C6.2 Flashcards
What are hydrocarbons?
Compounds that only contain hydrogen and carbon
Why do alkanes form a homologous series?
The same general formula and they are all saturated :
Cn H2n+2
List some alkanes
Methane - 1
Ethane - 2
Propane - 3
Butane - 4
Displayed formulae of alkanes
Carbon atoms form 4 covalent bonds and hydrogen atoms form one covalent bond
Butane : H - C - C - C - C -H (and the upper bit too)
| | | |
H H H H
Complete combustion of alkanes
Carbon atoms oxidised to form CO2
Hydrogen atoms oxidised to form water
Incomplete combustion of alkanes
Water is still formed BUT carbon is oxidised to carbon monoxide and this is a problem because CO is toxic (some carbon atoms may not be oxidised at all and release carbon atoms - soot)
Alkenes homologous series
Have a carbon=carbon double bond making them UNSATURATED
CnH2n
How to model alkenes?
On the far right have the double bond and above the carbon atom that starts the double bond (there is no hydrogen atom)
How do alkenes react?
Just like alkane combustion
BUT they have a functional group (an atom or a group of atom that allows them to take part in some reactions and in this case it is a carbon-carbon double bond) -> allows them to undergo addition reactions
Addition reaction
Alkenes react with other molecules to form a larger product
Test for in saturation :
Alkene + bromine -> dibromoalkene
CARBON CARBON DOUBLE BOND BREAKS
Bromine only reacts with alkenes because they are unsaturated (decolorises the bromine water)
Also undergo addition reaction with hydrogen : forms alkanes (presence of nickel catalyst)
Ethene + hydrogen -> ethane
What is crude oil?
Crude oil is a fossil fuel and it was formed from the remains of marine organisms that lived millions of years ago - buried deep in the sea bed. It is a finite resource and that is why it is non - renewable (being used up at a faster rate than it is being formed) - only replenished on a time scale of a million years
How to make crude oil useful?
Crude oil is a mixture of alkanes that are separate from each other because they have different boiling points - larger hydrocarbons = stronger intermolecular forces = higher boiling point
How does fractionating distillation work?
Crude oil is heated and vapours are piled in the bottom of a fractionating column (temperature gradient) = hot at bottom and cold at the top, cools as it rises through the column and they condense into the liquid state if they reach a part that is cool enough.
Parts of crude oil (fractions)
LPG - refinery gases (lowest boiling point - dont condense, just leaves as gases)
Petrol - liquid
Paraffin - liquid
Diesel - liquid
Heating oil - liquid
Fuel oil - liquid
Bitumen - highest boiling point (solid at room temperature)
What is cracking?
Cracking is a chemical reaction that converted large alkane molecules into smaller alkane molecules and alkenes
Octane -> hexane + ethene
How is it done in oil refineries?
Heating oil fractions to a high temperature and passing them over a hot catalyst of alumina or silica - covalent bonds between atoms in large alkane molecules BREAK - can be done in classroom with a broken pot as catalyst
Why is cracking carried out?
Large alkane molecules do not flow easily and are difficult to ignite - not useful as fuels (on the other hand, fractions;with smaller alkane molecules have lower boiling points and can flow easier + easier to ignite => useful as fuels)
Why do oil refineries carry out cracking?
Help oil refineries match its supply of useful products with its customers’ demand for them - also produces alkenes which are useful for making polymers
Why is crude oil important?
Vital in our lives + main source of fuel and feedstock for chemical industry
