#chainsenergy&resources

Alkenes are unsaturated hydrocarbons.

Unsaturated - contains a C=C

The Double Bond:

The Carbon=Carbon is formed from two types of bonds: 

sigma bond

pi bond

Sigma Bond: This is a type of bond present in all bonds. In an alkene it is formed between the two carbons.

Pi Bond: The pi bond is formed from the overlap of p-orbitals and forms above & below the sigma bond

It is the pi bond which provides the double bond with its reactiveness and fixes the carbon atoms in place causing restricted rotation.

Shape of the Alkene molecule:

The alkene molecule adopts the trigonal planar shape as there are three regions of electron density around the carbon atom. Therefore, the electron pairs must repel each other as far as possible. So, alkenes also have a bond angle of 120 degrees.

Reactivity of the C=C bond:

The C=C bond is more reactive than a C-C bond because of the additional pi bonds. 

Bond enthalpies show that a C=C bond is stronger than a C-C bond and that a pi bond is weaker than a sigma bond.

Hydrocarbon:A compound that only contains carbon and hydrogen atoms.

Crude oil is our main source for hydrocarbons. It is a form of fossil fuel and therefore,non-renewable.Itself cannot ignite easily but it's components can and therefore, we separate crude oil into its different fractions to use for fuel.

The process of separating these fractions is known asfractional distillation.We are able to use this process because the components of Crude Oil havedifferent boiling points.

Method for Distillation:

Crude oil is vapourized by heat

It is then passed through the column

The column is hotter at the bottom and cooler at the top. The longer chained hydrocarbons have a higher melting point and so they are found towards the bottom of the column.

This is because once the components meets a fraction at a temp lower than its boiling point than it will condense and collect at said fraction.

3. The condensed components are then tapped off from their corresponding fractions.

Components in a fractionating column:

Petroleum Gases

Gasolene

Kerosene

Diesel

Industrial Fuel Oil

Wax, Bitumen oil

Why do long chained carbon have higher boiling points?

More carbons= more points of contact

More Van der Waals forces present

Requires a larger amount of energy to overcome the increased Van der Waals

Why do branched chained molecules have lower boiling points?

Molecule more spaced out/ have less points of contact

weaker Van der Waals

CO2 can be good and bad. There are numerous uses which are advantageous for us.

CO2 in foam:

CO2 acts as a substitute for CFCs in polysterene.

CO2 as a solvent:

Solvents are used to dissolve chemical reactants/extract products. Normally, organic solvents are used however, they are volatile/flammable/toxic. And so, CO2 is used instead.

It is turned into a supercritical fluid, scCO2, where it can be used as a solvent with materials that would usually decompose at higher temps.

This scCO2 is used in decaf coffee as you can alter the pressure so that only caffine can be removed and not flavour.

CO2 in Dry Cleaning:

CO2 acts as a substitute for chlorinated hydrocarbons in dry cleaning as they are carcinogenic and can contaminate water. 

The scCO2 is used with a wetting agent where the safer solvent can remove the grease and oil from clothing.

CO2 for Toxic Waste:

 scCO2 can remove toxic waste by  the same way for making decaf coffee.

A Catalytic Converter is made from:

platinum

rhodium

palladium

They are supported on a honeycomb mesh so that it has a large surface area.  

The hot exhaust gases pass over the honeycomb mesh where the gases are converted into other products.

The Carbon Monoxide and unburnt hydrocarbons are oxidised.

2CO + O2 --> 2 CO2

C12H26 + 18.5 O2 --> 12CO2 + 13H2O

Whilst the Nitrogen Oxides are taken care of by a three-way catalyst . The NO reacts with CO to form N2 and CO2.

The CO and NO passes through over the catalyst where it is adsorped onto the surface of the catalyst.

This weakens bonds held between the NO and CO so they can react together.

