Unit 2 : The Atmosphere

Diamond Covalent Structure

Each C atom is covalently bonded to 4 other C atoms. Tetrahedral shape, and forms a crystal lattice structure. It's hard. Good thermal conductor as vibrates easily. H.m.p.t. Can't conduct electricity, insoluble in solvents.

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Silicon Oxide

Giant lattice. Covalently bonds with 4 O atoms. tetrahedral shape. Crystal lattice. O atoms only bond with 2 Si atoms. Hard crystalline solid, h.m.p.t. Insoluble in solvent. Doesn't conduct electricity.

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Difference between the structures of CO2 and SiO2.

CO2=small molecules, each C forms double bond with O atoms. Gas @ r.t. as weak i.d-i.d forces. Dissolves in H2O. SiO2=giant lattice. Si atom forms single bonds with 4 O atoms. H.b.p.t. insoluble.

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What affects rate of reaction.

Concn=Particles closer together, collide more -more chances to react. Pressure=Particles closer together, collide more-more chance of reacting. Surface area=More particles come in contact with other reactants, -speedier reaction.

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Activation enthalpy

Minimum amount of kinetic energy particles need to react

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Qualitative effect of temperature changes on rate of reaction

Particles have more kinetic energy, more likely to react when they collide.

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Homogenous catalysts

Form one+ intermediate compounds which the products are formed from. The activation enthalpy needed to form this is lower. It's enthalpy profile will have two humps in it.

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Dynamic equilibrium

The rates of the forward and backward reaction is equal. The concentration of reactants and products are constant. This takes place in an enclosed system.

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Le Chatelier's Principle

If there's a change in concentration, temperature or pressure, the equilibrium will move to help counteract the change.

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Le Chatelier's Principle = Temperature and Concentration

Concn=reactant-shifts to right, makes more product. Product-shifts to left, makes reverse reaction faster. Temp=Increase-shifts to endothermic direction, absorb heat. Decrease-shifts to exothermic direction, replace heat.

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Le Chatelier's Principle = Pressure + Catalysts

Increase=shifts to side with fewer gas molecules, reducing pressure. Decrease=shifts to side with most gas molecules, raises pressure. Catalysts=No effect, can't increase yield-but equilibrium reached faster.

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Gases present in the atmosphere

Nitrogen=78%, Oxygen=21%, Argon=1%, Carbon Dioxide=0.035%. Oxides of N=car engine, Oxides of S=burning fossil fuels, CO=Incomplete combustion of hydroC

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Calculating values for composition in parts per million

% x 10,000

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The depletion of Ozone due to halogenoalkanes

Chlorine free radicals form when CFCs are broken down by UV. Radicals=catalysts, react with ozone to form an intermediate + O molecule. The Cl molecule regenerates, + attacks other ozone molecules. R+O3-->RO+O2, O'+RO-->O2+R

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Heterolytic Fission

2 different substances are formed, a +ve cation and a -ve anion.

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Homolytic Fission

2 electrically uncharged 'radicals' are formed. Radicals have an unpaired electron, thus making them very reactive.

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Initiation reactions

Sunlight gives energy to break the Cl-Cl bond=photodissociation. Bond splits equally, each atom gets 1 electron=homolytic fission. Atom is highly reactive free radical.

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Propagation reactions

Cl' attacks a methane molecule. New methyl free radical attacks another Cl2 molecule. Which attacks a new molecule and so on.

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Termination reaction

If 2 free radicals join together, it makes a stable molecule.

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How did scientists realise the problem caused to the Ozone?

Recent measurements of ozone over Antarctic lower than previous measurements, thought instruments faulty. High-altitude flights in converted spy plane, satellite data confirmed their hypothesis.

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Advantages of CFCs

Unreactive. Non-flammable. Harmless.

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Advantages + Disadvantages of replacement compounds for CFCs

HCFCs-broken down in atmosphere in 10-20 yrs, smaller effect GHG!!! HFCs-don't contain chlorine, GHG!!

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UV radiation breaks Carbon-Halogen Bonds

Carbon-Halogen bond splits homolytically=2 free radicals. Carbon-iodine bond most likely to break, carbon-fluorine bond least likely, as has the highest bond enthalpy.

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How does ozone act as sunscreen

Ozone layer removes all UVC radiation and about 90% of the UVB. UVB damages the DNA in cells causing skin cancer, + main cause of sunburn.

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Polluting effects of ozone in the troposphere

In heavily industrialised areas, ozone mixes with solid particles of carbon and many other substances causing photochemical smog. Which affects the lungs, and can trigger asthma attacks.

