3.3.2 Alkanes

Question 1

_____, _____ and _____ have no isomers

Answer 1

Methane, ethane and propane

Question 2

Name 4 physical properties of alkanes

Answer 2

1. Non-polar
2. As chain length increases = boiling points increase
3. Insoluble in water
4. Relatively unreactive

Question 3

Explain why alkanes are almost non-polar & what does this mean

Answer 3

∵ electronegativities of carbon and hydrogen = similar ∴ intermolecular forces = weak van der Waals forces

(Larger the molecule = stronger the van der Waals forces)

Question 4

Explain why as the chain length increases, the boiling points increase

Answer 4

Intermolecular forces increase

Question 5

Alkanes with branched chains have _____ melting points than straight chain alkanes (with same no. of C)

Answer 5

LOWER melting points than straight chain alkanes (with same no. of C)

Question 6

Why do alkanes with branched chains have lower melting points than straight chain alkanes (with same no. of C)?

Answer 6

∵ less surface area & weaker van der Waals forces

Question 7

Solid alkanes have a ____ feel

Answer 7

waxy

Question 8

Shorter chains of alkanes are ___ at room temp.

Answer 8

gases

Question 9

Why are alkanes insoluble in water?

Answer 9

Hydrogen bonds holding water molecules together = much stronger than van der Waals forces between alkane molecules

Question 10

What do alkanes react with?

Answer 10

halogens (& burn under suitable conditions)

Question 11

What is petroleum (crude oil)?

Answer 11

Mixture of hydrocarbons - mostly alkanes

Question 12

How did crude oil form?

Answer 12

Formed millions of years ago by breakdown of plant and animal remains at high pressure and temperature

Question 13

Why does crude oil contains other elements?

Answer 13

Contains some elements that were in original plants and animals

Question 14

Describe the stages of fractional distillation

Answer 14

1. Crude oil = vaporised at 350°C
2. Vaporised crude oil goes into fractionating column & rises up through trays
3. Largest hydrocarbons don’t vaporise = run to the bottom & form a gooey residue
4. Crude oil vapour goes up = gets cooler
5. Each fraction condenses at different temperature
   
   • Drawn off at different levels in column
6. Hydrocarbon with lowest boiling points don’t condense
   
   • Drawn off as gases at top of column

Question 15

What is cracking?

Answer 15

Breaking long-chain alkanes into smaller hydrocarbons, involves breaking C-C bonds

Question 16

Give 2 conditions of thermal cracking

Answer 16

• 900°C - high temperature
• Up to 70 atm - high pressure

Question 17

What does thermal cracking produce?

Answer 17

Produces a lot of alkenes

Question 18

How do can we avoid the decomposition (of C and H) in thermal cracking?

Answer 18

Alkanes kept in these conditions for very short time (e.g. a second)

Question 19

Name 3 conditions of catalytic cracking

Answer 19

• Uses zeolite catalyst (hydrated aluminosilicate)
• 1-2 atm - slight pressure
• 450°C - high temperature

Question 20

What is the structure of a zeolites & why?

Answer 20

Have honeycomb structure = enormous surface area

& are acidic

Question 21

What is the benefit of using a catalyst?

Answer 21

• Cuts costs ∵ reaction can be done at a low pressure & low temperature
• & catalyst speeds up reaction - saving time (& time is money)

Question 22

What does catalytic cracking normally produce?

Answer 22

Mostly produces aromatic hydrocarbons and motor fuels (+ branched alkanes and cycloalkanes)

Question 23

What do aromatic compounds contain?

Answer 23

Benzene rings

Question 24

What is the structure of benzene rings?

Answer 24

Have 6 carbon atoms with 3 double bonds

Question 25

Why are benzene rings stable?

Answer 25

Because the electrons are delocalised around the carbon ring

Question 26

Why doesn't cracking produce 2 alkanes?

Answer 26

Since there isn't enough hydrogen atoms to produce 2 alkanes = one of new chains has C=C

Question 27

How are the products obtained from cracking separated?

Answer 27

By fractional distillation

Question 28

Why are alkanes are great fuels?

