Alkanes Flashcards
What is an alkane?
A saturated hydrocarbon containing only C-H bonds
What is the general formula of an alkane?
CnH2n+2
Explain why their bonds are non-polar
Carbon and hydrogen have similar electronegativities
Non-polar bonds means Van der Waals forces
What is the solubility and reactivity of alkanes?
Insoluble in water
Very reactive
How does fractional distillation work?
- Crude oil heated
- Passed into column that’s cooler at the top
- Vapours rise up the column and condense at their boiling points
- Shorter hydrocarbons condense closer to the top
What are the pro and cons of cracking?
+ reduces imported gas and electricity
+ gas supply for many years
- lots of traffic to local areas
- pollutes water supplies
Why are alkanes cracked?
To turn longer chain alkanes in shorter ones
More economically valuable
What are the conditions for thermal cracking?
Temperature = 1200K
Pressure = up to 700kPa
What are the conditions for catalytic cracking?
Temperature = 720K
Lower pressure
Zeolite catalyst
What are the main products of catalytic cracking?
Cycloalkanes
Aromatic hydrocarbons
Branched alkanes
What is a fuel?
Something which releases heat energy when combusted
What is incomplete combustion?
Combustion in a limited supply of oxygen
Produces carbon monoxide (toxic)
Typically longer chain hydrocarbons
What are the environmental impacts of some of these compounds?
Nitrogen oxides —> acid rain
Sulfur dioxide —> acid rain
Carbon / Soot —> global dimming
Unburnt hydrocarbons —> photochemical smog
Carbon dioxide —> global warming
Write an equation that desulfurises flue gas
CaCO3 + SO2 + 1/2 O2 —> CaSO4 + CO2
How do greenhouse gases contribute to global warming?
They trap infrared radiation in the atmosphere
Atmosphere heats up = global warming
Define carbon neutral activities
Activities that produce no net carbon dioxide emissions
Explain the 3 stages of free radical substitution
- Initiation = breaking halogen bond to form free radicals
- Propagation = products formed but free radicals remain
- Termination = free radicals removed, stable products formed
What conditions are needed for a free radical chlorine atom?
Presence of UV light
Write the free radical substitution of methane with chlorine to make CH3Cl
Initiation:
Cl2 —> 2 Cl* (presence of UV light)
Propagation:
Cl* + CH4 —> HCl + CH3
CH3 + Cl2 —> CH3Cl + Cl
Termination:
CH3 + Cl —> CH3Cl
2Cl —> Cl2
*CH3 + *CH3 —> CH3CH3
How do chlorine free radicals catalyse the decomposition of O3?
Cl2 —> 2 Cl* (presence of UV light)
Cl* + O3 —> ClO* + O2
ClO* + O3 —> Cl* + 2 O2
Overall : 2 O3 —> 3 O2
