A Level Chemistry - Organic Chemistry - Topic 2 - Alkanes
Is incomplete combustion more likely to occur with short or long chain alkanes?
Long chain hydrocarbons because they require more oxygen to burn efficiently.
What is the process of fractional distillation?
1. The crude oil is heated and the vapours are pumped into the bottom of a distillation column. 2. The vapours rise up the column, cooling as they do so. 3. Different hydrocarbons vapours condense to liquids a different point (when they reach a level which is at a temperature below their boiling point). 4. The liquids run out of the column along pipes.
What is the equation for flue gas desulfurisation?
2CaO(s) + 4H2O (l) + 2SO2 (g) + O2 -> 2CaSO4.4H2O(l)
What is free-radical substitution (formation of halogenoalkanes)?
A 3 step reaction mechanism where a halogen replaces a hydrogen atom in an alkane molecule.
What is a zeolite catalyst?
A catalyst made of mainly silicon dioxide and aluminium oxide with an enormous surface area and honeycomb structure. They are also acidic.
What conditions are needed for thermal cracking?
A high temperature between 700 and 1200K and a high pressure of up to 7000kPa.
How are halogenoalkanes formed?
A mixture of an alkane and a halogen is put in front of a form of UV light, where they then react to form a halogenoalkane.
What is petrol made out of?
A mixture of hydrocarbons (alkanes) with an approximate chain length of 8.
What is gypsum (calcium sulfate)?
A saleable product used to make builder's plaster and plasterboard.
How does flue gas desulfurisation work?
A slurry of calcium oxide and water is sprayed into the flue gas causing the sulfur dioxide to react with them to form calcium sulfite which can be further oxidised to form calcium sulfate (gypsum).
What conditions are needed for catalytic cracking?
A temperature of around 720K, and a zeolite catalyst.
What is the polarity like in alkanes?
Alkanes are almost non-polar because there is no significant difference in electronegativity between the H and C atoms.
What is the trend in solubility for alkanes?
Alkanes are not soluble in water. This is because the water molecules are held together by hydrogen bonding which are much stronger than the van der Waals forces and thus the van der Waals forces cannot overcome these forces meaning that alkanes do not dissolve. However, alkanes do dissolve in other non-polar solvents.
What are the chemical properties of alkanes?
Alkanes are relatively unreactive. They do not react with acids, bases, oxidising agents, and reducing agents. Alkanes however are easily combustible and react with halogens. Alkanes burn in a plentiful supply of oxygen to form carbon dioxide and water (or in a restricted supply to form carbon monoxide).
What is a carbon-neutral activity?
An activity that produces no carbon dioxide emissions overall.
What products are formed from thermal cracking?
An alkane and an alkene.
How does a catalytic converter work?
As polluting gases pass over the rhodium/platinum catalyst, they react with each other to form less harmful products.
What properties change in the alkanes as we go down the fractioning column?
As we go down the fractioning column, the alkanes boiling points increase making them more viscous, with a lower flammability and a darker colour.
How can we tell the alkane and halogen have reacted together?
At the end of the reaction between an alkane and a halogen, the mixture becomes colourless and misty white fumes are seen (from the hydrogen halide).
What physical property is used to separate crude oil?
Boiling point.
What products are formed from catalytic cracking?
Branched alkanes, cycloalkanes and aromatic compounds.
What fuel is propane used for?
Camping gas.
What greenhouse gases cause global warming?
Carbon dioxide, water vapour and methane.
What are some examples of atmospheric pollutants from alkane fuels?
Carbon monoxide, nitrogen oxides, sulfur dioxide, particulates, carbon dioxide, water vapour, unburnt hydrocarbons.
What is a catalytic converter?
Catalytic converters help remove nitrogen oxides, unburnt hydrocarbons and carbon monoxide from the car's exhaust using transition metals such as rhodium and platinum to convert them into less harmful chemicals. The catalytic converter has a honeycomb shape providing an enormous surface area so only a small amount of the metals is needed.
Why is the chain reaction in the formation of halogenoalkanes inefficient?
Chain reactions are not very useful because they produce such a range/mixture of products. They will also occur without light at high temperatures.
What type of chemical is destroying the ozone layer?
Chlorofluorocarbons (CFCs).
What equations are related to CFCs destroying the ozone layer?
Cl• + O3 -> ClO• + O2 ClO• + O3 -> 2O2 + Cl•
What is the general formula for an alkane?
CnH(2n+2) in a straight chain. CnH2n in a cyclical ring.
Why does complete separation not occur in fractional distillation?
Complete separation doesn't happen as there are hundreds of different alkane chain lengths in crude oil. The tower is not significantly different in temperature to allow each chain length to be collected separately so each fraction contains a mixture of hydrocarbons that are more similar in chain length than the mixture in crude oil.
How is crude oil formed?
Crude oil is made from plants and animals that lived millions of years ago. When they died, they fell onto the land/ in the sea and were covered in sediment. With increasing pressure and heat from the sedimentary rock, the organic matter was turned into crude oil.
What are the uses for alkanes?
Fuels, lubricants and the starting materials for a range of other compounds.
What are the three steps in free-radical substitution?
Initiation, propagation, termination.
What fuel is methane used for?
Natural gas.
How are CFCs destroying the ozone layer?
Once CFCs reach the stratosphere where the UV radiation is very high, the high radiation causes chlorine free radicals to be formed. These react with ozone molecules to create permanent oxygen molecules, thinning the protective ozone layer.
