Organic Chemistry 4.1-4.28

Question 1

4.1 What is a hydrocarbon?

Answer 1

A compound of hydrogen and carbon only

Question 2

4.2 How do you represent molecules using molecular formulae

Answer 2

Actual number of each type of atom present e.g. C2H4 = ethene

Question 3

4.2 How do you represent molecules using empirical formulae?

Answer 3

Simplest whole number ratio e.g. C2H6 = CH3

Question 4

4.2 How do you represent molecules using general formulae

Answer 4

General formula for the homologous series e.g. CnH2n

Question 5

4.7 What is crude oil

Answer 5

A mixture of hydrocarbons

Question 6

4.2 How do you represent molecules using a) structural formulae b) displayed formulae

Answer 6

a) How the atoms are joined together

b) Show all bonds in the molecule as lines (covalent bond)

Question 7

4.3 What is a homologous series?

Answer 7

Group of compounds with
- same general formula, trends in their physical properties, similar chemical properties, trends in their physical properties, each successive member differs by CH2, same functional group

Question 8

4.3 What is a functional group

Answer 8

An atom or group of atoms that determines the chemical properties of the compound

Question 9

4.20 What does saturated mean

Answer 9

Contains single bonds only

Question 10

4.19 What is the general formula of alkanes

Answer 10

CnH2n+2

Question 11

4. 4 What is the number of carbon atom(s) in
   a) propane
   b) ethane
   c) methane
   d) butane

Answer 11

3, 2, 1, 4

Question 12

4.6 Reaction of alkanes with bromine with ultraviolet light

Answer 12

A substitution reaction occurs as a hydrogen atom on the alkane has been replaced by a bromine atom

methane + bromine (red/brown) -> bromomethane + hydrogen bromide (colourless)
CH4 + Br2 -> CH3Br + HBr

More examples:
C3H8 + Br2 -> C3H7Br (Bromopropane) + HBr

Question 13

4.6 Combustion of alkanes

Answer 13

Complete combustion: alkane + oxygen -> carbon dioxide + water
e.g. CH4 + 2O2 -> CO2 +2H20

Question 14

4.6 and 4.12 Combustion of alkanes continued

Answer 14

Incomplete combustion occurs with an insufficient supply of oxygen
alkane + oxygen -> carbon monoxide + water
alkane + oxygen -> carbon + water

Question 15

4.3 What is structural isomerism

Answer 15

Structural isomers are molecules with the same molecular formula, but different structural formulae

Question 16

4.8 How does fractional distillation separate crude oil into fractions

Answer 16

1. Crude oil is heated and enters the fractioning column
2. The column is hotter at the bottom and gets cooler as you go up
3. Large molecules with high boiling points condense near the bottom
4. Small molecules with low boiling points remain as gases and leave at the tap

Question 17

4.9 What are the uses and names of the main fractions obtained from crude oil (6)

Answer 17

a) Refinery gases - liquefied petroleum gas for domestic cooking
b) Gasoline - fuel in cars
c) Kerosene - fuel for jet aircrafts, domestic heating oil and ‘paraffin’ for small heaters and lamps
d) Diesel- fuel for buses and lorries
e) Fuel oil - fuel for ships
f) Bitumen - road surface tar

Question 18

4.13 Why is carbon monoxide poisonous

Answer 18

Carbon monoxide is toxic because it reduces the capacity of blood to transport oxygen around the body

Question 19

4.11 What is a fuel

Answer 19

A substance that when burned releases heat energy

Question 20

4.10 What are the trends in the main fractions of a) colour b) boiling point and c) viscosity

Answer 20

Molecules get larger as you go down
Colour lighter at top and darker at the bottom
Boiling point lower at the top and higher at the bottom
Viscosity lower at the top (runny) and higher at the bottom (thicker)

Question 21

4.14 Why in car engines can nitrogen and oxygen react

Answer 21

The temperature reached is high enough to allow nitrogen and oxygen from air to react, forming oxides of nitrogen

N2 (g) + O2 (g) ->2NO (g) nitrogen oxide
N2 (g) + 2O2 (g) -> 2NO2 (g) nitrogen dioxide

Question 22

4.15 How does sulphur dioxide form via combustion

Answer 22

Fossil fuels contain sulfur as an impurity and when they burn, the sulfur is also oxidised to form:

sulfur dioxide = S(s) + O2 (g) -> SO2 (g)

and in water forms an acidic solution sulfurous acid =
SO2 (g) + H20 (l) -> H2SO3 (aq)

+ oxygen = 2H2SO3 + O2 -> 2H2SO4 (sulfuric acid)

Question 23

4.16 How does sulfur dioxide and oxides of nitrogen contribute to acid rain

Answer 23

They dissolve in water to form acidic solutions

Question 24

4.18 Why is cracking necessary

Answer 24

The supply of short chain hydrocarbons from crude oil is not enough to meet demand

Supply of long chain hydrocarbons is larger than demand

Question 25

4.17 How are long chain alkanes converted by cracking

Answer 25

Long chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking By heating fuel oil until it is a gas, then passing it over a catalyst, made of silica or alumina at a temperature in the range 600-700C Thermal decomposition

Question 26

4.21 How can you work out the name of unbranched-chain isomers

Answer 26

Which carbon the branch is on e.g. 2 = 2-.... Number of carbons in the branch e.g. 1 carbon = methyl Longest carbon chain e.g. 4 = propane

Question 27

4.23 What is the functional group of alkenes

Answer 27

C=C

Question 28

4.24 What is the general formula for alkenes

Answer 28

CnH2n

Question 29

4.25 Why are alkenes unsaturated hydrocarbons

Answer 29

Contains at least 1 double bond of carbon atoms and only carbon and hydrogen atoms

Question 30

4.28 How can bromine water be used to distinguish between an alkane and an alkene

Answer 30

Method: Place 5 drops of hydrocarbon into test tube Add 5 drops of bromine water Results: Hexane - orange Cyclohexene- colourless ....... Alkenes react with bromine water causing it to decolourise, alkanes do not

Question 31

4.27 What does the reaction of alkenes with bromine produce

Answer 31

Ethene + bromine -> dibromoethane ( Loses double bond between carbons and 2 bromine replace 2 hydrogen) Addition reaction