Alkanes and alkenes

How does fractional distillation work? (Use crude oil as an example)

• Used to separate a mixture of liquids that have different boiling points

• heat crude oil to evaporate it

• gas rises up the fractional column and cools

• Column is hot at the bottom, cold at the top

• Gas condenses at different heights based on boiling point as it enters the condenser

• Liquid collected in a conical flask

• Short chains have low boiling points whereas long chains have high boiling points

  
  

What do fractions mean?

Mixture of similar length hydrocarbons

What is a hydrocarbon?

A compound that only contains carbon and hydrogen

What is crude oil?

• a mixture of different length hydrocarbons

• The fossil fuel that is produced by the high pressure that compresses the organic matters underground

What are organic molecules?

Compounds that contain carbon

What is a homologous series?

Group of organic compounds that have similar properties and the same functional group

What are alkanes?

• saturated hydrocarbons that have no double bonds (every C atom has 4 single covalent bonds)

  
  

Why are alkanes a homologous series?

• Because they are groups of organic compounds that have similar chemical properties

• They have the general formula CnH2n+2

• They have graduated physical properties

• They are in the same functional group-a group of atoms within a molecule that determines its chemical properties and reactivity (C=C)

  
  

What is a molecular formula?

Exact number and type of atoms in a single molecule

What is a empirical formula?

The lowest whole number ratio between elements (e.g Butane-C2H5)

What are isomers?

Compounds that have the same molecular formula but different structural formula

Name of alkanes and their molecular formula

• Methane CH4

• Ethane C2H6

• Propane C3H8

• Butane C4H10

• Pentane C5H12

• Hexane C6H14

Rules for the IUPAC system to name isomers

• Count longest chain to determine the parent name

• Number the carbons starting from the end closest to any branching (for the lowest possible numbers)

• One carbon branch (CH3) sticking off the main chain = methyl

• Two extra carbon branches = dimethyl

• No space in names

What is a structural formula?

Formula that shows how the atoms are arranged

Boiling point from short chains to long chains

Increases from low to high

Colour of flame for short, medium (e.g. hexane) and long chains

• Short (1-4 carbons) - clean blue flame (no smoke)

• Medium (5-12 carbons) - yellow flame (slight smoke)

• Long (13-40+carbons) - smoky yellow flame

Flammability from short chains to long chains

Decreases from high to low

Viscosity (a fluid’s resistance to flow) from short to long chains

Increases from low(runny) to high (thick)

Volatility (how easy something evaporates) from short chains to long chains

Decreases from high to low

Explain the trend of bpt/viscosity/volatility/flammability in chains

• Long chains have more intermolecular forces

• More energy is needed to need to overcome the intermolecular forces

Explain the colour of flame in chains

Long chains (Smoky yellow flame)

• There is insufficient oxygen in the air to fully react with a long chain hydrocarbon

• This causes incomplete combustion

• Therefore unburnt carbon atoms (soot) are released

• Solid carbon particles get extremely hot inside the flame and glow yellow, creating a smoky flame

Short chain (Clean Blue Flame)

• Requires less oxygen, so they undergo complete combustion

• All carbon is fully oxidised into CO2, so no soot is released, resulting in a clean blue flame

What is a fuel?

A substance that releases heat energy when burned, e.g. hydrocarbon

What is combustion?

A chemical reaction where a fuel reacts with oxygen to release energy as heat and light

Metal + oxygen in complete combustion forms…

Metal oxide

Non metal + oxygen in complete combustion forms…

Non metal dioxide

Formula of hydrocarbon burning in complete combustion

hydrocarbon + oxygen → carbon dioxide + water

Balance C9H20 + O2 → CO2 + H2O

• 1st step - 9CO2

• 2nd step - 10H2O

• 3rd step - 14O2

• C9H20 + 14O2 → 9CO2 + 10H2O

Formula of hydrocarbon burning in incomplete combustion (lack of oxygen conditions)

• hydrocarbon + oxygen → carbon monoxide + water

• Hydrocarbon + oxygen → carbon + water

• Carbon monoxide binds to hameoglobin, red blood cells start carrying carbon monoxide instead of oxygen, causes you to suffocate

How is acid rain formed?

