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62 Cards in this Set

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What are CFCs used for?

Used as propellants in aerosols, refrigerants, blowing agent for making expanded plastics and as cleaning solvents.

CFCs are good because:

Low reactivity, low BP, low toxicity and high stability

Problem with CFCs

Lifetime in the troposphere of approximately 100 years. when they reach the stratosphere, photo dissociation occurs to produce Cl radicals which remove ozone.

Who predicted this would happen in the early 1970s?

Sherry Rowland and Mario Molina

Proof of ozone layer damaged

in 1985, a team identified a hole in the ozone layer over Antarctic using UV spectroscopy. Computers at NASA satellites had treated the very low ozone readings as anomalies. Further proof obtained from aircraft flying over Stratosphere in Antarctica which measured the conc of ClO and O3. As O3 fell, ClO rose.

Alternatives to CFCs?

HCFCs and HFCS and alkanes

Advantages

H-C bonds are broken down in troposphere before the compounds have a chance to reach the Stratosphere




Alkanes do not contain chlorine

Disadvantages

Greenhouse gases = global warming




Are flammable and greenhouse gases

The greenhouse effect

- Solar energy reaches Earth mainly as visible and UV light


- Earth absorbs some of this energy, heats up and radiates IR


- Greenhouse gases e.g. C02 and methane in the troposphere absorbs some of this IR, in the "IR" window


- Absorption of IR by greenhouse gases increases the vibrational energy of their bonds, the energy is transferred to other molecules by collisions thus increasing kinetic energy & raising temp (bonds vibrate more vigorously)


- greenhouse gas molecules also re- emit some of the absorbed IR in all directions (in space and towards Earth) heating up the Earth

Global warming

Increases CFCs (not naturally present) and CO2, naturally present gas conc, though in small amounts (CFCS) large gwp.


Leads to enhanced greenhouse effect



Reducing human activities

Reducing consumption of fossil fuels


Using alternative energy


Increasing photosynthesis


Burying or reacting CO2

Diamond

Made from Carbon atoms.


Giant network.


Each carbon atom is joined tetrahedrally to four other carbon atoms by v. strong covalent bonds


V. strong C-C bonds and highly symmetrical network structure make diamond hard

Silicon (IV) Oxide

Four bonds


Silicon bonds covalently to four oxygen atoms


Quartz is a pure form of silicon oxide. It is an extended network of SiO4 units.



Differences in CO2 and SiO2

CO2 has a molecular structure with three atoms bonded in a linear arrangement. O=C=O.


Has weak intermolecular bonds so little energy needed to pull the molecules apart, so solid co2 sublimes at low temp. Intramolecular bonds in co2 are polar, so dissolves easily in h20.




Silicon atoms are larger than C atoms = more electrons


Unable to make double bonds


Silicon has a giant network structure, lot's of energy needed to overcome intramolecular bonds = high BP and MP



Sections of the atmosphere

Three sections:


- The troposphere, the stratosphere and the ionosphere. (closest to furthest distance from Earth's surface)


T= 78% nitrogen, 21% Oxygen, 0.93% Argon and 0.038% CO2

PPM calc

Divide by 1 million x 100

if in percentage divide by 1000

Energy reacts with matter at increasing energy

Translational energy when the molecule moves around as a whole


Rotational energy associated with the molecule rotating as a whole


Vibrational energy associated with vibrations of bonds within molecule when they absorb IR radiation


Electronic energy associated with electrons moving from one level to another due to electrons being promoted to higher energy levels due to UV and visible radiation, causes bond breaking

Vibrational & electronic energies of molecules are quantised.

Exist at certain fixed levels at specific frequencies.

Electromagnetic spectrum

(at increasing frequencies): Infrared, Visible (red- blue), UV

Electronic changes when molecules absorb radiation

When UV or visible radiation absorbed:


electrons can be excited to higher energy levels, the electrons can return to their original energy levels in time, releasing the energy that has been absorbed.


Chemical bonds can break and radicals form - photo dissociation


An electron is ejected from a molecule which then becomes ionised.



Heterolytic fission

Both electrons of the shared pair go to just on of the atoms when bond breaks - forms ions

Homolytic fission

One of the two electrons in the shared pair goes to each of the atoms. Radicals are formed (species of one or more unpaired electrons): these are very reactive

Mechanisms of a radical chain reaction

3 Stages: Initiation, Propagation and Termination. Radical reactions are v. fast and are initiated by heat or light in gaseous phase.

Initiation

No radicals at the start of this stage but radicals are formed by the end of the stage e.g. Cl2 = 2CL in presence of hv

Propagation

There are radicals at the start at the stage and new radicals are formed by the end of the stage

Termination

The reaction is terminated when two free radicals collide

Reaction of alkanes and halogens

A halogen can substitute a hydrogen in alkane chain via a radical substitution.


Radicals formed due to presence of light is photo dissociation.

Formation and destruction of ozone

In stratosphere, O2 absorb UV radiation of the right frequency to split molecule apart - photodissociation.




O2--> 2O




Ozone is formed when an oxygen radical reacts rapidly with a dioxygen molecule:




O + O2 --> O3

Ozone

highly reactive.


destroyed by reacting with radicals present in the stratosphere.




X + O3 --> XO + O2


XO + O --> O2 + X


O + O3 --> 2O2




X is a radical and can be OH (formed from water), NO (internal combustion engines)or Cl (breakdown of CFCs used in cleaning solvents, refigerants or aerosol propellants). Acts as a catalyst - catalytic cycle. (it is recycled) This how one molecule of Cl can remove 1 million ozone molecules.




NB: equation repeats itself and ozone is lost from the system

Why depletion of ozone is a problem?

