chem revise Flashcards
fractional distillation of alkanes
Oil is pre-heated
* then passed into column. * The fractions condense at different heights
* The temperature of column decreases upwards
* The separation depends on boiling point. * Boiling point depends on size of molecules. * The larger the molecule the larger the van der waals forces
* Similar molecules (size, bp, mass) condense together * Small molecules condense at the top at lower temperatures
fractional distillation in the lab
- Heat the flask, with a Bunsen burner or electric
mantle - This causes vapours of all the components in the
mixture to be produced. * Vapours pass up the fractionating column. * The vapour of the substance with the lower boiling
point reaches the top of the fractionating column
first. * The thermometer should be at or below the boiling
point of the most volatile substance. * The vapours with higher boiling points condense
back into the flask. * Only the most volatile vapour passes into the
condenser. * The condenser cools the vapours and condenses to
a liquid and is collected.
what is cracking
conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
economic reasons for cracking
The petroleum fractions with shorter C chains are in more demand than larger fractions.
The products of cracking are more valuable than the starting
materials
two types of cracking
thermal and catalytic
thermal cracking conditions
High pressure (7000 kPa)
High temperature (400°C to 900°C)
thermal cracking conditions
produces mostly alkenes e.g. ethene used
for making polymers and ethanol
sometimes produces hydrogen used in the
Haber Process and in margarine manufacture.
catalytic cracking conditions
Slight or moderate pressure
High temperature (450°C)
Zeolite catalyst
products oc catalytic cracking
Produces branched and cyclic
alkanes and aromatic hydrocarbons
Used for making motor fuels
Cheaper than thermal cracking because it saves
energy as lower temperatures and pressures are used
so2
SO2 will dissolve in atmospheric water and can produce acid rain
SO2 can be removed from the waste gases from furnaces by flue gas desulfurisation
basic calcium oxide which reacts with the acidic
sulfur dioxide in a neutralisation
no
Nitrogen oxides form from the reaction between N2 and O2
inside the car engine.
The high temperature and spark in the engine provides sufficient energy to break strong N2 bond
NO is toxic and can form acidic gas NO2
catalytic converters
These remove CO, NOx and unburned hydrocarbons (e.g. octane, C8H18)
from the exhaust gases, turning them into ‘harmless’ CO2
, N2 and H2O.
Converters have a ceramic
honeycomb coated with a thin
layer of catalyst metals
platinum, palladium, rhodium– to give a large surface area
greenhouse effect
UV wavelength radiation passes through the atmosphere to the Earth’s surface and heats up Earth’s surface.
The Earth radiates out infrared long wavelength radiation.
The C=O Bonds in CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere.
This energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed.
why alkanes do not react with many reagents
This is because the C-C bond
and the C-H bond are relatively
strong
uv light
The UV light supplies the energy to break the Cl-Cl bond. It is
broken in preference to the others because it is the weakest.
free radical
A free radical is a reactive species which
possess an unpaired electron
Primary halogenoalkane
One carbon attached to the
carbon atom adjoining the
halogen
Secondary halogenoalkane
Two carbons attached to the
carbon atom adjoining the
halogen
Tertiary halogenoalkane
Three carbons attached to the
carbon atom adjoining the
halogen
Halogenoalkanes undergo either
substitution or elimination
Nucleophile
electron pair donator e.g. :OH-, :NH3
, CN
Aqueous silver nitrate
added to a halogenoalkane
halide leaving group combines with a silver ion to form a
silver halide precipitate
The quicker the precipitate is formed, the faster the substitution
reaction and the more reactive the halogenoalkane
AgI
(s) - yellow precipitate
AgBr(s) – cream precipitate
AgCl
(s) – white precipitate
Nucleophilic substitution with aqueous hydroxide ions
Change in functional group: halogenoalkane
alcohol
Reagent: potassium (or sodium) hydroxide
Conditions: In aqueous solution; Heat under reflux
Mechanism: Nucleophilic Substitution
Type of reagent: Nucleophile, OH-
Nucleophilic substitution with cyanide ions
Change in functional group: halogenoalkane
nitrile
Reagent: KCN dissolved in ethanol/water mixture
Conditions: Heating under reflux
Mechanism: Nucleophilic Substitution
Type of reagent: Nucleophile, :CN-
Nucleophilic substitution with ammonia
Change in functional group: halogenoalkane
amine
Reagent: NH3 dissolved in ethanol
Conditions: Heating under pressure (in a sealed
tube)
Mechanism: Nucleophilic substitution
Type of reagent: Nucleophile, :NH3
Elimination with alcoholic hydroxide ions
Change in functional group: halogenoalkane
alkene
Reagents: Potassium (or sodium) hydroxide
Conditions: In ethanol ; heat
Mechanism: Elimination
Type of reagent: Base, OH
Aqueous: substitution
Alcoholic: elimination
Primary tends towards substitution
Tertiary tends towards elimination
Uses of Halogenoalkanes
Chloroalkanes and chlorofluoroalkanes can be used as solvents.
Halogenoalkanes have also been
used as refrigerants, pesticides
and aerosol propellants CH3CCl3 was used as the solvent in dry cleaning.
Many of these uses have now been stopped due to the toxicity of
halogenoalkanes and also their detrimental effect on the atmosphere.
(CFC’s)
caused a hole to form in the ozone
e chlorine free radical
catalyse the
decomposition of ozone, due to these reactions,
because they are regenerated. (They provide an
alternative route with a lower activation energy)
These reactions contributed to the formation of a
hole in the ozone layer.
CH2FCF3 are now used for refrigerators
and air-conditioners why
These are safer
as they do not contain the C-Cl bond.
