organic chem; alkanes Flashcards
Alkanes
Are saturated hydrocarbons
Hydrocarbon
Molecule containing ONLY hydrogen & carbon
General formula for cycloalkanes
CnH2n
Cycloalkane
Are saturated & are a functional group isomer of alkenes
Where are alkanes found?
In crude oil
Crude oil
It’s a mixture of different length hydrocarbons
Fractional distillation
Vaporised oil enters the column & rises through the trays — the longest hydrocarbons don’t vaporise & run down to the bottom
Describe the column of fractional distillation
The column has a temp gradient — its cooler at the top. As the vapour rises, parts of the mixture condense at different temps
Why do some hydrocarbons condense at different temps?
As they have different chain lengths (& boiling points)
Fractional distillation for the shortest hydrocarbons
They come off as a gas at the top of the column
Products of fractional distillation (in order)
Gas — stove gas
Petrol — cars
Kerosene — jet fuel & heating
Diesel oil — fuel
Fuel oil — ships & power stations
Cracking
It’s the process of breaking a longer chain alkane to shorter hydrocarbons
Types of cracking
Thermal & Catalytic
Thermal Cracking
Conditions: high temp (1000˚C) & high pressure (70 atm)
Products: mainly alkenes
What are the products of thermal cracking used for?
Alkenes are used to make polymers (e.g. plastics)
Catalytic cracking
Conditions: High temp (450˚C) & slight pressure used & zeolite catalyst
Products: Mainly aromatic hydrocarbons, useful in fuels
Why is a catalyst used for catalytic cracking?
As it lowers the temp & pressure needed for cracking for — lowers cost & speeds up the process
Complete combustion of alkane
Alkanes burn in oxygen completely to produce carbon dioxide & water
Why are alkanes a good fuel?
Are good fuel as most burn readily to produce large amounts of energy
Uses of alkanes
Used to power vehicles & electricity
Incomplete combustion of alkenes
When alkanes burn with a limited supply of oxygen, carbon monoxide & carbon (soot) is produced
Effects of carbon monoxide
It’s poisonous as it bonds to haemoglobin in the blood & prevents oxygen bonding — can be removed using a catalytic converter
Effects of soot
Can cause breathing problems, makes buildings dirty & clog ups engines
Product of burning fossil fuel
Carbon dioxide — greenhouse gas
Greenhouse effect
This is where carbon dioxide absorbs infra red radiation (heat) from the sun but emit some of it back in to earth
Effects of photochemical smog
Harms the respiratory system in animals & damages plants
Formation of ozone
Ozone occurs at the lowest level of the atmosphere which exists as sunlight, hydrocarbons and nitrogen dioxide mix to from ozone
How is a photochemical smog formed?
Formed when solid carbon particulates & ozone mix
Formation of nitrogen dioxide gas
Oxide of nitrogen & oxygen found in the air combine under high pressure & temp — car engines provide these conditions
Importance of catalytic converters
Help to reduce the amount of unburnt hydrocarbons & oxides of nitrogen going into the atmosphere
Effect of burning fossil fuels
Can release sulfur dioxide which contributes to acid rain
Effects of acid rain
Causes damages to plants, kills fish & causes erosion of buildings
How is sulfur dioxide produced?
Some fossil fuels contain sulfur based impurities, so when burned the sulfur reacts with oxygen producing sulfur dioxide
Formation of acid rain
Sulfur dioxide is an acidic gas so when it reacts with water in the atmosphere it forms sulfuric acid, which falls as acid rain — nitrogen oxides have the same effect
Wet scrubbing
Process that removes sulfur dioxide from flue gases by using an alkali to neutralise the sulfur dioxide in flue gases
What substances does wet scrubbing use?
Involves dissolving calcium carbonate or oxide in water & spraying it on acidic sulfur dioxide gas
Stages of free radical chain reaction
Initiation
Propagation
Termination
