Unit 3: Section 2- Alkanes and Halogenoalkanes Flashcards
What is the general formula of alkanes?
CnH2n+2
What is the general formula of cycloalkanes?
Cn2n
What one word could you use to describe alkanes?
saturated
What does saturated mean?
it only contains single bonds
What is petroleum in other words?
crude oil
How could you describe a petroleum fraction?
a mixture of hydrocarbons with a similar chain length and boiling point range
What does petroleum consist of?
a mixture consisting of mainly alkane hydrocarbons
Describe how fractional distillation of crude oil works (4)
- oil is vapourised to about 350 C
- the vapourised crude oil goes into a fractionating column and rises up through the trays. the largest hydrocarbons don’t vaporise at all, because their boiling points are too high- they just run to the bottom and form a gooey residue
- as the crude oil vapour goes up the fractionating column, it gets cooler. because the alkane molecules have different chain lengths, they have different boiling points, so each fraction condenses at a different temperature. the fractions are drawn off at different levels in the column
- the hydrocarbons with the lowest boiling points don’t condense, they’re drawn off as gases at the top of the column
In a fractionating column, where would you find long chain hydrocarbons?
at the bottom of the column
In a fractionating column, where would you find short chain hydrocarbons?
at the top of the column
What would be a use of one of the short chain hydrocarbons in fractional distillation?
camping/ fuel gas
At what temperature would petrol be formed at in fractional distillation?
40 C
What could bitumen be used for?
road surfacing
Briefly describe vacuum distillation
Low pressures lower the boiling point of long chain hydrocarbons, this allows heavier fractions to be further separated without high temperatures which could break them down
Define cracking
the conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
What is the general reaction formula when breaking down high Mr alkanes?
high Mr alkanes –> smaller Mr alkanes + alkenes + (hydrogen)
Why does cracking need to be done at a high temperature?
it is a chemical process involving splitting strong covalent bonds
Name three economic reasons for cracking
- shorter chains more in demand
- make use of excess larger hydrocarbons
- products of cracking more valuable
What are the two types of cracking?
thermal
catalytic
What conditions are needed for thermal cracking?
high pressure (7000kPa)
high temperature (up to 1000C)
What does thermal cracking produce?
produces mostly alkenes e.g. ethene used for making polymers and ethanol
can sometimes produce hydrogen- used in Haber process
What conditions are needed for catalytic cracking?
slight or moderate pressure
high temperature (450 C)
zeolite catalyst
What does catalytic cracking produce?
branched and cyclic alkanes, aromatic hydrocarbons
What can the products of catalytic cracking be used for?
used for making motor fuels
What are the advantages of catalytic cracking compared to thermal cracking?
cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used
catalyst also speeds up reaction, and time=money
Why are alkanes useful as fuels?
burning small amounts releases lots of energy
What is the downside of burning alkanes and using them as fuels?
they produce lots of pollutants
What are the products of complete combustion?
CO2 H2O
What two dangerous products can form in incomplete combustion?
CO
C
Why is CO toxic?
CO molecules bind to the same sites on haemoglobin molecules in red blood cells as oxygen molecules, so oxygen cant be carried around the body
Why is C (soot) toxic?
can cause global dimming
cause breathing problems
can build up in engines
What do catalytic converters do?
they remove CO, NO and unburned hydrocarbons from the exhaust gases, turning them into less harmful chemicals, CO2, N2, and H2O
How do catalytic converters have a large surface area?
They have a ceramic honeycomb coated with a thin layer of catalyst metals e.g. rhodium
What happens when sulphur in fossil fuel is burnt?
sulphur dioxide will dissolve in atmospheric water and can produce acid rain
Why is acid rain so harmful?
it can destroy trees and vegetation, as well as corroding buildings and statues
Name a substance high in sulphur content
coal
How can sulphur dioxide be removed from power station flue gases?
flue gas desulphurisation
gases pass through scrubber containing basic calcium oxide, which reacts with the acidic sulphur dioxide in a neutralisation reaction to form calcium sulfite
What is formed in flue gas desulphurisation?
calcium sulfite
How does nitrogen oxide form in a car engine?
N2 and O2 react due to the high temperatures and pressure in a car engine
Why is NO and NO2 so toxic?
