3.3.2- Alkanes (PAPER 2)

Question 1

What are alkanes?

Answer 1

Saturated (only single bonds between the carbon atoms) hydrocarbons (compounds containing only hydrogen and carbon)

Alkane homologous series increases by CH2 each time.

Question 2

What is the general formula for alkanes?

Answer 2

CnH2n+2

Question 3

What are the 3 physical properties of alkanes?

Answer 3

1) Non-polar: electronegativities of carbon and hydrogen are very similar so the electrons are shared equally, C-C bonds require large amount of energy to overcome so also unreactive.
2) Insoluble in water as water is a polar solvent so will dissolve polar substances, they dissolve in other non-polar liquids. Attractions between alkane and water are VDW’s which do not provide enough energy to break H bonding in water.
3) Relatively low boiling points: 1st 4 alkanes are gas at room temp and pressure, larger alkanes are volatile liquids.

Question 4

How does boiling point change with with increasing size alkanes?

Answer 4

The larger the molecule- with increasing Mr, the stronger the vdw forces between the molecules and the higher the boiling point.

Question 5

What are the most volatile components of crude oil?

Answer 5

Shorter hydrocarbon molecules which have a lower Mr so lower boiling point and higher volatility.

Question 6

What are alkanes used for and why?

Answer 6

They are unreactive as their carbon-carbon bonds are strong BUT

Used mainly as fuels: the combustion of alkanes releases large amounts of energy which can be used to drive the cylinders in an engine.

Lubricants: reduce friction

Question 7

What is crude oil?

Answer 7

A fossil fuel made from the breakdown of plant and animal remains at high temp+ pressure under surface of earth.

Non-renewable: forms at such a slow rate

Mixture of different alkanes of different lengths and branching.

Question 8

Describe fractional distillation

Answer 8

NO COVALENT BONDS BROKEN!!!!!

Crude oil heated above its boiling point so it is vaporised, passed into fractionating column passing through trays until it is cool enough for them to condense back into a liquid, collect into the tray and leave as liquid alkane.
There is a negative temperature gradient.

Shorter alkanes rise further up column as they have lower boiling points, some do not condense at all. Largest alkanes leave as liquids that did not vaporise as it was not warm enough for them.

Question 9

What occurs with the residue from fractional distillation?

Answer 9

Further distillation.

The residue still contains useful materials, the residue is further distilled under reduced pressure (vacuum distillation) which reduces temperature and prevents decomposition, solutions boil at a lower temperature.

Question 10

Why is vacuum distillation helpful?

Answer 10

Less heat energy required so less energy to break C-C bonds and C-H strong covalent bonds preventing decomposition.

Question 11

Give the name, length of carbon chain and use of the fractions in crude oil

Answer 11

LPG= 1-4 C chain, calor gas
Petrol= 4-12 C chain, petrol for cars
Naptha= 7-14 C chain, petrochemicals
Kerosene= 11-15 C chain, jet fuel
Gas oil= 15-19 C chain, central heating fuel
Mineral oil= 20-30 C chain, lubricating oil
Fuel oil= 30-40 C chain, fuel for ships
Wax/grease= 40-50 C chain, candles
Bitumen= 70+ Carbon chain, roofing

Question 12

Properties of shorter alkanes

Answer 12

Lower boiling points: turn into gases more easily

More volatile: can turn from a liquid to a gas at normal temperatures

More flammable

Less viscous (thick) and more runny as the intermolecular forces are weaker

Less coloured than longer alkanes

Question 13

What is cracking + 2 types?

Answer 13

Cracking is a chemical process where C-C bonds are broken in large alkane molecules to make smaller more useful alkanes.

2 types: thermal and catalytic

Question 14

Describe thermal cracking

Answer 14

High temperatures (around 1000K) and high pressure (around 7000KPa).

C-C bonds break and one electron from each pair in covalent bond goes to each carbon atom- free radicals are formed through homolytic fission.

There are not enough H atoms to produce 2 alkanes: one of the new chains must have at least one C=C and is an alkene. The chain could break at any point- can limit the time the alkanes spend in these conditions to prevent decomposition.

Question 15

Write thermal cracking equation of C15H32 to form octane and ethene in 1:2 ratio with one other product.