• S + O2 → SO2

• 2SO2 + O2 + 2H2O → 2H2SO4 (sulfuric acid)

• N2 + O2 → NOx

• 4NO + O2 + 2H2O → 4HNO2 (nitrous acid)

• these can cause respiratory problems

Use of refinery gas (C1-C4)

• Domestic heating

What is petrol/gasoline (C5-C7) for?

A fuel in car

What is naptha (C8-C12) for?

• cracked to form alkenes

• Alkenes can be turned into polymers, which can make plastic

  
  

What is kerosene (C10-C16) for?

• Fuel for jet engines in aircraft

• Also called paraffin, a fuel burned for heating

What is diesel (C14-20) for?

• used as fuel in cars

• fuel in heavy duty vehicles like trucks

What is lubricating oil (C20-C50) for?

• stick to surfaces to reduce friction

• Also protects surface from rust

What is fuel oil (C50-C70) for?

Fuel for ships

What is Bitumen (C70+) for?

• waterproof so used to cover roofs

• Combine with stone chippings to help roads withstand destructive forces without significant deformation

What is a substitution reaction?

A chemical reaction where one atom in a molecule is replaced by another atom

When do alkanes react with the halogens?

Present of ultraviolet light

What are the products of CH4 + Br2 reacting in UV light?

• CH4 + Br2 → CH3Br + HBr

• A hydrogen is being substituted for a bromine

• CH3Br is called bromomethane (haloalkane)

Further substitutions : CH3Br + 2Br2 → ?

• 2Br2 - number in front of halogen represents number of substitutions

• Ch3Br + 2Br2 → CHBr3 + 2HBr

What is cracking?

• a decomposition reaction that breaks down long chain hydrocarbons into small chains

• Using a catalyst (Al2O3)

• Or 600-700 degrees heat

Why do we want small chains instead of long chains?

• Higher volatility and flammability

• Therefore easier to use as fuels

• To release energy

Describe how cracking works

• longer alkanes → smaller alkanes + alkenes

• Collect a boiling tube containing kerosene

• Place aluminium oxide in the tube

• Clamp boiling tube horizontally at a height where the end of delivery tube is under water in a tank

• Heat catalyst and kerosene alternatively to speed up reaction

• Collect gas by using three test tubes, using a plug to prevent gas escaping

• Bromine water to test if compounds contain C=C double bonds (colourless, alkenes)

What are alkenes?

• unsaturated hydrocarbons that have a double bond, which can break to allow atoms to be added to the molecule

• Added together to make polymers

• More reactive than alkanes

Why are alkenes a homologous series?

• contains same functional group (C=C)

• General formula = CnH2n

• Share similar chemical properties

• Graduated physical properties

Names of alkenes and their molecular formulas

Ethene - C2H4

Propene - C3H6

Butene - C4H8

Pentene - C5H10

Hexene - C6H12

Drawing and naming isomers of alkenes

• Swifting C=C, then shifting a methyl group

• E.g. But-1-ene (number depends on the position of double bond)

Reactions of alkenes with bromine water

• Bromine forms an orange-brown colour in water

• Addition reaction occurs, a double bond will break and the bromine atoms will be added to the alkene

• The product formed is called a dibromo(alkanes) and is colourless

• Small amount of bromine water will react with the dibromo(alkane) to form a colourless solution

• Alkanes are already saturated, no room for bromine to be added

• Therefore no reaction and solution remains orange-brown

What is an addition reaction

A chemical reaction where two or more molecules join together to form a larger molecule with no leftover products

What is an addition polymer?

• formed by many small molecules called monomers joining up together

What are rules when drawing addition polymers?

• need to draw monomers facing up and down rather than out at angles

• Repeating unit - draw the empty bonds pointing out to the left or right through the brackets

• (N)s after the brackets to present no of repeats of monomers

What do addition polymerisation require?

• high pressure and a catalyst

How to name an addition polymer?

poly(alkene)

What are problems with an addition polymer?

• Non-biodegradable because difficult to break the long chains of strong C=C double bonds

• Therefore takes up lots of space in landfill

• When burned, CO2 released and toxic CO gas in incomplete combustion

• Some addition polymers are thermosetting (doesn’t melt when heated)

• Therefore cannot be recycled