The C-F bond is stronger than the C-Cl bond and is not affected by UV
alkenes genral fromula
CnH2n
carbon- carbon double bond
The arrangement of bonds around the
>C=C< i
planar and has the bond angle 120o
C=C double covalent bond
consists of
one sigma (σ)
bond and one pi (π) bond.
π bonds are exposed and have high
electron density. They are therefore vulnerable to attack
by species which ‘like’ electrons: the
Stereoisomerism
Stereoisomers have the same structural formulae
but have a different spatial arrangement of atoms
how ez isomers raise
There is restricted rotation around the C=C double bond.
(b) There are two different groups/atoms attached both ends of
the double
Electrophile
an electron pair accepto
Reaction of bromine with alkenes
Change in functional group: alkene dihalogenoalkane
Reagent: Bromine
Conditions: Room temperature (not in UV light)
Mechanism: Electrophilic addition
Type of reagent: Electrophile, Br+
Reaction of hydrogen bromide with alkenes
Change in functional group: alkenehalogenoalkane
Reagent: HCl or HBr
Conditions: Room temperature
Mechanism: Electrophilic addition
Type of reagent: Electrophile, H+
why hbr is polar
HBr is a polar
molecule because
Br is more
electronegative
than H. The H δ +
is
attracted to the
electron
‘Markownikoff’s Rule’
In most cases, bromine will be added to the
carbon with the fewest hydrogens attached to it
If the alkene is
unsymmetrical,
addition of hydrogen
bromide can lead to
two isomeric
products.
Reaction of sulfuric acid with alkenes
Stage 1
Change in functional group
alkene alkyl hydrogensulfate
Reagents: concentrated H2SO4
Conditions: room temperature
Mechanism: Electrophilic addition
Type of reagent: Electrophile, H2SO
Direct industrial hydration of alkenes to form alcohols
Essential conditions
High temperature 300 to 600°C
High pressure 70 atm
Catalyst: concentrated H3PO4
poly chlorophyll ethene
water proof
used make uPVC window frame
coverings and guttering
If a plasticiser is added the intermolecular forces are weakened which
allows the chains to move more easily, resulting in more flexibility in the
polymer. In this form PVC is used to make insulation on electrical wires,
and waterproof clothing.
alkene reagent
Bromine water
aldehyde reagent
Tollens’ reagent Silver mirror formed
Fehling’s solution Blue solution to red precipitate
cabroxylic acid reagents
Sodium carbonate
Effervescence of CO2
evolved
primary secondary aldehydes
sod dichromate and sulphuric acid
orange to green
chloroalkane reagents
warm with silver nitrate
slow formations of white ppt of agcl
carboxylic acid
d can be tested by addition of sodium carbonate. It will fizz and produce carbon dioxide
primary alcohol
Primary alcohols are alcohols
where 1 carbon is attached to
the carbon adjoining the
oxygen.
secondary alcohol
Secondary alcohols are alcohols
where 2 carbon are attached to
the carbon adjoining the oxygen
tertiary alcohol
Tertiary alcohols are alcohols
where 3 carbon are attached to
the carbon adjoining the oxygen
Partial Oxidation of Primary Alcohols
Reaction: primary alcohol aldehyde
Reagent: potassium dichromate (VI) solution and dilute
sulfuric acid.
Conditions: (use a limited amount of dichromate) warm
gently and distil out the aldehyde as it forms:
Full Oxidation of Primary Alcohols
Reaction: primary alcohol carboxylic acid
Reagent: potassium dichromate(VI) solution and dilute
sulfuric acid
Conditions: use an excess of dichromate, and heat
under reflux: (distil off product after the reaction
has finished)
Oxidation of Secondary Alcohols
Reaction: secondary alcohol ketone
Reagent: potassium dichromate(VI) solution and
dilute sulfuric acid.
Conditions: heat under reflux
Reaction of Alcohols with Dehydrating Agents
Reaction: Alcohol Alkene
Reagents: Concentrated sulfuric or phosphoric acids
Conditions: warm (under reflux)
Role of reagent: dehydrating agent/catalyst
Type of reaction: acid catalysed elimination
fermentation
glucose ethanol + carbon dioxide
C6H12O6 2 CH3CH2OH + 2 CO2
The conditions needed are: *Yeast *No air *temperatures 30 –40oC
The optimum temperature for fermentation is around
38oC
At lower temperatures the reaction is too slow.
At higher temperatures the yeast dies and the enzymes
denature.
Fermentation is done in an absence of air because the
presence of air can cause extra reactions to occur.
It oxidises the ethanol produced to ethanoic acid
(vinegar).
Advantages
*sugar is a renewable resource
*production uses low level technology / cheap
equipment
Disadvantages
*batch process which is slow and gives high production
costs
*ethanol made is not pure and needs purifying by
fractional distillation
*depletes land used for growing food crops
ethene
high temperature 300 °C
high pressure 70 atm
strong acidic catalyst of conc
Advantages: *faster reaction
*purer product *continuous process (which means cheaper
manpower)
Disadvantages: *high technology equipment needed (expensive
initial costs) *ethene is non-renewable resource (will become
more expensive when raw materials run out) *high energy costs for pumping to produce high
pressures
Ethanol as biofuel
A biofuel is a fuel produced from plants
d from fermentation
e any carbon dioxide given off when the biofuel is
burnt would have been extracted from the air by photosynthesis when the
plant grew. There would be no net CO2 emission into the atmosphere
fingerprint”.
This part of the spectrum is unique for every compound
Aldehydes can be oxidised to
carboxylic acids, but ketones cannot be oxidised.
Oxidation of aldehydes
Reaction: aldehydecarboxylic acid
Reagent: potassium dichromate (VI) solution and dilute sulfuric acid.
Conditions: heat under reflux
