halogenoalkanes and alkanes past paper questions Flashcards
state two general features of a homologous series of compounds
- chemically similar or chemically the same or react in the same ways
2, sane general formula
- same gradual change in physical properties
explain what is means by the term fraction as applied to fractional distillation
- similar boiling points
- similar CHAIN length
describe the temperature gradient in the column
- higher temperature at the base
state one economic reason for cracking petroleum fractions
more useful products
crude oil is separated into fractions by fractional distillation
Outline how different fractions are obtained by this process
- crude oil is vapourised at high temps of 350C
- the vapours passes into the fractionating column
- vapours cool and condense at trays that are at or below their boiling points. They are therefore separated by their boiling points
explain why catalytic cracking of crude oil fractions is commercially important
- smaller molecules are in high demand
- and are more useful and valuable
the pollutant gas NO is found in the exhaust gases from petrol engines
Explain why NO is formed in petrol engines but is not readily formed when petrol burns in the open air
NO is formed by a reaction between N2 and O2 from the air (1)
OR N2 +O2 → 2NO
High combustion temperature or spark in the engine (1)
provides EA or sufficient heat/energy to break N≡N (1)
the pollutant gas CO is also found in the exhaust gases from petrol egines
Explain how CO and NO are removed from the exhaust gases and why the removal of each of them is desirable
Need to remove NO as forms acid rain or toxic product or causes
respiratory problems (1)
2NO + O2 → 2NO2 (1)
4NO2 + O2 + 2H2O → 4HNO3 (1)
Need to remove CO as it is poisonous (1)
Catalytic converter (1)
uses Pt / Rh / Pd / Ir (wrong answer cancels a correct one) (1)
Provides active sites / reduces EA (1)
Forms N2 + CO2 (1)
2NO + 2CO → N2 + 2CO
state why hydrocarbons from heavy fractions are cracked and explain why these hydrocarbons are less easy to ignite than those in petrol
Demand for heavy fraction: low or for petrol: high (1)
Supply of heavy fraction: high or of petrol: low (1)
larger Mr are less volatile/have higher bp (1)
due to stronger intermolecular forces (1)
what is the mechanism involved in cracking
free radical substitution
describe briefly how fractions can be used to convert crude oil into a small number of fractions
heated / vaporised / boiled
passed into column / tower
condense at different heights / liquefy at different heights
similar molecules (size, bp, mass) condense together / (1)
small molecules at the top and big molecules at the bottom
the residue from the fractional process distillation is usually vacuum distilled
This process allows the compounds in the residue to boil at temperatures much lower than their normal boiling points
State whether the sizes of the molecules of the compounds are smaller or larger than those in the other fractions and suggest why the process of vacuum distillation is used
larger (1)
reduces decomposition
what is important to remember about molecules that contribute to the greenhouse effect
their bonds are polar, water vapour would contribute to the greenhouse effect but N2 would not (nitrogen)
rather than climate change what should you say instead
global warming NOT climate change
x is branched therefore it is smaller/ it has less surface area
this means that the van der Waal forces between the molecule would be weaker
what is important to remember about free radical substitution
in free radical substitution, it is important to remember that the free radical is on the carbon
However, mark schemes do not seem to care unless your draw out the full structure