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The suns radiation

Gives out radiation cause of its nuclear processes in its core. Mainly gives out visible and infrared radiation, along with small amounts of UV.

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Molecules change in vibrational energy states

Some molecules absorb energy from IR, this extra energy makes their covalent bonds vibrate. Only molecules of diff atoms abosrb IR as polarities change when they vibrate.

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Frequencies of radiation correspond to particular amounts of energy absorbed

Gas molecules have certain fixed energy levels called quantised levels. A bonds energy can only jump from 1 level to another, so diff molecules absorb diff frequencies of radiation.

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UV and visible radiation give electrons more energy

When UV/visible radiation hit a molecule of gas, the electrons can absorb the energy and jump up to their next energy level, the energy is quantised so only specific frequencies are absorbed, if enough energy's absorbed, bonds break=free radicals.

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Energy from radiation calculation

∆E=hv, Energy=Planck's constant (6.63x10^-34) x frequency

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Greenhouse Effect

Solar energy reaches Earth mainly as visible/UV. Earth absorbs some of this energy, heats up and radiates IR. GHG in troposphere absorb some of this IR in the 'IR window'. Absorption of IR by GHG increase vibrational energy of bonds,

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Greenhouse Effect 1

This energy transferred to other molecules by collisions, thus increasing their kinetic energy and raising the temp. GHG also re-emit some of the absorbed IR in all directions, some heats up the earth. ^ conc of GHG ^greenhouse effect.

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Different approaches to control carbon dioxide emissions

Burn fewer fossil fuels, increase photosynthesis, and bury/react carbon dioxide.

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Hydrogen Bonds form

Only happens when hydrogen covalently bonded to F2, N2, O2, as they're electronegative so draw bonding electrons away from H. Bond is polarised and H has high density charge so weak H bonds with lone pairs form.

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What to look for to see if a H bond is present

-OH or -NH.

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Hydrogen bonds make high boiling points

Lots of energy is needed to overcome intermolecular forces and hydrogen bonds present.

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Hydrogen bonds mean some polymers dissolve in water

If OH present, strong hydrogen bonding so insoluble. Polymers are insoluble when very many/very few internal H bonds, soluble when an intermediate number of H bonds.

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Aldehydes

Carbonyl group at end of Carbon Chain e.g. CH2CH2CHO

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Ketones

Carbonyl group anywhere in the chain e.g. CH3COCH3

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Carboxylic acids

Contain -COOH

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Primary structure of alcohols

The carbon with the -OH attached is attached to one alkyl group e.g. propan-1-ol

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Secondary structure of alcohols

The carbon with the -OH attached is attached to two alkyl groups e.g. propan-2-ol

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Tertiary structure of alcohols

The carbon with the -OH attached is attached to three alkyl groups e.g. 2methylpropan-2-ol

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Heating under reflux for reactions involving volatile liquids

1) Heat ethanol with potassium dichromate(VI) solution + sulfuric acid in test tube, =ethanal. 2) Heat excess alcohol with a controlled amount of oxidising agent so aldehyde distilled off. 3) Mix alcohol with excess oxidising agent under reflux.

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Reflux

Increase the temperature of an organic reaction to boiling without losing volatile solvents, reactants or products.

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Elimination reaction

Small group of atoms break away from larger molecules. 1) Ethanol vapour passed over hot catalyst of pumice stone 2) Or reflux ethanol with excess concentrated sulfuric acid at 170 degrees,

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Addition polymerisation

The double bonds in alkenes can open up and join together to make long chains - polymers.

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Electrophiles

Electron-pair acceptors.

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Addition

Double bonds open up and atoms are added to the carbon atoms

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Carbocation

Positively charged cation

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E/Z isomerism

C=C bonds can't rotate when there are 2 different groups on each carbon. E=opposite sides of bond. Z=same sides of bond. E=Trans, Z=Cis.

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A polymers use depends on its properties

Poly(ethene)=bags, bottles. Poly(propene)=bottle crates, rope. Poly(chloroethene)=water pipes, building material. Poly(tetrafluoroethene)=coating 4 frying pans. Polystyrene=expanded polystyrene, insulator.

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Thermoplastic

No cross-linking between chains. Weak intermolecular forces. Easy to melt and easy to remould.

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Thermoset

Covalent cross-links. 3D giant covalent structure. Too much heat makes it char. Strong, hard, rigid and insoluble.

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Co-polymer

Made from more than one type of polymer in random order.

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Get Revising

Unit 2 : The Atmosphere

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