OCR AS Level Chemistry

For all those studying this cause too! Good Luck and I hope you find this useful!

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Reactions of Halogenoalkanes

Halogenoalkanes can undergo nucleophilic substitution due to the electron deficient carbon atom.

Therefore, it can also under hydrolysis when reacted with an aqeous solution of hot OH- ions.

Mechanism:

The hydroxide ion has a lone pair which is attracted to the carbon and donates itself to it. This forms a new covalent bond between the OH- and the carbon and breaks the C-Halogen bond by heterolytic fission. Therefore a Halide ion is formed.

·   Lone pair attracts and donates itself to C atom

·   New covalent forms between OH and C

·   C and halogen bond breaks by heterolytic fission

·   Halide ion and alcohol formed

Rate of Hydrolysis:

The rate of hydrolysis depends on two things: bond enthalpy & polarity

The more polar a bond is, the more the carbon atom is able to attract the nucleophile and react quicker.

The higher the bond enthalpy, the harder the bond can be broken.

However, bond enthalpy is more important than bond polarity. Therefore, the rate of reaction decreases on going down the group as the bond enthalpies do.

Halogen-containing polymers

PTFT aka Teflon: A polymer of tetrafluoroethene.

Carbon-fluorine bonds are very strong because of the high bond enthalpy.  Therefore, the PTFE is inert and resistant to chemical attack. Therefore this is why it is able to be non-stick and heat resistant.

    PVC aka Poly(vinylchlorine): A polymer of chloroethene

PVC is used for drainpipes, plastic window frames. Sports equipments etc.

Chlorofluorocarbons, CFCS:

CFCs were used as blowing agents in expanded polystyrene foam, refrigerants and solvents for dry cleaning. However, they were found to have lead to the depletion of the Ozone layer due to the Chlorine radicals CFCs can break down into. Therefore, they have been replaced with alternatives.

e.g. HFCs- hydrofluoroalkanes or HCFCs- hydrofluorohydrocarbons. However, these too can deplete the ozone layer but at a tenth of that of CFCs.

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Alkenes

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.

This is because the bond enthalpy of C=C bond is less than twice of C-C bond.

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Hydrocarbon in Crude Oil

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

Less energy to over come the weaker intermolecular forces

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Carbon Dioxide

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.

i.e. altering the pressure so that only the toxic materials are removed from the mixture.

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Catalytic Converter

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 products are then desorbed and diffused away.

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The Internal Combustion

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.

When humid & sunny, conc. of low level ozone can build up...

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Regulating CFCS

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. 

However scientists now have also revealled that HCFCs and HFCs also contribute to global warming

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Ozone Depletion

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

 Overall: O3 + O ---> 2 O2

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The Ozone-Oxygen Cycle

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.

O3 + O --> 2O2

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The Ozone Layer

Ozone:

Ozone is a molecule containing 3 oxygen atoms: O3.

Ozone can be both a good and bad thing within our environment. Whether it is good/bad, depends on where it is found.

GOOD = Found in the  Stratosphere (upper atmosphere) & protects living organisms from harmful UV rays.

BAD = Found in the Troposphere (lower atmosphere) & is an air pollutant. It has harmful effects against our respiratory system.

The Ozone Layer:

It is found in the Stratosphere (upper atmosphere) and is responsible for filtering out all UV-c rays and 95% of UV-b rays. These types of rays have short wavelengths which are harmful to the human body.

The ozone is able to convert the UV rays into heat and so the ozone layer is warmer than the rest of the stratosphere.

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Global Warming & Methods to Reduce It

The effects of global warming include:

melting ice caps

more frequent/destructive storms and floods

river overflow

extreme heat waves

Cause of Global Warming:

Essentially, our need for energy from burning coal/natural gas/oil. This process emits an enormous amount of CO2- a greenhouse gas. Therefore, increasing the amount of the greenhouse effect and raising the Earth's temperature. 

i.e. Burning fossil fuels --> more CO2 emitted --> enhanced greenhouse effect --> increase in the surface temp. of Earth

Methods to Reduce Global Warming

There are two ways to reduce the greenhouse effect:

Alternative fuels

Carbon Capture & Storage

Alternative fuels:

Pretty standard GCSE answers. Some include using:

- Wind Turbines

- Tidal Power

- Solar Panels

- Nuclear plants

Carbon Capture & Storage:

This is when the CO2 emitted from the power stations are captured and stored away. 

The power station burns the methane from natural gas as a fuel.

CH4 + 2 O2 --> CO2 + 2 H2O

The CO2 is then separated and piped off and stored into an underground oil field. 

When the CO2 is pumped in, some of the remaining oil in the oil field- that was once previously unable to be obtained- is able to be extracted. 

Another way the CO2 can be stored is by reacting with metal oxides to form metal carbonates.

