Alkanes

Question 1

Crude Oil

Answer 1

• FOSSIL FUEL
• arose from decomposition of dead sea plants & animals put under heat & pressure millions of years ago
• PETROLEUM alternative name
• Mixture of 150 HYDROCARBONS, unbranched alkanes, cycloalkanes & arenes
• THICK BLACK LIQUID
• Difficult to IGNITE, is of limited use in its native state.
• Refining ~ FRACTIONAL DISTILLATION

Individual hydrocarbons uses:
- FUELS
- PETROCHEMICALS ~ polymers & synthetic fibres

Question 2

Fractional distillation of crude oil STEPS

Answer 2

1. Crude oil is fed in
2. VAPOURISED in the furnace
3. GASEOUS crude oil enters the fractional distillation column.
4. The fractionating column is HOTTER at the BOTTOM and COOLER at the TOP.
5. SHORT-chained hydrocarbons have LOWER BP so condense near the TOP
6. LONG-chain hydrocarbons have HIGHER BP so condense near the BOTTOM
7. RESIDUE contains hydrocarbons which are TOO LONG , boil and are removed at the bottom.
8. GASEOUS (petroleum gas) do not condense because the temperature at the top is NOT LOW ENOUGH.

Question 3

Fractional Distillation

Answer 3

The SEPARATION of the components in a LIQUID mixture into FRACTIONS.

FRACTIONS~ mixtures of components with similar boiling points.

SEPARATION is achieved because individual components across the fractions all have DIFFERENT boiling points.

Question 4

The products from fractional distillation and the temperatures that they condense at.

Answer 4

20 degrees:
- Liquefied petroleum gas
- 1 to 4 carbons
Use ~ camping gas

150 degrees:
- Gasoline
- 4 to 12 carbons
Use ~ petrol

200 degrees:
- Kerosene
- 11 to 15 carbons
Use ~ jet fuel, petrochemicals and heating.

300 degrees:
- Diesel
- 15 to 19 carbons
Uses ~ fuel and petrochemicals.

370 degrees:
- Lubricating oil
- 20 to 30 carbons
Uses ~ lubricating oil and petrochemicals

RESIDUE:
Fuel oil ~ 30 to 40 carbons, power station fuel.

Paraffin wax ~ 40 to 50 carbons ,candles.

Bitumen ~ 50+ carbons, road surfacing.

Question 5

Effect of CHAIN LENGTH on boiling points of alkanes

Answer 5

An increased chain length means:

• More points of CONTACT between neighbouring molecules.
• STRONGER LONDON FORCES between the molecules.
• MORE ENERGY or higher temperatures are required to separate LONGER hydrocarbons.

This is how different alkanes in crude oil are separated as their BP vary due to their different CHAIN LENGTHS.

Question 6

Effect of BRANCHING on boiling points of alkanes

Answer 6

In comparison with UNBRANCHED isomers, BRANCHED isomers:

• FEWER CONTACT POINTS
• WEAKER LONDON FORCES
• LESS ENERGY or lower temperature are required for them to be separated.

Question 7

What is the trend in BOILING POINT going along the homologous series ALKANES.

Answer 7

• Number of CONTACT POINTS INCREASES
• Strength of LONDON FORCES INCREASES
• MORE ENERGY required to separate the molecules
• Boiling point INCREASES

Question 8

Reactions of alkanes

Answer 8

• Alkanes are relatively INERT (unreactive)
• C-C & C-H bonds are STRONG, requires a LOT OF ENERGY to break

C-C 347 KJ mol^-1
C-H 413 KJ mol^-1

• NON-POLAR
• Do not attract H+ , OH- or polar molecules.

