Alkanes and Alkenes (3.3.2 & 3.3.4- organic)

Question 1

Define saturated hydrocarbons

Answer 1

A molecule made only of carbon and hydrogen with only single bonds

Question 2

Define unsaturated hydrocarbons

Answer 2

A molecule made only of carbon and hydrogen that contains a double bond

Question 3

List the features of a homologous series

Answer 3

• Similar chemical properties
  Because they have same functional group
  -Same general formula
  -Trend in physical properties

Question 4

How do you separate a mixture of alkanes? (e.g. crude oil)

Answer 4

Fractional distillation

Question 5

The larger the hydrcarbon…

Answer 5

The higher the boiling point and the less volatile

Question 6

Define volatile

Answer 6

Easily turned into a vapour, low bpt

Question 7

Pollutant: Carbon Dioxide

Answer 7

Source: complete combustion of fuel
Problem: Greenhouse gas—-> global warming
Solution: CCS- carbon capture and storage, used in extinguishers

Question 8

Pollutant: Carbon monoxide

Answer 8

Source: incomplete combustion of fuel
Problem: Toxic, replaces oxygen and binds to haemoglobin, less respiration
Solution: catalytic converters

Question 9

What is the equation used in catalytic converters?

Answer 9

CO + NO —–> CO₂ + 1/2 N₂

Question 10

What catalyst is used and why is it spread in a ‘honeycomb support’?

Answer 10

Paladium, to maximise SA

Question 11

Pollutant : Carbon

Answer 11

Source: incomplete combustion of fuel
Problem: medical particulates lead to lung cancer/ environmental global dimming
Solution: Filter, solids get trapped

Question 12

Pollutant : SO2

Answer 12

Source: Sulfur impurities in fuel burning
Problem: Acid rain, lowers pH of ocean, corrodes limestone
Solution: neutralise with lime/ slaked lime (CaO)

Question 13

Pollutant : NOx

Answer 13

Source: N from air, very high temp causes spark
Problem: Acid rain, lowers pH of ocean etc.
Solution: Catalytic converter

Question 14

What mechanism is involved with the reaction of alkanes?

Answer 14

Free radical substitution

Question 15

Stage 1 of free radical sub.

Answer 15

Initiation

Cl₂ ———–> [UV light] 2Cl•

Question 16

Define free radical

Answer 16

A highly reactive species with an unpaired electron

Question 17

Stage 2 of free radical sub.

Answer 17

Propagation

a) Cl• + CH₄ ———–>•CH₃ +HCl
b) •CH₃+ Cl₂ ———-> CH₃Cl + Cl•

Question 18

Stage 3 of free radical sub.

Answer 18

Cl • + •CH₃ ———–> CH₃Cl + Cl •

Question 19

Thermal cracking

Answer 19

-free radical sub.
Conditions: High temp, 400-900 deg.C and high pressure (70atm)
Equation: C₈H₁₈ ——-> C₆H₁₄ + C₂H₄
Products: High % of alkenes for polymers

Question 20

Catalytic cracking

Answer 20

-carbocation
Conditions : High temp- 450 deg.C, Low press- ~>1 atm, Catalyst zeolite
Products: Motor fuels, branched alkanes

Question 21

Why do we do cracking?

Answer 21

• Supply and demand
• high demand for small molecules
• high supply of big molecules
• crack to make smaller from bigger

Question 22

Smaller molecules are…

Answer 22

• less volatile

- easier to ignite

Question 23

Alkenes test

Answer 23

Orange bromine water goes colourless

Question 24

Are alkenes reactive and why?

Answer 24

They are due to double bond that has a region of high electron density

Question 25

What type of reactions do alkenes do?

Answer 25

Electrophilic addition

Question 26

Where do the curly arrows go in alkene mechanisms?

Answer 26

From double bond to d+ atom, from single bond to Cl or Br

From lone pair on halogen to positive C

Question 27

Where is thte major product formed with unsymmetrical alkenes?

Answer 27

The more stable carbocation

Question 28

Order of stability and why?

Answer 28

Primary < Secondary < Tertiary

This is due to electron releasing effect of alkyl groups

Question 29

Alkenes + sulphuric acid, conditions?

Answer 29

Cold concentrated sulphuric acid
central S, H–O recieves pair of e- from double bond
(Sulphuric acid reformed at end so it acts as a catalyst)

Question 30

How do you draw H₂SO₄?

Answer 30

Central S, 2OH, and 2 =OS