3.3.4 Alkanes Flashcards
_____, _____ and _____ have no isomers
Methane, ethane and propane
Alkanes with branched chains have _____ melting points than straight chain alkanes (with same no. of C)
LOWER melting points than straight chain alkanes (with same no. of C)
Why do alkanes with branched chains have lower melting points than straight chain alkanes (with same no. of C)?
∵ less surface area & weaker van der Waals forces
Solid alkanes have a ____ feel
waxy
Shorter chains of alkanes are ___ at room temp.
gases
Why are alkanes insoluble in water?
Hydrogen bonds holding water molecules together = much stronger than van der Waals forces between alkane molecules
What do alkanes react with?
halogens (& burn under suitable conditions)
What is petroleum (crude oil)?
Mixture of hydrocarbons - mostly alkanes
How did crude oil form?
Formed millions of years ago by breakdown of plant and animal remains at high pressure and temperature
Why does crude oil contains other elements?
Contains some elements that were in original plants and animals
Describe the stages of fractional distillation
- Crude oil = vaporised at 350°C
- Vaporised crude oil goes into fractionating column & rises up through trays
- Largest hydrocarbons don’t vaporise = run to the bottom & form a gooey residue
- Crude oil vapour goes up = gets cooler
- Each fraction condenses at different temperature
- Drawn off at different levels in column
- Hydrocarbon with lowest boiling points don’t condense
- Drawn off as gases at top of column
Give 2 conditions of thermal cracking
- 900°C - high temperature
- Up to 70 atm - high pressure
How do can we avoid the decomposition (of C and H) in thermal cracking?
Alkanes kept in these conditions for very short time (e.g. a second)
Name 3 conditions of catalytic cracking
- Uses zeolite catalyst (hydrated aluminosilicate)
- 1-2 atm - slight pressure
- 450°C - high temperature
What is the structure of a zeolites & why?
Have honeycomb structure = enormous surface area
& are acidic
What is the benefit of using a catalyst?
- Cuts costs ∵ reaction can be done at a low pressure & low temperature
- & catalyst speeds up reaction - saving time (& time is money)
What do aromatic compounds contain?
Benzene rings
What is the structure of benzene rings?
Have 6 carbon atoms with 3 double bonds
Why are benzene rings stable?
Because the electrons are delocalised around the carbon ring
Why doesn’t cracking produce 2 alkanes?
Since there isn’t enough hydrogen atoms to produce 2 alkanes = one of new chains has C=C
How are the products obtained from cracking separated?
By fractional distillation
Why are alkanes are great fuels?
Burning small amount = huge amount of energy
Give the word equation for complete combustion
Alkanes + Oxygen → Carbon Dioxide + Water
Give the word equations for incomplete combustion
Alkanes + Oxygen → Carbon Monoxide + Water (+ sometimes Carbon Dioxide)
Alkanes + Oxygen → Carbon + Water
Why is carbon monoxide gas poisonous?
Binds to same sites on haemoglobin molecules in red blood cells as oxygen molecules ∴ oxygen cannot be carried around the body
Name 2 issues with soot (particulates)
Soot causes breathing problems & builds up in engines = can’t work properly
Describe the greenhouse effect
- Greenhouse gases in atmosphere = good at absorbing infrared energy (heat)
- Emit some energy they absorb back towards Earth, keeping it warm = greenhouse effect
How does rising temperature affect water vapour?
- Higher temps = more water vapour in air = more greenhouse effect
- Greater cloud formation and clouds reflect solar radiation
How can carbon monoxide be removed from exhaust gases?
By catalytic converters on cars
Where do unburnt hydrocarbons come from?
Engines (they don’t burn all fuel molecules = come out as unburnt hydrocarbons)
How are oxides of nitrogen (NOx) produced?
Produced when high pressure and temperature in car engine causes nitrogen and oxygen atoms in air to react together
Describe how smog (O3) is created?