NO- can form acidic gas NO2
NO2- toxic and acidic, can form acidic rain
Name three greenhouse gases
carbon dioxide
methane
water vapour
Describe the mechanism of the greenhouse effect
UV wavelength 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
What is the main greenhouse gas?
water vapour
Why have CO2 levels risen in recent years?
increased burning of fossil fuels
Define free radical
a particle with an unpaired electron
How do free radicals form?
when a covalent bond splits equally, giving one electron to each atom
What type of reaction is halogens reacting with alkanes?
photochemical reaction
What do chlorine and methane react to form?
chloromethane
What is the overall reaction of chlorine and methane?
CH4 + Cl2 –> CH3Cl + HCl
What type of mechanism is the reaction for chlorine and methane?
free radical substitution
What are the three steps in free radical subsitution?
- initiation
- propagation
- termination
What essential condition is needed for initiation in free radical substitution?
UV light
What happens to chlorine in the initiation stage?
the Cl-Cl bond is broken by homolytic fission, so each atom gets one electron from the covalent bond, forming free radicals
Why is chlorines bond broken in preference to the others?
it is the weakest bond
What happens in the propagation stage?
the chlorine free radicals are very reactive and remove a H from the methane, leaving a methyl free radical
the methyl free radical reacts with a Cl2 to produce the main product (CH3Cl) and another Cl free radical
What happens in the termination stage?
the free radical form stable molecules
What happens if two free radicals join together?
they make a stable molecule, the two unpaired electrons make a covalent bond
What are CFcs?
halogenoalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine atoms
How are halogenoalkanes named?
chloro-
iodo-
bromo-
What classifies a halogenoalkane as primary, secondary or tertiary?
the number of carbons attached to the C-X functional group
What are the two types of reaction that halogenoalkanes can undergo?
substitution
elimination
What is a nucleophile?
an electron pair donor
Give three examples of nucleophiles that can react with halogenoalkanes
OH-
CN-
NH3
What happens in a nucleophilic substitution reaction?
a nucleophile reacts with a polar molecule by kicking out the functional group and taking its place
What does the rate of substitution reactions depend upon?
the strength of the bond between the carbon and the halogen
Which is the strongest and weakest C-X bond in the halogens?
strongest- C-F
weakest- C-I
Define hydrolysis
the splitting of a molecule using water
How can water react with halogenoalkanes?
water is a poor nucleophile, but it can react slowly with a halogenoalkane in a substitution reaction
How can halogenoalkanes react to form alcohols?
react with warm, aqueous hydroxide ions e.g. sodium or potassium hydroxide
Why is important that aqueous solutions of hydroxides are used in nucleophilic substitution?
if the solvent is changed to ethanol, then an elimination reaction occurs
How can nitriles be formed?
by reacting halogenoalkanes with cyanide ions
must be warm
usual reagent: KCN dissolved in ethanol
What conditions do you need to form amines from halogenoalkanes?
warm a halogenoalkane with excess ethanolic ammonia under pressure
Why can the yield of amine decrease after a nucleophillic substitution reaction?
further substitution reactions can happen between the halogenoalkane and the amines formed leading to a lower yield of amine
How can you stop the yield of amine decreasing in a substitution reaction?
by using excess ammonia
What is an elimination reaction?
the removal of a small molecule (often water) from the organic molecule
What is the change in functional group in an elimination reaction with a halogenoalkane and alcoholic hydroxide ions?
the halogenoalkane forms an alkene
What are the conditions needed for an elimination reaction?
in ethanol, heat
What does under reflux mean?
heating a substance without loosing any of it
What type of bond forms in an elimination reaction?
C=C
What are the two types of reaction that can occur when a halogenoalkane is reaction with a hydroxide?
nucleophilic substitution
elimination
What reaction occurs when a halogenoalkane reacts with a hydroxide in aqueous conditions?
nucleophilic substitution
What reaction occurs when a halogenoalkane reacts with a hydroxide in anhydrous conditions?
elimination
What is a use of chloroalkanes and chlorofluoroalkanes?
solvents
Why is the naturally occuring ozone (O3) layer in the upper atmosphere beneficial?
it filters out much of the sun’s harmful UV radiation
Why is ozone in the lower atmosphere considered a pollutant?
it contributes to the formation of smog
Explain how CFCs form a hole in the ozone layer
Chlorine atoms are formed in the upper atmosphere when energy from UV radiation causes Cl-Cl bonds in CFCs to break.
The chlorine free radical catalyses the decomposition of ozone due to these reactions because they are regenerated, and O3 becomes 02.
