>>3.3 - Alkanes✔ Flashcards
What affect does the UV light have on the mechanism for chlorination of alkanes?
*Frequency of UV light causes Cl₂ bond to break apart (homolytic fission.)
In general terms describe the process of chlorination of alkanes:
*Alkanes react with halogens in the presence of UV light to produce halogenoalkanes.
*The UV light breaks down the halogen bonds producing reactive intermediates called free radicals, these attack the alkanes resulting in a series of reactions: Initiation, propagation and termination.
Explain how photochemical smog is caused:
*Caused by pungent, toxic gasses and minute solid particles suspended in the air.
*Smog is formed when NOₙ, SO₂ and unburnt hydrocarbons react with sunlight, also includes carbon atoms produced from combustion with a very limited O₂ supply.
What is the name of the mechanism for chlorination of alkanes?
*Free radical substitution (only ever happens with alkanes.)
What are the conditions and reagents for the mechanism for chlorination of alkanes?
*UV Light.
*Alkane + Chlorine OR Alkane + Bromine.
How are free radicals shown and what is a radical?
*Using a dot e.g: Cl *
*Radicals are very reactive species.
Chlorine and Methane:
What takes place in initiation in the mechanism of chlorination of alkanes?
- The halogen is broken down into 2 radicals.
- Cl₂ → 2Cl *
Chlorine and Methane:
What takes place in propagation in the mechanism of chlorination of alkanes?
- Chain reaction, P1 makes hydrogen halide, P2 makes halogenoalkane.
- P1: CH₄ + Cl* → *CH₃ + HCl
- P2: *CH₃ + Cl₂ → Cl * + CH₃Cl
- Overall Equation: CH₄ + Cl₂ → CH₃Cl + HCl
Chlorine and Methane:
What takes place in termination in the mechanism of chlorination of alkanes?
*Two radicals join to end the chain reaction and form a stable product.
- *CH₃ + *CH₃ → C₂H₆
- Cl* + Cl* → Cl₂
- Cl* + *CH₃ → CH₃Cl
Give the order of substances extracted as you go down the fractionating column:
*Refinery gases.
*Petrol.
*Naphtha (petrochemicals, chemical feedstock.)
*Kerosene (jet fuel.)
*Diesel.
*Heavy fuel oil (boat fuel.)
*Waxes.
*Bitumen.
What is cracking?
*A process where less useful long chain hydrocarbons are broken down into more useful products that lack availability and are in high demand.
*Involves breaking c-c bonds.
What is a benzene group?
*C₆H₆
*Aromatic compound.
Thermal Cracking:
What is the raw material, temperature, pressure, time, catalyst, products and use of products?
- Crude oil.
- 1000-1200K.
- 70atms.
- 1s.
- None.
- Alkenes and short chain alkanes.
- Manufacture of chemicals e.g: naphtha and manufacture of polymers from alkenes.
Catalytic Cracking:
What is the raw material, temperature, pressure, time, catalyst, products and use of products?
*Crude oil.
*800K.
*1atms.
*2-4s.
*Aluminium silicate zeolite.
*Branched and cyclo-alkanes and aromatic compounds.
*Fuel for motor vehicles.
What is the test for unsaturated hydrocarbons and what are the results for alkenes and alkanes?
*Add bromine water and shake.
*For alkenes the orange solution turns colourless.
*For alkanes the orange solution remains orange.
What are the health issues surrounding photochemical smog?
*Irritation to eyes and severe lung disease.
Explain the structure of a catalytic converter and what do they do?
*Honeycomb (giving large SA) clay structure coated in one of three metals: platinum, rhodium or palladium acting as catalysts.
*Thin layer makes it cheaper and uses less natural resources and increases SA.
*CO and NOₙ can both be removed from systems by converting them to more stable products such as CO₂ or H₂O.
What can CO₂ / SO₂ / NO₂ / NO all be classified as?
*Acidic non-metal oxides.
How can SO₂ be removed from flue gasses?
*Needs to be reacted with a base, either CaO or CaCO₃
Give the two possible equations for the removal of SO₂ from fumes:
- CaO + SO₂ → CaSO₃
- CaCO₃ + SO₂ → CaSO₃ + CO₂
What can calcium sulphite be used for?
*CaSO₃ can be used to make plaster boards.
Deduce the equations for the formation of: CO₂, CO, NO and NO₂
- C + O₂ → CO₂
- 2C + O₂ → 2CO
- N₂ + O₂ → 2NO
- N₂ + 2O₂ → 2NO₂ (2NO + O₂ → 2NO₂)
What is the origin, conditions causing it to form and the pollution caused of the following?
≫CO₂:
≫CO:
≫C:
≫NOₙ:
≫SO₂:
≫Hydrocarbons:
≫H₂O:
*C in fuel, excess O₂, global warming.
*C in fuel, limited O₂, toxic gas.
*C in fuel, very limited O₂, particles exacerbate asthma.
*N₂ in air, extremely high temperatures, acid rain + photochemical smog causing respiratory problems.
*S from fuel impurities, acid rain.
