UNIT 3: SECTION 2 - ALKANES & HALOGENOALKANES Flashcards
What is an alkane?
Hydrocarbon with formula CnH2n
Forms maximum bonds with hydrogen
What are the order of fractions?
GAY - Gas
POLICEMEN - Petrol
NEVER - Naptha
KILL - Kerosene
GAY - Gas Oil
MEN - Mineral Oil
BITCH - Bitumen
Conditions for thermal cracking?
High temp (1000°C) High pressure (70atms) Mostly alkenes - polymerisation -> polymers such as plastic: poly(ethene).
Conditions for catalytic cracking AND one advantage?
High temp (450°C)
Slight pressure
Mostly aromatic HC’s and motor fuels
Uses zeolite catalyst
Cost-Efficient!
One use of alkanes,
and how they are used in this way?
One risk to this use?
Alkanes are good fuels.
Products are CO2 (POLLUTANT) and H2O and a lot of energy.
- Used in power-stations, car engines etc.
- If not enough O2, incomplete combustion occurs, leading to more pollutants.
4 Pollutants other than CO2 which are produced during the incomplete combustion of alkanes?
1) CO - Poisonous gas, binds to haemoglobin -> lowers O2 saturation.
[REMOVED VIA CAT. CONVERTERS]
2) Carbon - Soot, causes respiratory issues and engine buildup.
3) Smog - Unburnt HC + NOx
[NOx made in engine; high pressure high temp]
[REMOVED VIA CAT. CONVERTERS]
4) Acid Rain - SO2 from burning of fossil fuels.
SO2 dissolves in air -> H2SO4 -> RAINS.
- Acid rain destroys vegetation, buildings, aquatic life.
What are free radicals?
Atoms with unpaired electrons, eg. *Cl
How are free radicals formed?
UV light + Halogen molecule -> 2Rads.
Photochemical reaction
How are halogenoalkanes formed and what type of reaction is this?
Halogen + Alkane ———> Halogenoalkane + Hydrogen
Free-rad substitution
The 3 stages of free radical reactions?
1) Initiation:
Cl2 (+UV) ———> 2Cl*
2) Propagation: Chain reaction…
Cl* + CH4 —> CH3* + HCl
CH3* + Cl2 —> CH3Cl + Cl*
And so on.. until all molecules of Cl/CH4 used up.
3) Termination: Free rads used up..
Cl* + CH3* —> CH3Cl
OR
CH3* + CH3* —> C2H6
What is a CFC? Why are they bad?
What is the reaction between CFC’s and O3?
CFC’s are chlorofluorocarbons -
All H’s replaced.
They were used in refrigerator coolants, aerosol sprays etc.
Damaged the Ozone layer.
1) CFCl3 (+UV) —> CFCl2* + Cl*
2) Cl* + O3 —> ClO* + O2
3) ClO* + O3 —> 2O2 + Cl*
Overall: 2O3 —> 3O2 (Cl* Catalyst)
What is O3?
How is it useful?
Ozone; found in Ozone layer -
ACTS AS CHEMICAL SUNSCREEN;
PREVENTS SKIN CANCER BY ABSORBING LOTS OF HEAT.
What are CFC alternatives?
HFC’s
Is the bond between a halogen and carbon polar?
If so, what causes this polarity?
It is polar because there is a high difference in ELECTRONEGATIVITY.
What is a nucleophile?
Electron donor.