Answer 15

C15H32—-> C8H18 + 2C2H4 + C3H6

Question 16

Describe catalytic cracking

Answer 16

Still high temperatures of around 723K, pressure only slightly above atmospheric.

Zeolite catalyst (silicon dioxide + aluminium oxide) with a honeycomb structure which has a large SA.

Produces branched alkanes, cycloalkanes and aromatic compounds- mainly produces motor fuels.

Ions are formed through heterolytic fission- the covalent bond breaks and the shared pair of electrons are taken by one of the atoms= uneven distribution.

Question 17

What are the economic reasons for cracking?

Answer 17

-Shorter chain products found in crude oil are more useful as fuels
-Shorter chain products are more economically valuable than long chain products.
-Alkenes are converted to a wide range of products like polymers- eg ethene used for poly(ethene)

Question 18

What is the difference between complete and incomplete combustion?

Answer 18

Complete combustion occurs when the alkane is burnt in excess oxygen to form CO2 (g) and H20 (l) whereas incomplete combustion occurs when the alkane is burnt in a limited supply of oxygen producing CO (g) or C (s) but still producing H20 (l)

Question 19

What are the pollutants that can be made when burning alkanes? (7)

Answer 19

carbon monoxide- poisonous gas made through incomplete combustion,

carbon particulates (soot) can exacerbate asthma,

carbon dioxide, water are greenhouse gases,

sulfur dioxide if alkanes have sulfur dissolved in them S+O2–> SO2= can react with water to cause acid rain,

nitrogen oxides are most commonly produced in petrol engines due to high temperatures N2+ O2—-> 2NO can react with water to cause acid rain,

unburnt hydrocarbons can cause photochemical smog if they react with nitrogen oxides to create airborne particles
which can interfere with sunlight.

Question 20

How to remove sulfur dioxide from the atmosphere?

Answer 20

SO2 + 1/2 O2 + H20 —–> H2SO4

When gases leave the flue they produce flue gases.

Neutralisation reaction can occur between CaO and these gases to eventually form gypsum: CaSO4.2H2O which can be used to make plaster and plaster boards.

CaO + H20 + SO2 + O2 —-> CaSO4.2H2O

Question 21

How can catalytic converters reduce the escape of toxic gases?

Answer 21

The gases (eg CO, NOx, unburnt hydrocarbons) enter the catalytic converter and it is a honeycomb structure which has a large SA, maximising the opportunity for collision between exhaust gases and catalyst.

Expensive catalysts used: eg platinum, palladium, rhodium.

The exhaust gases react with eachother to form less harmful products.

Eg- 2CO(g) + 2NO (g) —–> N2 (g) + 2CO2 (g)

Question 22

What is carbon footprint?

Answer 22

the total amount of carbon dioxide (and other greenhouse gases such as carbon dioxide, methane, nitrous oxide and CFCs (chlorofluorocarbons) which are emitted over the full life cycle of a product, service or event.

Question 23

What is carbon neutral?

Answer 23

fuel is described as carbon-neutral if the production and use of the fuel has no net increase on the amount of carbon dioxide in the atmosphere.

Question 24

What are chloroalkanes?

Answer 24

Compounds in which some H atoms of alkanes have been replaced by one or more chlorine atoms.

Question 25

What are the conditions and reagents for the reaction between chlorine and methane?

Answer 25

Reagents: chlorine and methane
Conditions: UV light/ sunlight

Question 26

What is the overall equation for the reaction between chlorine and methane?

Answer 26

CH4 + Cl2 —-> CH3Cl + HCl

Question 27

What occurs during the initation?

Answer 27

Chlorine radicals formed in the presence of UV light.

Cl2 —-> 2Cl*

Question 28

What occurs during propagation? 2 steps

Answer 28

Chlorine radicals react with methane molecules to form new radicals and molecules.

CH4 + Cl* —-> HCl + C*H3

CH3 + Cl2 —-> CH3Cl + Cl

Question 29

What occurs during termination?

Answer 29

Radicals react with eachother to form molecules.

CH3 + Cl —–> CH3Cl

OR

CH3 + CH3 —-> C2H6

Question 30

How to stop further reactions from occurring?

Answer 30

Use excess alkane