3.3.2 Alkanes knowledge Flashcards
what is an alkane?
saturated hydrocarbon that contains only C-H and C-C single bonds
what does saturated mean?
compound contains single bonds only
what is a hydrocarbon?
compound contains carbon and hydrogen only
what is surface contact?
the extent to which 2 molecules are touching one another
what is crude oil?
a fossil fuel (non-renewable resource); mixture of hydrocarbons with different chain lengths
what is a fraction?
a mixture of molecules with a similar size/boiling point
what is a zeolite?
catalyst for catalytic cracking; complex aluminosilicates, and are large lattices of aluminium, silicon and oxygen atoms carrying a negative charge.
what is a fuel?
something that can be used to produce useful energy
why do Different alkanes have different boiling points?
the boiling point is determined by the carbon-chain length and the number of branches (side-chains)
how does C-chain length impact the boiling point of an alkane?
- The boiling point of alkanes increases with increasing C chain length
- alkanes have induced dipole-dipole intermolecular forces
- the strength of these attractions increases with carbon chain length
- as a larger molecule will have more electrons and more electrons means a greater intermolecular force
how does the number of branches impact the boiling point of an alkane?
- The boiling point of alkanes decreases with increasing number of branches
- increased number of branches means the molecules have less surface contact so the induced dipole-dipole intermolecular forces are weaker
what is the main source of hydrocarbons?
crude oil
what is fractional distillation of crude oil used for?
to separate hydrocarbons based on their differing boiling points, as different hydrocarbons have different uses.
what is the process of fractional distillation of crude oil?
- fractionating column is cooler at top than bottom
- larger molecules have higher boiling points
- larger molecules therefore condense at the bottom of the column
- small molecules condense at the top
what is cracking used for?
- can be used after fractional distillation to further refine crude oil
- breaks larger fractions (for which supply exceeds demand) into smaller fractions (for which demand exceeds supply); the purpose of cracking is to make alkenes/shorter chain molecules
how does cracking make shorter chain molecules?
Cracking involves breaking C-C bond in long alkanes to make shorter alkanes
what are the 2 types of cracking?
thermal and catalytic
what are the coniditons needed for thermal cracking?
Uses high temperature and pressure
what is the temperature range used for thermal cracking?
700-1200K
what is the pressure used for thermal cracking?
7000kPa
what does thermal cracking produce?
Thermal cracking always produces one alkane, with all other molecules being alkenes
what are the conditions needed for catalytic cracking?
Uses high temperature (lower than thermal), slight pressure, and catalyst
what is the temperature used for catalytic cracking?
720K
what is the pressure used for catalytic cracking?
100kPa
what is the catalyst used for catalytic cracking?
zeolite
what does catalytic cracking produce?
Catalytic cracking produces a high percentage of branched alkanes and cycloalkanes
what are cycloalkanes used for?
cycloalkanes are widely used in motor fuels
when does complete combustion occur?
in a plentiful supply of oxygen
when does incomplete combustion occur?
in a limited supply of oxygen
what do alkanes undergo complete combustion to form?
CO2 and H2O
what do alkanes undergo incomplete combustion to form?
CO(g) or C(s), and H2O
why is production of CO dangerous?
it is highly poisonous
what makes CO poisonous?
it prevents haemoglobin in red blood cells from binding with oxygen
what polluting chemicals are produced by the combustion of alkanes?
- CO2 is a greenhouse gas
- H2O is a greenhouse gas
- CO is a poisonous gas
- Solid C damages the lungs
how is sulfur dioxide produced in the combustion of alkanes?
Most crude oil deposits contain sulfur as an impurity, so some sulfur is present in most hydrocarbon fuels; when the fuel is burned, the sulphur also burns, producing sulphur dioxide
give the equation for the formation of sulfur dioxide?
S(s) + O2(g) ⇒ SO2(g)
why is sulfur dioxide dangerous?
Sulphur dioxide gas dissolves in rainwater, forming a very acidic solution of sulphuric acid - acid rain
give the equation for the formation of acid rain from sulfur dioxide
SO2 + 0.5O2 + H2O ⇒ H2SO4
why is acid rain dangerous?
Acid rain erodes buildings and statues, kills plants, trees, and fish
how are nitrogen oxides produced?
At high temperatures, like in car engines, some of the nitrogen in the air reacts with oxygen to produce nitric oxide, nitrogen dioxide, and dinitrogen tetroxide
give the equation for the production of nitric oxide
N2(g) + O2(g) ⇒ 2NO(g)
give the equation for the production of nitrogen dioxide
NO(g) + 0.5O2(g) ⇒ NO2(g)
why are nitrogen oxides dangerous
Nitrogen dioxide dissolves in rainwater, to form an acidic solution of nitric acid - acid rain
give the equation for the formation of acid rain from nitrogen dioxide
4NO2 + 2H2O + O2 ⇒ 4HNO3
how are unburnt hydrocarbons produced as pollutants?
some of the hydrocarbon fuel is vaporised in the engine, but escapes before it is burned
why are unburnt hydorcarbons dangerous
Unburnt hydrocarbons are toxic and can cause cancer is breathed in
what are 2 Methods to reduce pollution
- Flue Gas Desulphurisation
- Catalytic Converters
how does flue gas desulpharisation work?
- Any gases produced by power stations are called flue gases
- In desulphurisation, the acidic sulfur dioxide gas is neutralised by an alkali
what are 2 compounds used in flue gas desulpharisation?
calcium oxide and calcium carbonate
how is calcium oxide used in flue gas desulphurisation
A mixture of calcium oxide and water is sprayed into the flue gas which reacts to make solid calcium sulphate.
give the equation for the use of calcium oxide in flue gas desulphurisation
CaO(s) + 2H2O(l) + SO2(g) + 0.5O2 ⇒ CaSO4.2H2O(s)
how is calcium carbonate used in flue gas desulphurisation
The flue gas is passed through calcium carbonate, which produces solid calcium sulphate.
give the equation for the use of calcium carbonaye in flue gas desulphurisation
CaCO3(s) + 0.5O2 + SO2 ⇒ CaSO4 + CO2
how can the byproduct of flue gas desulphurisation be used
Solid calcium sulphate can be sold to make builders’ plaster
how do catalytic converters work
Catalytic converters are designed to convert some of the more harmful gases present in car exhausts into less harmful ones
describe the structure of a catalytic converter
- It has a honeycomb structure made of platinum and rhodium
why does a catalytic converter have a honeycomb structure
- the honeycomb structure increases the effective surface area, increasing the reaction rate
what are 3 compounds removed by catalytic converters
nitrogen monoxide, carbon monoxide, unburnt hydrocarbons
give an equation for the removal of carbon monoxide using a catalytic converter
2CO + 2NO ⇒ N2 + 2CO2
give an equation for the removal of a hydrocarbon using a catalytic converter
C8H18 + 25NO ⇒ 8CO2 + 9H2O + 12.5N2
