Alkanes Flashcards
What is the general formula of alkanes?
Cn H2n+2
e.g. methane octane
Are alkanes saturated?
- Yes every carbon atom in an alkane has four single covalent bonds around it.
Are Alkanes polar?
- no as there is no distinct dipole moment present in the molecules.
Are alkanes water soluble?
- they are not water soluble as there is no distinct dipole moment. this means that its particles will not dissolve in polar water.
Are alkanes reactive?
- they are unreactive because its strong non-polar covalent bonds make them resistant to attack by other reactive species.
What is the melting and boiling point of alkanes like?
- the melting and boiling point increases with the carbon chain length
why do alkanes with longer hydrocarbon chains have a higher melting and boiling point?
- because stronger induced - dipole intermolecular forces in larger molecules as more electrons are involved. this means that more energy is needed to overcome these intermolecular forces.
how does branching affect the melting and boiling point in alkanes?
- branching generally lowers the boiling and melting points as molecules have a lower surface area
- induced dipole forces are weaker between the molecules fewer points of contact
- Unbranched alkanes have greater van der waals due to their larger surface area
what is a structural isomer?
- these are aka chain isomers
- molecules with the same molecular formula but a different structural formula
Which alkanes don’t have structural isomers?
- CH4 C2H6 C3H8
are corresponding cyclical alkanes structural isomers?
- no because they don’t have the same amount of hydrogen bonds.
what is crude oil?
“unrefined”
- mixture of hydrocarbons
- majority is saturated hydrocarbons
how does fractional distillation work?
- crude enters a furnace
- heated to 450 C to vaporize
- hydrocarbons with the longest chains come out of the bottom of the fractionating column (bitumen tar)
- at the first fraction fuel oil with 30 -40 carbon chain length 400C
- gas oil , kerosene , Nadtha gasoline
- the top of the column is the coolest point
- short chain carbons are the most useful
- at the top we have liquified petroleum gases 1-3 carbon chain length
what is cracking?
the process of breaking up larger less useful hydrocarbons into smaller more useful ones.
- an alkene is always a product used to make polymers
- in general : longer alkanes -> shorter alkanes + alkenes +(hydrogen sometimes)
what is thermal cracking?
- high temp 900 c and pressure 7000kpa are applied to the long chain hydrocarbons
-homolytic fission occurs and breaks the hydrocarbon into smaller chains to get free radicals - this makes alkenes and smaller alkanes
- useful for making polymers
what is catalytic cracking?
- lower temperatures of 500C
- catalyst zeolite ( sulphur dioxide + aluminium oxide) applied
- heterolytic fission occurs to form carbocations.
- go on to form smaller alkanes
- zeolites have microscopic pores and channels giving a larger surface area
- branched hydrocarbons , cyclic hydrocarbons and aromatic hydrocarbons
- usefull as fuels
what is combustion? (alkanes)
- the reaction of an alkane with oxygen
- energy is released so it is exothermic
What is complete combustion?
- combustion where there is an excess of oxygen
- e.g. C3+ O2 -> 3Co2 + 4 H2O
what is incomplete combustion?
- combustion where there is a deficiency of oxygen
- the leads to the production of carbon monoxide
- e.g. C3H8+O2 -> 3CO +4H2O
- occurs in longer chain alkanes
- some pure C can be produced
- impurities such as S, N may also be present producing SO2 +NOx
what happens when you increase the chain length of alkanes in combustion?
- more energy released
- but more energy is needed to react so they are harder to burn
What impact does Co2 have on the envrionment and what is the solution?
- Global warming
- solution is to use carbon neutral fuel sources e.g. biofuel
What impact does H2O have on the envrionment and what is the solution?
- global warming
- no definite solution
What impact does SO2 have on the envrionment and what is the solution?
- acid rain
- 2SO4 +O2 ->2SO3
-SO3+ H20 -> H2SO4 - the soluion is desulfurisation using Cao or Cao3
What impact does NO ,CO and unburned alkanes have on the envrionment and what is the solution?
- NO causes acid rain
- CO can cause health issues in smog
- unburned alkanes cotribute to global warming
- the solution is catalytic convertors
What impact do carbon particulates have on the envrionment and what is the solution?
- smog , health issues such as cancer
- the solution is to use fuels that release fewer particulates e.g. petrol over diesel
Why are catalytic convertors needed?
- internal combustion in engines produce a number of polluting gases
- e.g. soot, unburned alkanes ,sulfure dioxide and various nitrogen oxides.