Answer 28

Burning small amount = huge amount of energy

Question 29

Give the word equation for complete combustion

Answer 29

Alkanes + Oxygen → Carbon Dioxide + Water

Question 30

Give the word equations for incomplete combustion

Answer 30

Alkanes + Oxygen → Carbon Monoxide + Water (+ sometimes Carbon Dioxide) Alkanes + Oxygen → Carbon + Water

Question 31

Why is carbon monoxide gas poisonous?

Answer 31

Binds to same sites on haemoglobin molecules in red blood cells as oxygen molecules ∴ oxygen cannot be carried around the body

Question 32

Name 2 issues with soot (particulates)

Answer 32

Soot causes breathing problems & builds up in engines = can't work properly

Question 33

Describe the greenhouse effect

Answer 33

* Greenhouse gases in atmosphere = good at absorbing infrared energy (heat) * Emit some energy they absorb back towards Earth, keeping it warm = greenhouse effect

Question 34

How does rising temperature affect water vapour?

Answer 34

* Higher temps = more water vapour in air = more greenhouse effect * Greater cloud formation and clouds reflect solar radiation

Question 35

How can carbon monoxide be removed from exhaust gases?

Answer 35

By catalytic converters on cars

Question 36

Where do unburnt hydrocarbons come from?

Answer 36

Engines (they don't burn all fuel molecules = come out as unburnt hydrocarbons)

Question 37

How are oxides of nitrogen (NOx) produced?

Answer 37

Produced when high pressure and temperature in car engine causes nitrogen and oxygen atoms in air to react together

Question 38

Describe how smog (O₃₎ is created?

Answer 38

Hydrocarbons and nitrogen oxides react in presence of sunlight to form ground-level ozone (O₃) = major component in smog

Question 39

Why is ground-level ozones bad? (3x)

Answer 39

Irritates people's eyes and increases respiratory problems + lung damage

Question 40

How can we remove unburnt hydrocarbons and NO_x from exhausts?

Answer 40

By catalytic converters

Question 41

What is a catalytic converter?

Answer 41

Honeycomb made of ceramic material coated with platinum and rhodium metals

Question 42

Describe how acid rain occurs

Answer 42

1. Some fossil fuels contain sulfur: when burnt, sulfur reacts to form sulfur dioxide gas (SO₂) 2. Sulfur dioxide in the atmosphere reacts with water vapour and oxygen to form sulfuric acid = acid rain

Question 43

Name 3 things acid rains destroys

Answer 43

* Trees and vegetation * Corroding buildings/statues * Kills fish in lakes

Question 44

What are flue gases?

Answer 44

Gases given out by power stations

Question 45

Describe flue gas desulfurisation (how flue gases can be removed from power stations before it gets into atmosphere)

Answer 45

* Powdered calcium carbonate (limestone) or calcium oxide is mixed with water to make an alkaline slurry * When flue gases mix with alkaline slurry = acidic sulfur dioxide gas reacts with calcium compounds to form harmless salt (calcium sulfate) * Oxides 1st to produce calcium sulphite & then oxidises again to form calcium sulfate (aka gypsum)

Question 46

What is calcium sulfate (AKA gypsum) used for?

Answer 46

Used to make builders' plaster and plasterboard

Question 47

Give the symbol equation for forming NO

Answer 47

N_{2 (g)} + O_{2 (g)} → 2NO_(g)

Question 48

Give the symbol equation for forming sulfuric acid

Answer 48

SO_2(g) + 1/2 O_2(g) + H₂O_(l) → H₂SO_4(l)

Question 49

Give 2 word equations of a catalytic converter converting polluting gases into less harmful products

Answer 49

Carbon Monoxide + Nitrogen Oxide → Nitrogen + Carbon Dioxide Hydrocarbons + Nitrogen Oxide → Nitrogen + Carbon Dioxide + Water

Question 50

What are free radicals?

Answer 50

Particles with unpaired electrons

Question 51

When do free radicals form?

Answer 51

When a covalent bond spilts equally, giving one electron to each atom

Question 52

Why are free radicals very reactive?