DO NOT REACT with:
- Acids
- Bases
- Oxidising or reducing agents
- Nucleophiles
- Electrophiles
- Polar reagents

They DO REACT with:
- Oxygen
- Halogens

Question 9

Complete combustion of alkanes

Answer 9

• Occurs when an alkane is burned in EXCESS OXYGEN.
• Products are CARBON DIOXIDE & WATER.
• OXIDATION REACTION as each element present is combining FULLY with oxygen.
• SHORT alkanes usually burn CLEANLY which means they undergo COMPLETE combustion.
• NATURAL GAS is used for HEATING and COOKING in homes.
• The LARGEST component in natural gas is METHANE:

CH4(g) + 2O2(g) ——— CO2(g) + 2H2O(l)

Question 10

INCOMPLETE combustion of alkanes

Answer 10

• Occurs when burning the alkane in LIMITED OXYGEN.
• Products are CARBON MONOXIDE & WATER.
• The shortage of oxygen in car engines means some incomplete combustion is inevitable , i.e when burning OCTANE:

C8H18(l) + 8 1/2 02(g) —– 8CO(g) + 9H2O(l)

Question 11

Properties of CARBON MONOXIDE

Answer 11

• Produced when any fossil fuel is burnt in LIMITED OXYGEN.
• Odourless
• Colourless
• Poisonous
• Irreversibly reduces the amount of oxygen that HAEMOGLOBIN can carry in red blood cells.

Question 12

Incomplete & complete combustion with a BUNSEN BURNER

Answer 12

HOLE OPEN = complete combustion
- BLUE flame

HOLE CLOSED = incomplete combustion

Products ~ CO & C (s)

• YELLOW flame & luminous because of the carbon particles being produced.
• Heating apparatus with a luminous flame is AVOIDED as it gets covered in BLACK SOOT , solid carbon.

CH4(g) + O2(g) ——– C(s) + 2H2O(l)

Question 13

How can our HIGH DEMAND for short chained hydrocarbons be produced with our low demand , SURPLUS long chained hydrocarbons?

Answer 13

CRACKING:

• Long chain alkanes are broken down into a mixture of :

SHORTER CHAIN ALKANES ~ fuels

ALKENES ~ polymers

• RANDOM process of BOND BREAKING
• Both C-H and C-C bonds are broken in the process

Question 14

Reaction conditions of CRACKING process

Answer 14

• 450 degrees

ZEOLITE CATALYST:

• Speeds up the reaction WITHOUT being used up
• LOWERS temperature requirement ~ reduces ENERGY COST
• Zeolites are crystalline alumino -silicates with PORES of EXACT SIZE.
• Long chain alkanes fit into the pores of the catalyst

Question 15

Halogenation of Alkanes

Answer 15

• Halogen atom replaces a HYDROGEN atom in the alkane
• SUBSTITUTION reaction~ two moles of reactant produces two moles of product.

First step:

• HOMOLYTIC FISSION
• produces two radicals
• requires of UV LIGHT
• Type of reaction: RADICAL SUBSTITUTION

Example:

C6H12 + Br2 ————— C6H11Br + HBr
alkane + halogen ————–haloalkane + hydrogen halide

Question 16

Reaction mechanisms

Answer 16

Shows the STEP-BY-STEP MOVEMENT of an electron pair during a chemical reaction

Question 17

REACTION MECHANISM of a radical substitution reaction:

METAHNE + CHLORINE

Answer 17

INITIATION ~ the production of radicals

  UV light Cl2 ----------- 2Cl .
homolytic fission

PROPAGATION ~ rapid chain reactions until no reactants theoretically remain.

STEP 1 ~ CH4 + Cl . —— . CH3 + HCl
STEP 2~ . CH3 + Cl2 —— CH3Cl + Cl .

Add steps 1 & 2 and cancel out radicals to give overall reaction:

CH4 + Cl2 ——– CH3Cl + HCl

TERMINATION ~ two radicals can combine and stop the reaction prior to all the reactants being used up.

. CH3 + Cl . —– CH3Cl (chloromethane)

2CH3 . —— C2H6 (ethane)

2Cl . —— Cl2 (chlorine) Reverse of initiation

Question 18

BROMINE WATER added to an alkane vs alkene

Answer 18

Alkane ~ remains orange / not decolourised

Alkene ~ orange to colourless

Question 19

During Radical Substitution how could other ORGANIC IMPURITIES form?

Answer 19

Further SUBSTITUTION