O- Alkanes Flashcards
What is the main source of alkanes?
crude oil
Uses of alkanes
- fuels
- lubricants
- starting materials for other compounds
Bond angle of straight chain alkanes
109.5
General formula for cyclic alkanes
CnH2n
** cyclic isomers to alkenes
Polarity
non-polar
∵ C & H - similar electronegativities
Boiling points
- longer chain ↑ bp
–↑ vdW BETWEEN MOLECULES - straight chain ↑ bp
–√ packed ↑ tightly tgt–> ↑ vdW
Solubility
- insoluble
– strong H bonds in water, stronger than vdW between alkanes (↑ stable- x break to form vdW to mix)
Reactivity
- unreactive ∵ strong C-C & C-H bonds
3 conditions for crude oil to form
- ↑↑ heat & pressure
- millions of years
- biomass (eg. plants & animals)
What is crude oil?
Mixture of diff. (branched & unbranched) alkanes of diff. lengths
What is a fraction?
Group of alkanes w/ similar chain lengths & bp
What is bitumen used for?
- road surfacing
- further processed
What are alkenes from fractional distillation used for?
- polymers
- starting materials for other compounds
2 types of cracking
- thermal
- catalytic
Conditions for thermal cracking
- high temp- {700-1200K}
- high pressure- {7000kPa}
Describe how each type of products are formed in thermal cracking.
bond between 2 carbons in hydrocarbon chain splits into 2 molecules each w/ a free radical (unbonded e-)
Alkanes:
- free radical reacts w/ H –> alkane
Alkenes:
- x enough H –> loses 1 H–> alkene
Conditions for catalytic cracking
- lower temp- {720K}
- ↓ pressure
What is a zeolite catalyst + how is it adapted for its function?
- silicon dioxide + aluminium oxide
- honeycomb structure w/ ↑ s.a.
- acidic
Types of products formed in catalytic cracking
- branched alkanes
- cyclic alkanes
- aromatics
What is the effect of no. of carbons in the amount of energy released in complete combustion?
↑ no. of carbons ↑ heat energy released
Products of incomplete combustion
- CO
- carbon particulates (soot)
As the chain length increases, what happens to the likelihood of incomplete combustion?
↑
∵ ↑ carbons require ↑ O2 for complete combustion
How are Nitrogen oxides (NO, NO2, N2O4) formed?
What are the negative consequences?
- nitrogen & O2 from atmosphere react
- high temp
-
✔ react w/ water vapour & O2 in air–> nitric acid
– 1. photochemical smog
– 2. acid rain
Negative consequences for CO
- toxic
- haemoglobin has a high affinity for it
Negative consequences for carbon particulates
- cancer
- exacerbate asthma
Negative consequences of unburnt hydrocarbons
- photochemical smog
Negative consequence of SO2
- sulfur in fossil fuels–> SO2
– reacts w/ O2 & H2O in atmosphere –> sulfuric acid–> acid rain
SO2 (g)+ 1/2 O2 (g) + H2O (l) –> H2SO4 (l)
How is SO2 formed?
- sulfur-containing impurities in crude oil
- sulfur in fossil fuels
What is flue gas?
Fossil fuels burnt at power stations
Describe how flue gas desulfurization is carried out
- 2 methods
1. spray slurry of lime (CaO) & water on flue gas
— SO2 forms calcium sulphite–> oxidised–> calcium sulfate (gypsum) - sold to make builder’s plaster/ plasterboard
CaO (s) + 2H2O (l) + SO2 (g) + ½ O2 (g) –> CaSO4.2H2O (s)
- calcium carbonate xlime
CaCO3 (s) + ½ O2 (g) + SO2 (g) –> CaSO4 (s) + CO2 (g)
What does flue gas desulfurization get rid of?
sulfur dioxide
What do catalytic converters get rid of?
- nitrogen oxides
- carbon monoxide
- unreacted hydrocarbons
Describe what a catalytic converter is made of
- honeycomb structure
- ceramic material
- expensive metals eg. platinum & rhodium
Important feature of a catalytic converter
↑ s.a.
↓ amount of metals for same effect
Chemical equation for removal of carbon monoxide
2CO (g) + 2NO (g) N2 (g) + 2CO2 (g)
Word equation for removal of unburnt hydrocarbons
Unreacted hydrocarbons + nitrogen oxide nitrogen + carbon dioxide + water
What is carbon neutrality?
- process/ activities that release no CO2 emissions overall
What is the reaction forming a halogenoalkane called and what is the condition?
- substitution reaction- H substituted by halogen
- condition- UV light
3 steps in free radical substitution
- initiation- halogen absorbs a single quantum of UV light
- propagation- the chain part (repeats)
- termination- getting rid of radicals
Product of initiation
2 radicals
eg. 2Cl.
Why do C-H bonds not break under UV light?
wrong wavelength- only breaks bonds between halogens
Products of each step in propagation
eg. reacting Cl w/ methane (CH4)
1. CH3 radical
2. Cl radical
– both reactants for the other reactions–> repeating chain
Why are chain reactions not selective?
- any product can be formed
- ∵ any C-H bond could be broken in alkane
–> positional isomers
List the 3 possible products in termination
👑 1. halogenoalkane
2. halogen
3. bigger alkane
Why is a hole in the ozone (O3) layer a problem?
- harmful UV radiation from the sun goes through the hole
- sunburn + skin cancer
