation steps in free radical substitution?

CH3CH3 + Cl(fr) --> CH3CH2(fr) + HCl
CH3CH2(fr) + Cl2 --> CH3CH2Cl + Cl(fr)
where (fr) is a free radical.

27

6B.16
Give an example of a termination step in free radical substitution?

2CH3CH2(fr) --> CH3CH2CH2CH3
or
2Cl(fr) --> Cl2
where (fr) is a free radical.

28

6B.17
Suggest a limitation of free radical substitution.

A mixture of products is formed due to further substitution of products.