Answer 52

∵ electrons are unpaired

Question 53

Example: Reacting Chlorine with Methane What happens after the termination reactions?

Answer 53

Depends on whether there's more chlorine or methane around: 1. Chlorine's in excess: Cl• free radicals = start attacking chloromethane, producing dichloromethane CH₂Cl, trichloromethane CHCl₃ & tetrachloromethane CCl₄ 2. Methane's in excess: product mostly be chloromethane

Question 54

What are chlorofluorocarbons (CFCs)?

Answer 54

Halogenoalkane molecules where all of hydrogen atoms have been replaced by chlorine and fluorine atoms

Question 55

What does Ozone (O₃) in upper atmosphere do?

Answer 55

Acts as a chemical sunscreen → absorbs UV radiation from Sun

Question 56

How is ozone formed naturally?

Answer 56

1. When oxygen molecule (O₂) is broken down into 2 free radicals by UV radiation 2. Free radicals attack other oxygen molecules forming ozone

Question 57

Explain how CFCs are creating holes in ozone layer (include symbol equations)

Answer 57

* Chlorine free radicals (Cl•) = formed in upper atmosphere when C-Cl bonds in CFCs are broken down by UV e. g. CCl₃F → CCl₂F• + Cl• * These free radicals = catalysts → react with ozone to form an intermediate (ClO•) & oxygen molecule Cl• + O₃ → O₂ + ClO• ClO• + O₃ → 2O₂ + Cl• * Overall reaction: 2O₃ → 3O₂ & Cl• = catalyst

Question 58

Name 3 properties of CFCs

Answer 58

Unreactive, non-flammable & non-toxic

Question 59

CFCs are \_\_\_\_\_

Answer 59

BANNED

Question 60

What can we now use instead of CFCs?

Answer 60

e.g. HFCs (hydrofluorocarbons) & hydrocarbons

Question 61

For the conversion of methane to tetrachloromethane: Give the overall equation

Answer 61

CH₄ + 4 Cl₂ → CCl₄ + 4 HCl

Question 62

Free Radical Substitution Condition

Answer 62

UV light neeed to break halogen bond

Question 63

For the conversion of ethane to 1,1-dibromoethane Give the overall equation

Answer 63

C₂H₆ + 2Br₂ → C₂H₄Br₂ + 2HBr

Question 64

For the conversion of ethane to 1,1-dibromoethane Give the pair of propagation steps to form bromoethane from ethane

Answer 64

C₂H₆ + Br• → C₂H₅• + HBr C₂H₅• + Br₂ → C₂H₅Br + Br•

Question 65

For the conversion of ethane to 1,1-dibromoethane: Give the pair of propagation steps to form 1,1-dibromoethane from bromoethane

Answer 65

C₂H₅Br + Br• → C₂H₄Br• + HBr C₂H₄Br• + Br₂ → C₂H₄Br₂ + Br•

Question 66

For the conversion of ethane to 1,1-dibromoethane: Give the termination step that forms butane

Answer 66

2C₂H₅• → C₄H₁₀

Question 67

Write the symbol equation for when carbon monoxide reacts with nitrogen oxide

Answer 67

2CO_(g) + 2NO_(g) → N_2(g) + 2CO_2(g)

Question 68

Represent how calcium oxide is used to remove sulfur dioxide from flue gases by using an equation

Answer 68

Question 69

Represent how calcium carbonate is used to remove sulfur dioxide from flue gases by using an equation

Answer 69

Question 70

Deduce an equation to show how nitrogen dioxide reacts with oxygen and water to form nitric acid

Answer 70

4NO₂ + 2H₂O + O₂ → 4HNO₃

Question 71

Explain the main economic reason why alkanes are cracked (1)

Answer 71

To produce substances which are more in demand / products with high value

Question 72

Suggest why calcium oxide reacts with sulfur dioxide (1)

Answer 72

It is basic/SO₂ is acidic | (neutralises the gas)

Question 73

Write an equation to show how gypsum (calcium sulfate) is made

Answer 73

CaSO_3(s) + 1/2O_2(g) → CaSO₄ Calcium sulfite is oxidised to calcium sulfate (gypsum)