CO2 (g) + CaO (s) --> CaCO3 (s)

However, the process of storing the carbon dioxide into a carbonate is very slow and requires a large amount of energy to speed the rate of reaction.

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Global Warming Potential, GWP

Definition: 

The ability of a trace gas to cause global warming.

The GWP is also related to the lifetime of the gas within the atmosphere. 

e.g. Some CFCs are able to absorb more than CO2. However, since they are no longer produced, the effect of the CFCs are diminishing. 

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Greenhouse Gases

Greenhouses gases are naturally occuring. 

They include:

Water H2O - Evaporation from rivers/lakes/seas

Carbon Dioxide, CO2- Volcanic eruptions, respiration, combustion

Methane, CH4- Excrements, emitted during production of coal/natural gas/oil

Factors contributing to a gas's contribution to Global Warming:

Gas's ability to absorb infrared radiation

Concentration of the gas within our atmosphere

When a  molecule of a greenhouse gas absorbs the infrared radiation, it vibrates. After some time, this vibrating molecule emits radiation which then triggers another molecule of the greenhouse gas to absorb the radiation.

In H2O, the O-H bond is responsible for absorbing the radiation.

In Methane, the C-H bond is responsible.

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Greenhouse Effect

Definition: 

The process in which the absorption and subsequent emission of infrared radiation by atmospheric gases warms the lower atmosphere and the planet's surface.

What is the Greenhouse Effect?:

The Sun emits energy in the form of electromagnetic radiation to the Earth. The radiation is mostly absorbed  by the Earth or reflected back into space.

The absorbed radiation is used to heat up  the Earth, raising the temperature of our planet to the surface temp  of 13 celsius. 

The Earth then emits infrared radiation back into space. However this time, there are some gases- aka greenhouse gases- that do absorb this type of radiation and reflect it in all directions (including back towards Earth). 

Why do we need the Greenhouse effect?:

This process is considered to effectively trap most of the heat in the lower atmosphere.

The effect creates an equilibrium where 

rate of absorption by surface/atmospheric gases = rate of Earth emitting energy

This enables the Earth to have a constant temperature.

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Hydrocarbons From Crude Oil

Hydrocarbons: Compounds made up of only carbon and hydrogen atoms

Alkanes are saturated hydrocarbons: C-C single bonds only.

Crude Oil

Crude oil is a mixture of hydrocarbons -mainly alkanes. As crude oil itself is hard to ignite, it must be separated into its flammable components.

Fractional distilation is used to separate the components as they have different boiling points.

Fractional Distillation: A method used to separate a mixture of liquids with different melting/boiling points.

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Basic Concepts of Organic Chemistry

Definitions

General Formula: The simplest algebraic formula for a compound in a homologous series.

Structural Formula: The minimal detail to show the arrangement of atoms of an element in a compound.

Displayed Formula: The relative position of atoms of an element in a compound and the bonds attached to them

Skeletal Formula: The simplest organic formula in which the hydrogen atoms are removed from the alkyl chains leaving a carbon skeleton.

Homologous Series: A group of compounds with the same functional group with each successive member differing by -CH2.

Functional Group: A group of atoms that are responsible for the characteristic reactions of a compound

Structural Isomer: Compounds with the same molecular formula but different structural formulae

Stereoisomers: Compounds with the same molecular formula but have a different arrangement atoms in space.

E/Z Isomers

E/Z isomers are a form of stereoisomerisation. They occur due to the restricted rotation around the C=C double bond.

i.e. A Carbon to Carbon DOUBLE bond must be present.

They also require two different functional groups surrounding each carbon atom.

A E isomer has each of the groups on opposite sides of the Carbon atoms.

A Z isomer has each of the groups on the same side of the Carbon atoms.

A special case of E/Z isomerisation is Cis/Trans Isomers.

Cis/Trans Isomers

Cis/Trans isomers not only require a C=C double bond but also require one of each of the groups attached to C atom to be a HYDROGEN atom.

The cis isomer is a variation of the Z isomer.

The trans isomer is a variation of the E isomer.

Remember, the C=C double bond can be placed on the first two carbon atoms as well!

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Patterns in First Ionisation Energies

Across a Period

e.g. Li --> Ne

The First I.E. tend to gradually increase going across the period. This is due to the fact that all the elements in the period all have the same complete quantum shells. Therefore, they have similar electron shielding. 

However, the nuclear charges of the elements increase as each time one more proton is added to the nucleus. Therefore, there is an increase in the electrostatic forces of attraction and ionisation energy.

Down a Group

e.g. Li --> Na

There is a gradual decrease in First I.E. on going down a group. This is due to the increase in electron shielding and distance in atomic radius (the distance between outershell electron and the nucleus) outweighing the fact that there is an increase in nuclear charge.