topic 2: alkanes Flashcards
what are alkanes and petroleum, how are they seperated?
-alkanes are saturated (single bonds) hydrocarbons
-petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation
fractional distillation steps
- Crude oil is vaporised at 350o.
- It goes into a fractioning column and rises up.
- The column has a temperature gradient, it is hot at the bottom and cooler at the top.
- Hydrocarbons condense at different points as they move up the column depending on chain length and boiling point.
- The largest hydrocarbons don’t vaporise as they have a very high boiling point, this is the residue at the bottom
- The shortest hydrocarbons don’t condense so are fuel gases at the top.
- This produces fractions (mixture of hydrocarbons with similar boiling points).
what is cracking + name 2 types of cracking?
-cracking involves breaking C-C bonds in alkanes, long chains into smaller molecules
-thermal cracking and catalytic cracking
thermal cracking
- takes place at high pressure and high temperature and produces a high percentage of alkenes
catalytic cracking
-takes place at a slight pressure, high temperature and in the presence of a ZEOLITE catayst is used mainly to produce motor fuels and aromatic hydrocarbons
economic reasons for cracking alkanes
-to make products for the industry such as motor fuels and aromatic hydrocarbons which have higher economical value
-smaller chains have more economical value
what are alkanes used as?
fuels
what does the internal combustion engine produce
a number of gaseous pollutants including NOx (nitric oxide), CO (carbon monoxide), carbon and unburned hydrocarbons
- how is nitric oxide formed?
- equation formaton of nitric oxide
- what does nitric oxide cause?
- ways to reduce nitric oxide?
- Reaction between nitrogen and oxygen in the air at very high temperatures (in engines and furnaces)
- N2+ O2 → 2NO
- The nitrogen oxides can also react in the atmosphere and become nitric acid cause acid rain which destroys trees and vegetation, corrodes buildings, statues and limestone and kills fish in lakes.
- Catalytic converters can remove unburnt hydrocarbons and NOx’s from the exhaust.
- what does the combustion of hydrocarbons containing sulfur lead to and what does it cause?
- sulfur dioxide causes
- explain why sulfur dioxide can be removed from power station flue gases using calcium oxide or calcium carbonate
- Fossil fuels containing sulfur impurities. When they are burned they react to form sulfur dioxide gas (SO2).
S + O2 → SO2 - sulfur dissolves in moisture and is converted into sulfur dioxide which causes acid rain
SO2 + H2O →H2SO4 - -powdered calcium carbonate (limestone) or calcium oxide is mixed with water to make an alkaline slurry
-when the flue gases mix with the alkaline slurry an acid base reaction occurs, the acidic sulfur dioxide gas reacts with the calcium compounds to form a harmless salt (calcium sulfite)
CaCO3 + SO2 →CaSO3 + CO2
CaO + SO2 → CaSO3
- how is unburned hydrocarbons formed?
- unburned hydrocarbons caused
- ways to reduce problem of unburned hydrocarbons
- Some fuel may not actually burn.
- Wastes fuel and Reacts with NO2
- -Ensure engines are well-tuned and there is a good supply of oxygen.
-They can be removed from exhaust gases by catalytic converters on cars.
- how is carbon particulates (soots) formed?
- carbon particulates problem caused
- ways to reduce carbon particulates
- Incomplete combustion of fuels containing carbon.
- -Carbon particulates (soot) can cause breathing/ respiratory problems
-Build up in engines meaning they don’t work properly
-Global dimming
-Blackens buildings - Ensure a good supply of oxygen when burning fuels.
how can gaseous pollutants produced from internal combustion engines can be removed and name an example
using catalytic converters
example: platimum, palladium, rhodium, iridium
- how is carbon monoxide formed?
- carbon monoxide problem caused
- ways to reduce carbon monoxide
- Incomplete combustion of fuels containing carbon.
- Carbon monoxide is poisonous and toxic.
- -Ensure a good supply of oxygen when burning fuels.
-It can be removed from exhaust gases by catalytic converters on cars.
catalytic converters equations
1. carbon monoxide is removed in a catalytic converter
2. hydocarbon is removed in a catalytic converter
-Carbon monoxide + nitrogen oxides → nitrogen + carbon dioxide
2CO + 2NO → N2 + 2CO2
Hydrocarbons + nitrogen oxides → nitrogen + carbon dioxide + water
C8H18 + 25NO → 12½ N2 + 8CO2 + 9H2O
combustion of alkanes: complete
COMPLETE (burned in enough oxygen):
alkane + oxygen → carbon dioxide + water
CxHx(g) + O2(g) → CO2(g) + H2O(l)
combustion of alkanes: incomplete
INCOMPLETE (not enough oxygen):
-alkane + oxygen → carbon monoxide + water
CxHx(g) + O2(g) → CO(g) + H2O(l)
-alkane + oxygen → carbon + water CxHx(g) + O2(g) → C (s) + H2O(l)
chlorination of alkanes - stage 1: initiation
1. the reaction of methane with chlorine forms chloromethane
-the reaction of methane with chlorine forms chloromethane
CH4 + CL2 —> CH3CL + HCL
STAGE 1: INITIATION REACTIONS - free radicals are produced
-UV sunlight provides enough energy to break cl-cl bonds
Cl2 —-> 2Cl.
UV
-the bond splits equally and each atom gets to keep one electron
-the atom becomes a highly reactive free radical, Cl . because of its unpaired electron
chlorination of alkanes - stage 2: propagation
1. equation when CL . attacks a methane molecule
2. equation when the new free radical can attack another molecule
STAGE 2: PROPAGATION REACTIONS - free radicals are used up and created in a chain reaction
-CL . attacks a methane molecule:
Cl . + CH4 —> CH3 . + HCl
CH3. + Cl2 —-> CH3Cl + Cl.
chlorination of alkanes - stage 3: termination
-give 2 examples from the propagation step
-overall equations of chlorine and methane
STAGE 3: TERMINATION REACTIONS - free radicals are mopped up
-2 free radicals joining together form a stable molecule
-the 2 unpaired electron form a covalent bond
-example of termination reactions:
Cl. + CH3. —-> CH3Cl
CH3. + CH3. —-> C2H6
overall equation: CH4 + Cl2 —> CH3CL + HCl
