Alkanes

Question 1

Fractional distillation

Answer 1

1. Crude oil is heated to above 350°C in a furnace to vaporise the hydrocarbon mixture.
2. These vapours enter a fractionating column that has higher temperatures at the bottom and lower temperatures near the top
3. As the hot vapours rise through the column, they cool down. When the vapour temperature drops below the boiling point of a hydrocarbon in the mixture, it condenses from gas to liquid on the tray surface.
4. The condensed hydrocarbon liquids that accumulate on each tray are drawn off at specific intervals as fractions (mixtures of hydrocarbons with similar boiling points).

Question 2

What kind of process is fractional distillation

Answer 2

A physical process involving the splitting of VDWs forces between molecules

Question 3

What does vacuum distillation allow

Answer 3

Heavier fractions to be further separated without high temps which could break them down

Question 4

Vacuum distillation

Answer 4

• heavy residues from the fractioning column are distilled again under a vacuum
• lowering the pressure over a liquid will lower its boiling point

Question 5

Fractional distillation in a lab

Answer 5

1. Heat the flask with a Bunsen burner
2. This causes vapours of all the components in the mixture to be produced
3. Vapours pass up the fractioning column
4. The vapour of the substance with the lower bp reaches the top of the fractioning column first
5. The thermometer should be at or below the boiling point of the most volatile substance
6. The vapours with higher boiling points condense back into the flask
7. Only the most volatile vapour passes into the condenser
8. The condenser cools the vapours and condenses to a liquid and is collected

Question 6

What is cracking

Answer 6

Conversion of large hydrocarbons to smaller hydrocarbon molecules by breaking C-C bonds

Large alkanes to smaller alkanes and alkenes and hydrogen

Question 7

What kind of process is cracking

Answer 7

A chemical process involving the splitting of strong covalent bonds so requires high temps

Question 8

Economic reasons for cracking

Answer 8

• the petroleum fractions with shorter C chains are in more demand than larger fractions

• the make use of excess larger hydrocarbons and to supply demand for shorter ones, larger hydrocarbons are cracked

• the products of cracking are more valuable than the starting materials

Question 9

What are the 2 main types of cracking

Answer 9

Thermal and catalytic

Question 10

Thermal cracking

Answer 10

Conditions:
* high pressure (70atm)
* high temperature (~1000*C)

• This process generates a high yield of alkenes, which are vital for creating numerous valuable products.
• For example, ethene, a common product of thermal cracking, can be polymerised to manufacture polyethene, a widely-used plastic

Question 11

Catalytic cracking

Answer 11

Conditions:
* moderate pressure
* high temperature (450*C)
* zeolite catalyst

• This process primarily produces aromatic hydrocarbons and fuels for vehicles. Aromatic hydrocarbons contain highly stable benzene rings with delocalised electrons.
• Produces branched and cyclic alkanes and aromatic hydrocarbons
• Branched and cyclic hydrocarbons burn more cleanly

Question 12

Why is catalytic cracking cheaper than thermal

Answer 12

Be chase it saves energy as lower temps and pressures are used

Question 13

Fuel

Answer 13

Release energy when burnt

Question 14

Complete combustion

Answer 14

In excess oxygen alkanes will burn with complete combustion
Products: CO2 and H2O

Question 15

Incomplete combustion

Answer 15

If there’s a limited amount of oxygen then incomplete combustion occurs, producing CO and/or C

Question 16

Why are alkanes used as fuels

Answer 16

They readily burn in the presence of oxygen making their combustion highly exothermic

Question 17

What is a free radical

Answer 17

A reactive species which possesses an unpaired electron

Question 18

How is SO2 produced

Answer 18

Sulfur containing impurities are found in petroleum fractions which produce SO2 when burnt

S + O2 -> SO2

Question 19

How is acid rain produced

Answer 19

• SO2 will dissolve in atomospheric water and can produce acid eain

Question 20

How can SO2 be removed from the waste gases from furnaces

Answer 20

By flue gas desulfurisation
-> the gases pass through a scrubber containing basic calcium oxide which reaches with the acidic SO2 in a neutralisation reaction

SO2 + CaO -> CaSO3

This calcium sulfite produced can be used to make calcium sulfate for plasterboard

Question 21

How do nitrogen oxides form

Answer 21

From the reaction between nitrogen and oxygen inside the car engine
-> the high temp and spark in the energy provides sufficient energy to break strong nitrogen bond