The Internal Combustion Engine emits three main air pollutants:

CO

Oxides of Nitrogen

Unburnt Hydrocarbons

Carbon Monoxide: 

CO is a toxic/colourless/odourless gas. It can cause respiratory problems as it binds to the haemoglobin in our blood instead of oxygen. Therefore, the organs in the body will begin to be starved of oxygen and suffocate, leading to death.

Oxides of Nitrogen:

When the fuel is burned in the engine, air is draw in into the cylinder along with the fuel. This causes the fuel to be burnt in air creating  heat. This high temperature enables Nitrogen oxides to form.

Two types of Nitrogen Oxides are formed when this happens:

Nitrogen Monoxide, NO

Nitrogen Dioxide, NO2

NO2 is a contributer to asid rain whilst other nitrogen oxides are respiratory irritants and affects asthma.

Unburnt Hydrocarbons:

Many VOCs (Volatile organic compounds) are emitted in exhaust gas. However, the two problematic ones are:

Benzene

Buta-1,3-diene

These are found in petrol at small quantities and are carcinogenic.

Once in the atmosphere, the hydrocarbons react with NO2 to form a low level ozone. (Sunlight is required). Low level ozone is a pollutant which causes difficulty in breathing and increases contracting infections.

Montreal Protocol:

The Montreal Protocol was put in place to control the emission of CFCs around the world.

The restrictions include: 

Productions of CFCs stopped by 2000

Productions  of tetrachloromethane stopped by 2000

Production of halons (used in fire extinguishers) phased out by 2000.

Production of trichloroethane phased out by 2005

Use of HCFCs & HFCs to replace CFCs. 

Ozone Depletion Potential:

This is a scale in which measures the relative amount a substance can breakdown the ozone layer. 

The ODP of a substance is compared to the ODP of CFCl3 - Trichlorofluoromethane.

How does the Ozone Layer Deplete?:

CFCs (chlorofluorocarbons) can be broken down by UV radiation in the stratosphere. They form a chlorine radical and a CFCl2 radical.

Initiation:

CFCL3 ---> Cl・+ ・CFCl2

As radicals are rather reactive, they then immediately react with the ozone molecule.

Propagation:

Cl・ + O3 ---> ClO・ + O2

ClO・+ O ---> Cl・ + O2

Overall = O3 + O ---> 2O2

Therefore, the Cl・ acts as a catalyst in the breakdown of Ozone and creates a cycle.

A similar process occurs with Nitrogen Oxide radicals, ・NO, found in lightning and air craft engines.

・NO + O3 ---> ・NO2 + O2

・NO2 + O ---> ・NO + O2

UV Radiation:

There are three types of UV radiation that the Ozone interacts with.

UV- a (320-400nm)- Is not very damaging

UV -b (280-320 nm) - Can cause sunburn and skin cancer

UV- c (200-280 nm) - Does not reach the surface of the Earth

5% of UV-a, 95% of UV-b and 100% of UV-c is filtered out by the Ozone layer. 

Formation of Ozone:

When O2 molecules absorb UV-c waves with less than 240nm, the molecule breaks down into 2 Oxygen atoms

O2 + (radiation <240nm) ---> 2O

The Oxygen atom then reacts with the molecule to form Ozone & heat.

O2 + O --> O3 + Heat

The heat is absorbed by air molecules in the stratosphere, therefore, increasing  the temp. 

How the Ozone Layer Works:

The ozone molecules that absorb wavelengths less than 310nm break down back into O2 and O.

O3 + (radiation < 310nm) --> O2 + O

This O produced immediately then reacts with another O2 molecule, creating a cycle.

Therefore, we can say that the UV rays are converted into heat as it is a by product of the reaction. 

This cycle allows the ozone layer to remain balanced and protect organisms from the harmful UV-c.

Removal of Ozone:

When an Oxygen atom reacts with Ozone, they form two oxygen molecules. However the removal rate is slow because the conc. of O atoms is low and often reacts with O2 before O3.