Ozone absorbs UV region radiation. This is damaging to the skin and can cause skin cancer. Ozone is thinning is some parts of the world, incidences of skin cancer is increasing.

Ozone in trophosphere

ground level. Irritant, toxic gas which weakens immune system. Some is formed by action of sunlight of primary pollutants in photochemical smogs, causing breathing problems

Dynamic equilibrium

When the rate of the forward reaction is the same as the rate of the backward reaction in a closed system. Concentration of reactants and products remain constant.

When a dynamic reaction has been established

The concentration of reactants and products remain unchanged.


The forward and backward reactions do not stop, they continue at the same rate.

Reversible reaction

If it has both a forward and backward reaction.

Where does a chemical equilibrium occur?

In a closed system.

Steady state

In an open system, equilibrium does not occur but will eventually reach a point where ozone is being produced as fast at is being used up. Concentration stays the same.




Model ans: In an open system, C02 g moves away from surface

Position of equilibrium

Used to describe one set of equilibrium concentration for a reaction.




If most of the reactants become products before the reverse reaction increases it lies to the right and VV

Le Chatelier's principle

The position of the equilibrium can be altered by changing the concentration of solutions, the pressure of gases or the temperature.




states "if a system is at equilibrium, and a change is made in any of the conditions, then the system responds to counteract the change as much as possible"

Concentration

increasing reactants --> equilibrium shifts the the right (decreases reactants), but products increased


Increasing products --> equilibrium shifts to the left (decreases products)

Pressure

Increased pressure --> shifts to the side with fewer gas molecules


Decreasing pressure --> shifts to the side with more gas molecules




Yield increases or decreases

Temperature

Increased temperature --> Position of equilibrium shifts in the direction of the endothermic reaction


Decrease in temperature --> shifts to the direction of exothermic reaction.

Rate of reaction affected by

Concentration, pressure, catalyst, particle size, temperature, surface area, intensity of radiation

Reactions occur when:

particles of reactants collide with a certain minimum kinetic energy

Higher temp for collision

A much higher proportion of colliding particles sufficient energy to react and more particles are able to overcome the activation enthalpy barrier




Model ans: Rate is greater when temp is higher as at higher temp more particles have energy therefore more particle collisions per unit of time and more collisions have total energy of successful collisions

Higher conc and pressure for collision

The particles are closer together and encourage more frequent collisions. (more collisions per unit of time) - increases rate

Smaller particles for collisions

Larger surface area on which the reactions can take place, so there is a greater chance for successful collisions. More particles can bond to a surface per unit of time.

Heterogeneous catalysts for collisions

Provide a large surface area to volume ratio where more successful collisions take place as it provides a surface onto which reactants are adsorbed

Activation enthalpy

Is the minimum kinetic energy required by a pair of colliding atoms or molecules before a reaction will occur.




The energy needed to overcome the energy barrier is called the activation energy barrier.

Enthalpy profile

Show how the enthalpy changes as a reaction proceeds. *insert image*

Effect of temperature on rate

As temperature increases so does the rate of a chemical reaction. this is because of the distribution of energies among the reacting particles - Maxwell - Boltzmann distribution




Reactions go faster at higher temperature because a larger proportion of the colliding particles have the minimum AE needed to react

Role of Catalysts

Provide an alternative reaction pathway for the breaking and making of bonds. This alternative path has a lower activation enthalpy then the uncatalysed pathway.




Products produced quicker.

Homogeneous Catalysts

The reactants an catalyst are in the same physical state. Remains chemically unchanged.

Homogeneous catalyst in terms of the formation of intermediates

Shown as two humps on enthalpy diagram.


1) The activation energy barrier is overcome and an intermediate is formed


2) The intermediate breaks down to give a product and reform the catalyst

Explain why reactions between atoms may occur more quickly at the top of the troposphere than middle of the stratosphere even though temp is the same (2)

The concentration of gas is higher in the troposphere/ more pressure so more collisions per second

Suggest why, even though reaction has a low activation enthalpy, it still occurs slowly in the atmosphere (1)

The conc on reacting particles are low therefore fewer collisions

Polluting gases and their sources (2)

CO2 generated from electricity, power stations, fermentation of waste. This is a greenhouse gas


Carbon monoxide is toxic and causes photochemical smog formed from incomplete combustion


Nitrogen oxide is toxic/ greenhouse effect/ smog


Methane from growing rice/ livestock farming

Why CFCs e.g. CFCl3 not broken down in troposphere and how CFCs contribute to breakdown of ozone in the stratosphere (4)

- Bonds are too strong to be broken in the troposphere


- Bonds are broken in stratosphere uder influence of high energy UV radiation


- To form radicals (Cl.)


- Radicals catalyse the breakdown of ozone

Why C-F bonds not broken in stratosphere (2)

UV radiation does not have enough energy

Suggest a meaning of half curly arrow (1)

Movement of one electron

Chloromethane causes ozone depletion in stratosphere and suggest why bromomethane has a lower ozone depleting potential (5)

Chloromethane is not broken down in troposphere but is photodissociated in the stratosphere at high energy UV which causes breakdown of Chlorine atoms and products of chloromethane catalyse ozone breakdown.




C-Br bond is weaker than the C-Cl so can be broken in troposphere

Why presence of ozone in the stratosphere is important to humans and the natural processes where ozone is formed in the atmosphere (7)

Filters any type of UV, Radiation occurs at high frequency and causes skin cancer and can damage eyes, the immune system and crops. Oxygen molecules are split and dissociate to form oxygen atoms. UV radiation causes the formation of oxygen radicals which react with O2 for ozone.

Disadvantage of build up of tropospheric zone

Causes photochemical smog/ breathing problems/ respiratory problems