3.3.2 Alkanes

Question 1

What are alkanes?

Answer 1

1. Alkanes have the general formula C_nH_2n+2
2. They only contain carbon and hydrogen atoms, so they are hydrocarbons

Question 2

Why are alkanes “saturated”

Answer 2

Every carbon atom in an alkane has four single bonds with other atoms. It’s impossible for carbon to make more than four bonds, so alkanes are saturated

Question 3

What are cycloalkanes?

Answer 3

• A ring of carbon atoms with two hydrogens attached to each carbon
• Cycloalkanes have a diiferent general formula to normal alkanes, C_nH_2n
• Cycloalkanes are still saturated

Question 4

What is crude oil?

Answer 4

Crude oil (petroleum) is a mixture of hydrocarbons mostly made of alkanes, ranging from small chain alkanes to long chain alkanes (50 carbons long).

Crude oil has limited uses if not seperated into fractions.

Question 5

Describe the process of fractional distillation

Answer 5

1. Crude oil is vaporised at ~350^oC
2. Vaporised curde oil goes into a fractionating column, and rises up through trays
3. As crude oil vapour rises through the column, fractions start to condense as it gets cooler
4. Fractions with the lower boiling points (short chain hydrocarbons) don’t condense or condense later so are removed further up the column , Fractions with higher boiling points don’t vaporise or condense earlier so are lower down the column

The column has multiple trays to collect a range of fractions.

Question 6

Explain why cracking is used to make hydrocarbons shorter

Answer 6

There is higher demand for shorter chain hydrocarbons (e.g. petrol), therefore it is more valuable. There is a lower demand for long chain hydrocarbons (e.g. bitumin), therefore its less valuble.

Cracking breaks C-C bonds making hydrocarbons shorter, and more useful.

Question 7

Describe thermal cracking

Answer 7

• Takes place at a high temperature (1000^oC) and high preasure (up to 70 atm)
• It produces a lot of alkenes
• These alkenes are used to make valuable products such as, polymers (plastics) e.g. poly(ethene)

Question 8

Describe catalytic cracking

Answer 8

• Uses a zeolite catalyst (hydrated aluminosilicate), at a slight preasure and high temperature (~450^oC)
• It mostly produces aromatic hydrocarbons and motor fuels

Question 9

What zeolite catalyst is used in catalytic cracking?

Answer 9

Hydrated aluminosilicate

Question 10

How does catalytic cracking reduce costs?

Answer 10

The reaction can be performed at a lower temperature and a lower preasure. The catalyst also speeds up the reaction.

Question 11

Give the equation for the complete combustion of propane

Answer 11

C₃H_8(g) + 5O_2(g) -> 3CO_2(g) + 4H₂O_(g)

Question 12

Give the equation for incomplete combustion of methane with CO as the product

Answer 12

CH_4(g) + 1 1/2 O_2(g) -> CO_(g) + 2H₂O_(g)

Question 13

Why is carbon monoxide produced by incomplete combustion bad?

Answer 13

Carbon monoxide is poisonous, CO blocks the site on haemoglobin where O₂ binds so oxygen cannot be carried around the body

Question 14

What can be used to remove CO from car exhaust fumes?

Answer 14

Catalytic converter

Question 15

Give the equation for the incomplete combustion of methane with carbon (soot) as the product

Answer 15

CH_4(g) + O_2(g) -> C_(s) + 2H₂O_(g)

Question 16

Why is soot as a product of incomplete combustion bad?

Answer 16

• Soot is thought to cause breathing problems
• Soot can build up in engines causing them to not work properly

Question 17

How does burning fossil fuel contribute to global warming?

Answer 17

1. Burning fossil fuels produes CO₂, which is a greenhouse gas
2. Greenhouse gasses in the atmosphere absorb infrared energy, and emit some of that energy back towards earth keeping it warm (this is called the greenhouse effect)

Most scientist agree that increasing the amount of CO₂ is making earth warmer, this is known as global warming.

Question 18

How do unburnt hydrocarbons and oxides of nitrgogen cause smog?

Answer 18

1. Engines don’t burn all the fuel molecules, some are left unburnt
2. Oxides of nitrogen are produced when the high preasure and temperature in a car engine cause nitrogen and oxygen atoms in the air to react
3. Hydrogens and nitrogen oxides react in the presence of sunlight to form ground level ozone (O₃), which is a major component of smog.

Question 19

Why is smog a problem?

Answer 19

The ground level ozone that is part of smog irritates peoples eyes, aggrevates respiritory problems and can cause lung damage

Question 20

How can smog from unburnt hydrocarbons and oxides of nitrogen be reduced?

Answer 20

Catalytic converters on cars removes unburnt hydrocarbons and oxides of nitrogen from the exhaust so they aren’t released into the atmosphere where they can form O₃

Question 21

What causes acid rain?

Answer 21

• Fossil fuels contain sulfer
• When burnt sulfer reacts to form sulfer dioxide gas (SO₂)
• Sulfer dioxide dissolves in the moisture in the atmosphere and is converted into sulfuric acid, which then falls as acid rain

Question 22

Why is acid rain a problem?

Answer 22

Acid rain destroys trees, vegetations, as well as corroding buildings and statues, acid rain also kills fish in lakes

Question 23

What are free radicals?

Answer 23

• A free radical is a particle with an unpaired electron
• Free radicals form when a covalent bond splits equally, giving one electron to each atom
• The unpaired electron makes them very reactive

Free radicals can be shown by putting a dot next to it

Question 24

How do halogens react with alkanes to form halogenoalkanes?

Answer 24

1. Halogens react with alkanes in photochemical reactions (reactions that are started by ultraviolet light)
2. A hydrogen atom is substituted by chlorine or bromine

This is called a free-radical substitution reaction.

Question 25

Describe stage 1 of free radical substitution

Using reaction between methane and chlorine as an example

Answer 25

Initiation reactions - free radicals are produced
1 - Sunlight provides enough energy to break the Cl-Cl bond - this is called photo dissociation

Cl₂ -UV-> 2Cl^.

2 - The bond splits equally and each atom get to keep one electron. The atom becomes a highly reactive free radical, Cl^., because of its unpaired electron

Question 26

Describe stage 2 of free radical substitution

Using reaction between methane and chlorine as an example

Answer 26

Propagation reactions - free radicals are used up and created in a chain reaction

1. Cl^. attacks a methane molecule: Cl^. + CH₄ -> CH₃^. + HCl
2. The new methyl free radical, CH₃^. can attack another Cl₂ molecule: CH₃^. + Cl₂ -> CH₃Cl + Cl^.
3. The new Cl^. can attack another CH₄ molecule, and so on, until all the Cl₂ or CH₄ molecules are used up

Question 27

Describe stage 3 of free radical substitution

Using reaction between methane and chlorine as an example

Answer 27

Termination reactions - free radicals are used up
* If two free radicals join together, they makes a stable molecule. The two unpaired electrons form a covalent bond.
* There are many termination reactions for example:

Cl^. + CH₃^. -> CH₃Cl

or

CH₃^. + CH₃^. -> C₂H₆

Question 28

What happens to the product after free radical substitution?

Answer 28

What happens next depends whether there’s more halogen or alkane left:
* If the halogen is in excess, X^. free radicals will start attacking the halogenoalkane, producing for example dichloromethane CH₃Cl₂, trichloromethane CHCl₃, or tetrachloromethane CCl₄
* If the alkane is in excess then the product will mostly a halogenoalkane with a single halogen e.g. chloromethane CH₃Cl

X = halogen