Hydrocarbons and nitrogen oxides react in presence of sunlight to form ground-level ozone (O3) = major component in smog
Why is ground-level ozones bad? (3x)
Irritates people’s eyes and increases respiratory problems + lung damage
How can we remove unburnt hydrocarbons and NOx from exhausts?
By catalytic converters
What is a catalytic converter?
Honeycomb made of ceramic material coated with platinum and rhodium metals
Describe how acid rain occurs
- Some fossil fuels contain sulfur: when burnt, sulfur reacts to form sulfur dioxide gas (SO2)
- Sulfur dioxide in the atmosphere reacts with water vapour and oxygen to form sulfuric acid = acid rain
Name 3 things acid rains destroys
- Trees and vegetation
- Corroding buildings/statues
- Kills fish in lakes
What are flue gases?
Gases given out by power stations
Describe flue gas desulfurisation
(how flue gases can be removed from power stations before it gets into atmosphere)
- Powdered calcium carbonate (limestone) or calcium oxide is mixed with water to make an alkaline slurry
- When flue gases mix with alkaline slurry = acidic sulfur dioxide gas reacts with calcium compounds to form harmless salt (calcium sulfate)
- Oxides 1st to produce calcium sulphite & then oxidises again to form calcium sulfate (aka gypsum)
What is calcium sulfate (AKA gypsum) used for?
Used to make builders’ plaster and plasterboard
Give the symbol equation for forming NO
N2 (g) + O2 (g) → 2NO(g)
Give the symbol equation for forming sulfuric acid
SO2(g) + 1/2 O2(g) + H2O(l) → H2SO4(l)
Give 2 word equations of a catalytic converter converting polluting gases into less harmful products
Carbon Monoxide + Nitrogen Oxide → Nitrogen + Carbon Dioxide
Hydrocarbons + Nitrogen Oxide → Nitrogen + Carbon Dioxide + Water
What are free radicals?
Particles with unpaired electrons
When do free radicals form?
When a covalent bond spilts equally, giving one electron to each atom
Why are free radicals very reactive?
∵ electrons are unpaired
Example: Reacting Chlorine with Methane
What happens after the termination reactions?
Depends on whether there’s more chlorine or methane around:
- Chlorine’s in excess: Cl• free radicals = start attacking chloromethane, producing dichloromethane CH2Cl, trichloromethane CHCl3 & tetrachloromethane CCl4
- Methane’s in excess: product mostly be chloromethane
What are chlorofluorocarbons (CFCs)?
Halogenoalkane molecules where all of hydrogen atoms have been replaced by chlorine and fluorine atoms
What does Ozone (O3) in upper atmosphere do?
Acts as a chemical sunscreen → absorbs UV radiation from Sun
How is ozone formed naturally?
- When oxygen molecule (O2) is broken down into 2 free radicals by UV radiation
- Free radicals attack other oxygen molecules forming ozone
Explain how CFCs are creating holes in ozone layer (include symbol equations)
- Chlorine free radicals (Cl•) = formed in upper atmosphere when C-Cl bonds in CFCs are broken down by UV
e. g. CCl3F → CCl2F• + Cl• - These free radicals = catalysts → react with ozone to form an intermediate (ClO•) & oxygen molecule
Cl• + O3 → O2 + ClO•
ClO• + O3 → 2O2 + Cl•
- Overall reaction: 2O3 → 3O2 & Cl• = catalyst
Name 3 properties of CFCs
Unreactive, non-flammable & non-toxic
CFCs are _____
BANNED
What can we now use instead of CFCs?