How does a catalytic convertor work?
- ceramic honeycomb structure is coated with platnum and rubidium (catalysts) that reacts with our gases
- there is a honeycomb structure due to an increased surface area that reducs the need for Pt /PH metals
- cheaper
what reactions do platnum and rubidium catalyse?
- 2CO + 2No -> N2 + 2CO2
- (Alkane) + NO -> N2 +CO2 +H20
What is free radical substitution?
- the substitution of alkane H atoms with Halogn free radicals
What is a free radical?
- an unpaired electron
- represented by a dot and are higly reactive
what is initiation?
- the formation of free radicals
- addition of UV LIGHT turns Cl2 into Cl * + Cl *
What causes the production of free radicals?
- in the case of Cl2 one electron goes to the Cl atom
- and the other goes to the other. this is homolytic fission
- UV light causes this to happen it has the correct frequency that breaks the bond between Cl2
What is propagation?
- chlorine free radical starts a chain reaction.
- products are formed but free radical remains
- e.g. reacts with CH4
what products are made in the propagation stage of free radical substitution?
- GH4+ CL* -> *Ch3 +HCL (methyl radical)
- CH3 + CL2 -> CH3 CL +CL ( chloromethane product)
What is termination?
- free radicals combine
- Cl* + Cl* -> Cl2
- CH3 + Cl -> CH3 Cl
- CH3 + *CH3 -> C2 H6 (tiwice the size of the original)
- CH4+ Cl2 -> Ch3Cl +HCL
what are the problems with free radical substiution?
- will not occur in the dark
- substitution is random and so there is no controll over which h is substituted in karger alkanes
- if left to run, multiple Cl gas substitutions will occur
- multiple products are made so its not a precise process.
what the relationship between temperature and energy during state changes?
Between 1 & 2, the particles are vibrating and gaining kinetic energy and the temperature rises
2. Between 2 & 3, all the energy goes into breaking intermolecular attractions – there is no increase
in kinetic energy or temperature
3. Between 3 & 4, the particles are moving around and gaining in kinetic energy
4. Between 4 & 5, the substance is boiling, so intermolecular attractions are breaking and there
is no increase in kinetic energy or temperature
5. From 5 & 6, the particles are moving around rapidly and increasing in kinetic energy
what is flue gas desulphurisation?
-during the combustion of fuel sulphur dioxide is formed
- removed after burning
- waste passes through scrubber
- this is calcium oxide or calcium carbonate
Cao + So2 -> CaSO3
- caoco3 + so2-> caso3 +co2
- this is an acid based reaction that forms gypsum calcium sulphate and this can make plaster board
how does the greenhouse effect work?
-Greenhouse gases absorb and emit infra-red radiation.
IR radiation excites the bonds C=O, C-H and O-H in carbon dioxide, methane and water vapour.
This causes the bonds to vibrate more vigorously because they vibrate at the same frequency as IR.
The molecules then re-emit the energy in all directions as heat. This heats up the atmosphere
what is meant by carbon neutral?
making or resulting in no net release of carbon dioxide into the atmosphere, especially as a result of carbon offsetting:
what is a carbon footprint/?
Carbon footprint, amount of carbon dioxide emissions associated with all the activities of a person
what is a biofuel?
a fuel derived immediately from living matter.
what is the ozone layer?
- a very high layer in the atmosphere rich in ozone
- absorbs uv c and b and prevents it from reaching the earth.
how is ozone formed?
- ozone is made by ultraviolet light reacting with oxygen molecules to form oxygen free radicals
- o2 -> .o. + .o.
- the oxygen free radical reacts with molecular oxygen to form ozone.
- o2 + .o.-> o3
overall 3o2 -> 2o3
how is ozone depleted?
- ozone molecule absorbs uv light to form an oxygen molecule and an oxygen atom
- 03 -> .o. +o2
this is reversible - 2o3 ->-< 302
what are chlorofluorocarbons?
halogenoalkane molecules where some of the hydrogens in the alkane have been replaced by chlorine and fluorine atoms.
- these are useful as refrigerants and inert molecules.
how do cfc’s and uv light interact?
UV light can break C-Cl bonds in CFCs to form chlorine free radicals.
The C – F bond is too strong to be broken by UV.
The chlorine radicals react with ozone and decompose it:
Cl + O3 →CIO + O2
The chlorine oxide radical can react with ozone and decompose it:
CIO* + O3→ 2O2 + *Cl
A chlorine radical is then produced which can go on to continue this reaction many thousands of time.