*Fuel compounds that remain unburned, react with NOₙ to form ground level ozone which causes respiratory issues.
*H in fuel.
Deduce the equations for the formation of: HNO₃ and H₂SO₄, both of which are acid rain:
- 2NO₂ + H₂O + 0.5O₂ → 2HNO₃
- SO₂ + H₂O + 0.5O₂ → H₂SO₄
Deduce the equation for converting carbon monoxide into safer carbon dioxide:
- 2CO + O₂ → 2CO₂
Deduce the equation for turning carbon + nitrogen monoxides into safer products in a catalytic converter:
- 2CO + 2NO → 2CO₂ + N₂
Deduce the equation for turning unburnt octane and nitrogen (II) oxide into safer products in a catalytic converter:
- C₈H₁₈ + 25NO → 8CO₂ + 9H₂O + 12.5N₂
What are the affects of acid rain?
*pH of soil.
*pH of waterways.
*Corrosion of CaCO₃ and metals.
Deduce the equations for H₂SO₄ on limestone, HNO₃ on limestone and H₂SO₄ on iron:
- H₂SO₄ + CaCO₃ → CaSO₄ + H₂O + CO₂
- 2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂
- H₂SO₄ + Fe → FeSO₄ + H₂
What are the conditions for complete combustion and incomplete combustion?
*Plentiful O₂
*Lack of O₂
What is the equation for complete combustion of ethane?
- C₂H₆ + 3.5O₂ → 2CO₂ + 3H₂O
Complete Combustion:
As we increase chain length by 1 carbon what happens to the amount of O₂?
*Increases by 1.5 moles.
Incomplete Combustion (CO):
As we increase chain length by 1 carbon what happens to the amount of O₂?
*Increases by 1 mole.
Incomplete Combustion (C):
As we increase chain length by 1 carbon what happens to the amount of O₂?
*Increases by 0.5 moles.
What is the equation for incomplete combustion of ethane?
- C₂H₆ + 2.5O₂ → 2CO + 3H₂O
What are the two possible products for incomplete combustion?
- CO + H₂O
OR - C + H₂O
Why is CO dangerous?
*Carbon monoxide is a toxic gaseous product that has no odour or colour.
Briefly describe how the internal combustion engine works:
*A small amount of fuel with a large excess of air is drawn into a combustion chamber, the mixture is compressed and ignited with an extreme temperature spark, the mixture burns explosively forcing movement of the engine parts.
How are NOₙ’s formed in the internal combustion engine?
*Due to the pressure and extreme temperatures the nitrogen in the air combines with oxygen producing a series of nitrogen oxides.
What is the equation for the formation of nitrogen (II) oxide?
- N₂ + O₂ → 2NO
What is the equation for the formation of nitrogen (IV) oxide?
- N₂ + 2O₂ → 2NO₂
How is SO₂ formed in the internal combustion engine?
*Sulphur originates from impurities in crude oil which end up in the fuel, when burned with oxygen it reacts to form sulphur dioxide.
What is the equation for the formation of sulphur dioxide?
- S + O₂ → SO₂
What is the general formula of an alkane?
*CₙH₂ₙ₊₂
Why do unbranched alkanes have a higher melting point?
*Molecules can pack closer together so there are more and stronger VDW forces.
Why do branched alkanes have a lower melting point?
*Molecules can’t pack close so VDW forces have to act over a greater distance so they are weaker and less of them.
What are the states of matter up to hexane?
*CH₄ (g)
*C₂H₆ (g)
*C₃H₈ (g)
*C₄H₁₀ (g)
*C₅H₁₂ (l)
*C₆H₁₄ (l)
What is crude oil?
*A mixture of hydrocarbons.
*It can be separated into separate molecules by fractional distillation.
What is the definition of a fraction?
*A group of compounds which have similar boiling points and are removed from the same level of the fractionating column.
Describe the process of fractional distillation:
*Crude oil is heated until it boils.
*The molecules evaporate.
*The gas particles rise up the fractionating column.
*A range of compounds of similar BP’s condense at different parts of the column.
*The temperature gradient of the column means that fractions of the lowest BP condense higher up the column except for the gasses.
What happens as you move down the fractionating column?
*Temperature increases.
*Size of hydrocarbons increases/MP+BP of hydrocarbons increases.
*Viscosity increases.
*Flammability decreases.
Describe the solubility of alkanes:
*Alkanes are insoluble in water but are soluble in organic solvents.
What is petroleum?
*A mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.
Suggest one important substance manufactured on a large scale from propene
Polypropylene
Define hydrocarbons
A compound of carbons and hydrogens ONLY
What is the equation for Pb in free radical substitution?
*h/c radical + halogen -> hydrogen radical + halogenoalkane
What is the equation for Pa in free radical substitution?
*Alkane + Halogen radical -> hydrogen halide + h/c radical
What is Homolytic fission?
*It is a reaction where 1 molecule breaks into 2 of the same molecule
What is free radical substitution?
*It is a series of reactions which allows Alkanes to react with halogens or halogenoalkanes to form a hydrogen halide and a new halogenoalkane