e.g. HFCs (hydrofluorocarbons) & hydrocarbons
For the conversion of methane to tetrachloromethane:
Give the overall equation
CH4 + 4 Cl2 → CCl4 + 4 HCl
Free Radical Substitution
Condition
UV light neeed to break halogen bond
For the conversion of ethane to 1,1-dibromoethane
Give the overall equation
C2H6 + 2Br2 → C2H4Br2 + 2HBr
For the conversion of ethane to 1,1-dibromoethane
Give the pair of propagation steps to form bromoethane from ethane
C2H6 + Br• → C2H5• + HBr
C2H5• + Br2 → C2H5Br + Br•
For the conversion of ethane to 1,1-dibromoethane:
Give the pair of propagation steps to form 1,1-dibromoethane from bromoethane
C2H5Br + Br• → C2H4Br• + HBr
C2H4Br• + Br2 → C2H4Br2 + Br•
For the conversion of ethane to 1,1-dibromoethane:
Give the termination step that forms butane
2C2H5• → C4H10
Write the symbol equation for when carbon monoxide reacts with nitrogen oxide
2CO(g) + 2NO(g) → N2(g) + 2CO2(g)
Represent how calcium oxide is used to remove sulfur dioxide from flue gases by using an equation
Represent how calcium carbonate is used to remove sulfur dioxide from flue gases by using an equation
Deduce an equation to show how nitrogen dioxide reacts with oxygen and water to form nitric acid
4NO2 + 2H2O + O2 → 4HNO3
Explain the main economic reason why alkanes are cracked (1)
To produce substances which are more in demand / products with high value
Suggest why calcium oxide reacts with sulfur dioxide (1)
It is basic/SO2 is acidic
(neutralises the gas)
Write an equation to show how gypsum (calcium sulfate) is made
CaSO3(s) + 1/2O2(g) → CaSO4
Calcium sulfite is oxidised to calcium sulfate (gypsum)
what are the steps of the process?
initiation (formation of free radicals)
propagation (chain reactions)
termination (free radicals combine)
homolytic fission
splitting an atom into 2 free radicals
heterolytic fission
splitting of an atom into a cation and an anion
cons of reaction:
needs UV light for reaction to start
substitution is random soon control over which H is substituted in longer alkanes- not a precise process
if left, multiple substitutions can occur-not a precise process
why is propagation a chain reaction?
further reactions produce more free radicals
(5) properties of alkanes and why
saturated: every carbon atom has 4 single covalent bonds around it
non-polar (and non-soluble in water): no distinct dipole movement in the molecules therefore will not dissolve in polar water
: electronegatives of C and H = similar therfore IMF = weak van der waals
unreactive: strong, non-polar covalent bonds make them resistant to attack by other reactive species
varied mp/bp: increases with increased carbon chain length (Mr)
: stronger induced-dipole IMF in larger molecules as more e- are involved therefore more E required to overcome IMF i.e IMF increases
: more branching genenerally lowers mp/bp as molecules have a lower SA therefore ID forces are weaker between molecules
why is the boiling point important to know in fractional distillation
more Mr = increased bp
stronger ID
therefore energy needed to separate them
= higher condensing point (g => l)
process of fractional distillation
1) crude oil is vapourised
2) vapour passed into a tower with a temperature gradient
3) vapour rises and it cools
4) molecules condense at different heights cause different bp
5) larger the molecule and the lower it condenses
what is cracking
the process of breaking up larger, less useful, hydrocarbons into smaller, more useful ones
what does thermal cracking form?
homolytic fission occurs forms 2 free radicals
goes onto form : mostly alkenes (makes polymers)
: smaller alkanes (makes fuels)
: H2 (useful in industry/fuel)
what does catalytic cracking form?
(greener than other method)
heterolytic fission occurs which forms carbocations
goes on to form : mostly smaller alkanes (motor fuels)
: aromatic hydrocarbons
: branched alkanes and cycloslkanes
what is reforming
where straight chain alkanes
which forms either: branched chain alkanes (fuels) at random length therfore further distillation is required
: benzene
combustion of alkanes
conditions: reaction with O2 and exothermic
what are pollutant made in combustion
CO2, H2O, SO2, NO, CO, unburned alkanes and carbon particulates
what does catalytic convertor remove
removes some polluting gases: CO, unburned alkanes and various nitrogen oxides made by internal combustion engines
ways in which alkanes mp/bp changes?
increases with carbon chain (bp) because stronger induced-dipole foces IMF in larger molecules as more e- involved to overcome IMF
more branching decreases mp as molecules have higher SA therfore ID forces are weaker between molecules