Why are ozone molecules in the upper atmosphere important?
Ozone molecules in the upper atmosphere absorb damaging UV rays, protecting life on Earth.
What are alkanes?
Saturated hydrocarbons (only have C-C and C-H single bonds).
What alkanes combust easily?
Shorter chain alkanes burn completely in a plentiful supply of oxygen to give carbon dioxide and water.
As well as the main products in the reaction, what other products can be formed when forming halogenoalkanes?
Some amount of an alkane can be produced at the termination stage. A dihalogenoalkane molecule can be formed at the propagation stage if a halogen free radical reacts with some halogenoalkane molecules that have already formed. With longer-chain alkanes, there will be many isomers formed because the halogen free radical can replace any of the hydrogen atoms.
Why do the C-H bonds in alkanes not break in the initiation step?
The C-H bond in the alkane needs more energy to break than what is available in a quantum of UV radiation, so this bond does not break.
What is the UV light's role in the reaction to form halogenoalkanes?
The UV lights role is to break up the covalent bonds, creating the free radicals that cause the chain reaction which forms the halogenoalkane.
What is the second step in propagation (formation of halogenoalkanes)?
The alkyl free radical goes on to react with the other halogen molecule. This creates another halogen free radical and a molecule of a halogenoalkane - a stable compound. This takes us back to the original type of free radical, ready to start the chain reaction again.
What is the trend in boiling point for alkanes?
The boiling point increases as the alkane chain gets longer as the increasing van der Waals forces are responsible for boiling point. Straight chain isomers have higher boiling points than branched chains. This is because the latter are not able to pack together so closely, so van der Waals are less effective.
What is the process of initiation (formation of halogenoalkanes)?
The breaking of the halogens covalent bond to form two halogen atoms which are free radicals and thus highly reactive. The halogen molecule absorbs one quantum of UV light which's energy is much greater than the covalent bond energy, so the bond breaks. Since the molecule is diatomic, the covalent bond breaks homolytically with one electron going to each halogen atom.
What is the function of the chlorine free radical in depleting the ozone layer?
The chlorine free radical is not destroyed in the chain reaction to form permanent oxygen molecules; thus it is a catalyst in breaking down the ozone molecules.
What is the enthalpy of combustion?
The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions, and all reactants and products are in their standard states.
What is the process of termination (formation of halogenoalkanes)?
The final step is destroying the free radicals by termination. In every case, two free radicals react to form a stable compound with no unpaired electrons. This can happen with: - Two halogen free radicals reacting to form the halogen. - Two alkyl group free radicals reacting together to form an alkane. - One halogen free radical and one alkyl group free radical react to form a halogenoalkane.
What is the process of propagation (formation of halogenoalkanes)?
The halogen free radical reacts with the alkanes in two stages creating a different free radical and another halogen free radical.
What is the first step in propagation (formation of halogenoalkanes)?
The halogen free radical takes a hydrogen atom from the alkane to form a hydrogen halide and an alkyl free radical. The alkyl group is a different free radical which can go on to react. The hydrogen halide is a stable compound and thus is no longer involved in the reaction.
Why are other products formed alongside the main products in the reaction to form halogenoalkanes?
The highly reactive nature of free radicals means they will react with any molecule they collide with.
What intermolecular force is seen between alkanes?
The intermolecular force involved is van der Waals forces which's strength is expected to increase with the increase chain length of the alkane with increased electrons.
What is the reaction to form a halogenoalkane?
The main reaction is: alkane + halogen -> halogenoalkane + hydrogen halide
What fraction is the highest in demand and why?
The naphtha fraction is huge demand for petrol and the chemical industry.
What happens when alkanes burned in a limited supply of oxygen?
The poisonous gas carbon monoxide is produced (CO) as well as carbon (soot).
Why are many of the longer chain hydrocarbon fractions cracked?
The shorter chain products are far more economically valuable than the longer chain hydrocarbons. Cracking is driven by demand.
What makes alkanes fuels?
Their high negative enthalpy of combustion which causes them to release heat energy when they undergo combustion as well as their ability to store a large amount of energy for a small mass.
What are the two methods of cracking?
Thermal cracking and catalytic cracking.
Why is propagation (formation of halogenoalkanes) a chain reaction?
These two steps end up forming a halogen halide, a halogenoalkane and a new halogen free radical which is ready to react with more alkanes and repeat the two steps. Thus, this is the chain part of the chain reaction and can potentially occur thousands of times before the free radicals are destroyed.
What properties of chlorofluorocarbons made them seem as no risk to the atmosphere?
They are chemically inert and non-toxic.
What damage can UV rays cause to humans?
UV rays can cause skin cancer by damaging DNA.
In a cracking reaction, how can we test for alkenes?
Use bromine water. If the water goes from orange to colourless, alkenes are present.
Do alkanes have a positive or negative enthalpy of combustion?
When burned in a plentiful supply of oxygen, alkanes have a large negative enthalpy of combustion. The greater the number of carbons present, the greater the heat output.
How do carbon monoxide and nitrogen oxide react in a catalytic converter?
carbon monoxide + nitrogen oxide -> nitrogen + carbon dioxide
How do hydrocarbons and nitrogen oxide react in a catalytic converter?
hydrocarbon + nitrogen oxide -> nitrogen + carbon dioxide + water
