3.2 Alkanes Flashcards
Describe the industrial method for fractional distillation
- Oil is pre-heated
- then passed into column.
- The fractions condense at different heights
- The temperature of column decreases upwards
- The separation depends on boiling point.
- Boiling point depends on size of molecules.
- The larger the molecule the larger the van der waals forces
- Similar molecules (size, bp, mass) condense together
- Small molecules condense at the top at lower temperatures
- and big molecules condense at the bottom at higher temperatures.
Describe Fractional Distillation In the laboratory
• Heat the flask, with a Bunsen burner or electric
mantle
• This causes vapours of all the components in the
mixture to be produced.
• Vapours pass up the fractionating column.
• The vapour of the substance with the lower boiling
point reaches the top of the fractionating column
first.
• The thermometer should be at or below the boiling
point of the most volatile substance.
• The vapours with higher boiling points condense
back into the flask.
• Only the most volatile vapour passes into the
condenser.
• The condenser cools the vapours and condenses to
a liquid and is collected.
What is Cracking. And the general formula for Cracking
Cracking: conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
High Mr alkanes → smaller alkanes+ alkenes + (H2)
What are some of the economical reasons for cracking ( list three)
• The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions.
• To make use of excess larger hydrocarbons and to supply demand
for shorter ones, longer hydrocarbons are cracked.
• The products of cracking are more valuable than the starting
materials (e.g. ethene used to make poly(ethene), branched alkanes
for motor fuels, etc.)
What are the two types of cracking?
- Thermal Cracking
* catalytic cracking
Outline the conditions for thermal cracking ( 2 conditions) and what type of products do they produce?
Conditions:
•High pressure (7000 kPa)
•High temperature (400°C to 900°C)
•produces mostly alkenes e.g. ethene used
for making polymers and ethanol
•sometimes produces hydrogen used in the
Haber Process and in margarine manufacture.
C8H18 → C6H14 + C2H
Outline the conditions for catalytic cracking (3
conditions) and what type of products do they produce?
Conditions:
•Slight or moderate pressure
•High temperature (450°C)
•Zeolite catalyst
Produces branched and cyclic alkanes and aromatic hydrocarbons
Used for making motor fuels
Branched and cyclic hydrocarbons burn more
cleanly and are used to give fuels a higher octane
number
Cheaper than thermal cracking because it saves
energy as lower temperatures and pressures are used
write an equation for complete combustion of CH3SH and how can it lead to air pollution
•CH3SH+ 3O2 → SO2 + CO2 + 2H20 •SO2 will dissolve in atmospheric water and can produce acid rain. •Coal is high in sulfur content, and large amounts of sulfur oxides are emitted from power stations.
How can SO2 be removed from industrial furnaces during combustion ( a reaction with something )
SO2 can be removed from the waste gases from furnaces (e.g. coal fired power stations) by flue gas desulfurisation. The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction.
SO2 + CaO → CaSO3
Write two equations for the production of Nitrogen oxides in an internal combustion engine and why does it occur.
Nitrogen Oxides NOx
•Nitrogen oxides form from the reaction between N2 and O2 (Nitrogen and oxygen come from the air) inside the car engine.
•The high temperature and spark in the engine provides sufficient energy to break strong N2 bond
- N2 + O2 → 2NO
- N2 + 2O2 → 2NO2
Environmental consequences of Nitrogen oxides (formed when N2 in the air reacts at the high temperatures and spark in the engine)
- NO is toxic and can form acidic gas NO2
* NO2 is toxic and acidic and forms acid rain
Environmental consequences of carbon monoxide
toxic- causes respiratory problems
Environmental consequences of carbon dioxide
Contributes towards global warming
Environmental consequences of unburnt hydrocarbon
Contributes towards formation of smog
Environmental consequences of carbon soot
Global dimming and respiratory problems
