7. Organic Chemistry Flashcards
What is a hydrocarbon?
A fuel; a molecule that consists of hydrogen and carbon
What is crude oil?
A fuel made from remains of marine plants and animals to form sedimentary rock. When pressure and heat was added the rock was broken down to form crude oil and natural gas
What are many of the compounds in crude oil?
Hydrocarbons - mostly alkanes
What is an alkane?
Contains a single chains of carbon atoms with hydrogen atoms bonded along the side
What is the formula for alkanes?
CnH2n+2
What is the simplest alkane?
Methane
What are the four simplest alkanes?
Methane, ethane, propane and butane
What is a molecular formula?
Gives number of atoms of each element in a molecule
What is a structural formula?
Shows the atoms molecule by molecule
What is the displayed formula?
Shows how atoms are arranged and the bonds between them
What happens as the number of carbons in a molecule increases?
Boiling point increases because more energy is needed to break the intermolecular forces
Gases in fractional distillation which have the lowest boiling point also are…
The most volatile and flammable
What is viscosity?
The resistance of flow that a liquid has
What happens in fractional distillation as molecule size decreases in terms of colour?
Becomes lighter: black, yellow, transparent
Why are hydrocarbons cracked?
Long chains are not useful
What types of cracking are there?
Thermal and catalytic cracking
Why are fractions with large hydrocarbon molecules more viscous?
Longer chains are easily entangled. The amount of intermolecular force also affects viscosity
How is thermal cracking carried out?
Long hydrocarbon chains are vaporised, and placed under high pressure and temperatures (~750c) and thermal decomposition takes place - chains are split apart
How is catalytic cracking carried out?
Long hydrocarbon chains are vaporised and passed over a hot catalyst - heat to a high temperature (~500c). Chains cracked as thermal decomposition takes place
What is a disadvantage of thermal cracking?
There is not a lot of control over what products are formed
What is an advantage of catalytic cracking?
Longer chains are split to smaller chains - about 8-10 carbons in length
What does cracking hydrocarbons produce?
A mixture of alkanes and alkenes
Why are porcelain chips used in catalytic cracking in the lab?
As a catalyst - where thermal decomposition takes place
What are alkenes used to produce?
Polymers (plastics)
What is the general formula for alkenes?
CnH2n
What bond does an alkene contain?
A double carbon-carbon bond
What is the simplest alkene?
Ethene - C2H4
What are the four simplest alkenes?
Ethene, propene, butene, pentene
An example of a saturated compound?
Alkanes
An example of an unsaturated compound?
Alkenes
What is a saturated compound?
Contains only single covalent bonds
What is an unsaturated compound?
Contains at least one double covalent bond
What is the test alkenes?
Will turn bromine water from orange to clear
Why do alkenes react with bromine water but alkanes don’t?
Alkenes have a double bond which can open up to react with bromine
What is a functional group?
The part of a molecule responsible for reactions
Example of a functional group?
The double bond in alkenes
What is a homologous series?
A series of compounds that have similar properties and the same general formula
How do the successive members of alkenes differ from each other?
As the carbon number increases by one, the number of hydrogens increases different numbers
What type of combustion do alkenes tend to undergo?
Incomplete, but some may be complete
How do alkenes burn compared to alkanes?
With a smokier, yellow flame
Which releases more energy per mole during combustion: alkenes or alkanes?
Alkanes
Why is it better to use octane in a car engine and not octene?
- octene undergoes incomplete combustion and so is very smoky and produces more pollution
- octane releases mor energy per mole so is more efficient
How do we know that halogens and alkenes undergo addition reactions?
Bromine water turning colourless
Why do addition reactions of alkenes occur?
- the double carbon bond in alkenes make them more reactive than alkanes (its electron dense)
- the double bond can open and react with other the carbons to add to the molecule
What does ethene + bromine give?
Dibromoethane
What does propene + bromine give?
Dibromopropane
What does ethene + chlorine give?
Dichloroethane
What does propene + iodine give?
Diiodopropane
How can alkenes become saturated?
By adding hydrogen
What conditions are required for the hydrogenation of alkenes?
- 60*C
* nickel catalyst
What does ethene + hydrogen give?
Ethane
Why would an alkene needed to be turned into an alkane?
To straighten the molecules - the closer they can get to each other means that they have stronger intermolecular forces
Why bother turning an alkene to an alkanae? i.e. from unsaturated to saturated
To increase melting point
Practical application of hydrogenation of alkenes?
To make margarine - enough hydrogen should be added to make it spreadable straight from the fridge - but not too little to make it a liquid
What is the process of hydrogenation?
- addition of hydrogen across the alkene’s double bond
- replaces double carbon bond to a single
- nickel catalyst at 60 degrees
- causes melting point to increase so become solid at room temperature
What is the addition reactions of hydrogen to alkenes also known as?
Hydrogenation of alkenes