N2 + O2 -> 2NO
N2 + 2O2 -> 2NO2

Question 22

Environmental consequence of nitrogen oxides

Answer 22

NO is toxic and can form acidic gases NO2

NO2 is toxic and acidic and forms acid rain

Question 23

Environmental consequence of carbon monoxide

Answer 23

Toxic

Question 24

Environmental consequence of carbon dioxide

Answer 24

Contributes towards global warming

Question 25

Environmental consequence of unburnt hydrocarbons

Answer 25

Contributes towards formation of smog

Question 26

Environmental consequence of soot (carbon)

Answer 26

Global dimming and respiratory problems

Question 27

Catalytic converters

Answer 27

These remove NO2, NOx and unburied hydrocarbons from the exhaust gases turning them into harmless CO2, N2 and H2O

Question 28

What are converters coated with and why

Answer 28

A ceramic honeycomb coated with a thin layer of catalyst metals platinum, palladium and rhodium to give a large surface area

Question 29

What are the greenhouse gases

Answer 29

CO2, CH4 and water vapour

Question 30

Mechanism of the greenhouse effect

Answer 30

• UV wavelength radiation passes through the atmosphere to earths surface and heats up earths surface • the earth radiates out long wavelength radiation • the C=O bonds in CO2 absorb infrared radiation so the IR radiation does not escape the atmosphere • this energy is transferred to other molecules in the atmosphere by collision so the atmosphere is warmed

Question 31

Why have CO2 levels risen greatly in recent years

Answer 31

Due to increasing burning of fossil fuels

Question 32

How carbon chain length affects boiling point of alkanes

Answer 32

1. Longer carbon chain means more electrons present 2. Results in stronger VDWs between molecules 3. So more energy is needed to overcome these forces

Question 33

How branching affects the boiling point of alkanes

Answer 33

1. Straight chain alkanes can pack together more closely creating stronger intermolecular forces 2. Branched alkanes can’t pack together as much and so have weaker intermolecular forces 3. More energy is needed to overcome the forces in straight chain alkanes

Question 34

Cracking Key Points

Answer 34

- Cracking transforms less useful long-chain alkanes into more valuable smaller hydrocarbons. - The point at which the chain breaks is random, leading to a variety of product combinations. - Cracking is also essential for producing alkenes, which are used in manufacturing plastics and polymers.

Question 35

Catalysts enhance the efficiency of the cracking process in two ways:

Answer 35

- They allow for lower temperatures and pressures, reducing energy consumption and production costs. - They increase the rate of reaction, enabling faster production of desired products.

Question 36

However, alkanes do undergo certain reactions, including:

Answer 36

- Combustion - the reaction of alkanes with oxygen to release energy. - Substitution - the replacing of H atoms with halogens in the presence of light.

Question 37

Alkanes are relatively unreactive due to their..

Answer 37

- Non-polar nature and their strong covalent bonds. - The electronegativity difference between carbon and hydrogen in alkanes is minimal, leading to no significant partial charges - This makes it difficult for alkanes to attract nucleophiles or electrophiles.

Question 38

How does carbon-halogen enthalpy change down group 7

Answer 38

It decreases down

Question 39

The carbon-halogen bond enthalpy decreases down group 7 due to several factors:

Answer 39

-Increasing atomic radius of the halogens. -Increasing carbon-halogen bond length. -Decreasing electrostatic attraction between bonding electrons and nuclei. -As a consequence, the energy needed to break these longer, weaker bonds decreases, leading to faster reaction rates.

Question 40

Which halogenoalkanes react the fastest

Answer 40

Iodoalkanes

Question 42

Why is K the major product

Answer 42

1. It’s formed from the more stable carbocation intermediate 2. The major product is from the tertiary carbocation rather than the secondary carbocation 3. The stability is greater as there are more electron releasing alkyl groups

Question 43

Why is ozone important in the atmosphere

Answer 43

Absorbs UV radiation that is harmful/causes cancer

Question 44

What is a fraction

Answer 44

Mixture of compounds/substances with similar boiling points

Question 45

Give a reason why CO2 absorbs infrared radiation

Answer 45

The bonds vibrate/ bonds are polar

Question 46

Explain why stereoisomerism wouldn’t occur on but-1ene

Answer 46

Two groups the same on one of the C=C carbons

Question 47

Give the 2 names of the stereoisomers of butene

Answer 47

Z-but-2-ene E-but-2-ene

Question 48

Why are but-2-ene stereoisomers

Answer 48

Because lack if rotation around C=C bond