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177 Cards in this Set
- Front
- Back
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What is the atmosphere? |
The atmosphere is a relatively thin layer of gas covering the Earth‟s surface to a depth ofapproximately 100km. |
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What can the atmosphere be divided by into a series of regions |
temperature variations. The temperature variations havereasons to do with physical andchemical processes occurring indifferent parts of the atmosphere. |
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What will we be looking at in the stratosphere? |
The ozone layer andprotection from UV radiation (further away from earth than the troposphere) |
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What will we be looking at in the troposphere? |
Global warming (interaction ofIR radiation with thetroposphere) troposphere closest to earth |
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What is air? |
a mixture of gases |
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what are the major components of air? |
= Nitrogen 78%, Oxygen 21% and Argon 1% |
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why is % of water vapour not stated |
very variable in amounts |
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What percentage is carbon dioxide |
0.04%, not major |
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Are there many other gases? |
yes, but in very small amounts expressed in parts per million |
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how do we turn parts per million into a percentage |
divide parts per million by 1million (6 0s) and then times by 100 |
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what are common sources of carbon dioxide |
Combustion of hydrocarbon fuels |
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methane |
Cattle farming, landfill sites |
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dinitrogenoxide (N20) |
fertilisers used on fields |
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carbon monoxide |
incomplete combustion of hydrocarbons |
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nitroxides? |
Reaction of N2+O2 during combustionin high temps of engines. |
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What absorbs UV light and what doesn't |
Glass absorbs UV light but waterand Perspex do not. Sunscreenscontain chemicals that absorbharmful UV waves. Theatmosphere protects us from muchharmful radiation from the sun . |
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What do molecules have? |
quantised energy levels (exist at fixed levels) |
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What causes changes in energies (ie move them from one fixed level to another) |
radiation interacting with matter at specific frequencies |
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What are the energy changes in the electromagnetic spectrum? |
Radio frequency, microwave, I, visible, UV, Xr rays, Gamma rays increasing frequency (Hz) from radio to gamma decreasing wavelength, though (m) |
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what do different types of electromagnetic radiation have ? |
different photons of energy associated with them |
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what happens when electromagnetic radiation interacts with matter? |
it will bring about changes associated with that matter |
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what does the type of energy change depend on? |
tpe of radiation absorbed |
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what is there between each electronic level? |
range of vibrational energy levels |
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what is there within each vibrational energy level? |
range of rotational energy levels and within rotational is tranlational |
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What is electronic energy? |
electrons moving from one energy to another |
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What behaviour does vibrational energy exhibit? |
vibration of bonds within a molecule |
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What behaviour does rotational energy exhibit? |
molecule rotates as a whole |
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What behaviour does translocational energy exhibit? |
molecule moves around as a whole As electronic energy levels are the furthest apart the highest energy radiation will be neededto excite a molecule into a higher electronic energy level. |
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What does visible or UV light cause? |
(photons with high energy) causechanges to the electronic energy levels of a molecule. |
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What does IR radiation cause? |
Infra-red radiation will cause changes to the vibrational energylevels of a molecule. As the vibrational energy levels are closer together, radiation with less energy will be required to excite a molecule to a higher vibrational energy level. |
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what does microwave radiation cause? |
Microwave radiation will cause a change to the rotational energylevels of a molecule. As rotational energy levels are very close together, it will take only very low energy radiationto cause a change in these. |
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Why is infra-red radiation associated with warming? |
The absorption of infra-red radiation causes bonds in moleculesto vibrate. This increases the kinetic energy of the molecule. Collisions pass this kinetic energy onto other molecules raisingthe average kinetic energy of a population of molecules. A rise in the average kinetic energy of molecules = a rise inTEMPERATURE. Some molecules may re-radiate IR causing more warming. |
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how can water be heated? |
(A similar argument explains how microwaves can heat water by increasing the rotational energy of each moleculeand raising the kinetic energy of the whole sample of water. Only microwaves of the correct frequency will be able tochange the rotational energy of water to cause heating |
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What does the frequency of IR needed to make a particular bond vibrate depend on? |
dependupon the strength of the bond. A strong bond will need a higher frequency ofIR to make it vibrate than a weak one |
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what is the calculation for the difference in energy levels? |
E=hv E = energy of a single photon ( in J not kJ), h = Planck’s constant, v =frequency of photon(Hz) |
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What happens when Ultraviolet radiation is absorbed by a molecule? |
UV radiation has a very high energy and will cause changes in electronicenergy levels of molecules. |
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least energy? |
An electron may besimply excited to ahigher energy leveland later return to itsoriginal level byemitting photons |
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What happens because this is not usually 1 step? |
radiation emitted is often of a lower frequency than that absorbed,sometimes leading to “fluorescence”, = UV is absorbed but frequencies including visible lightare emitted as the electron falls back to a lower level again |
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When is visible light usually emitted? |
visible light isusually only emitted by very hot objects or if high energy UV has been absorbed by acool object. |
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What happens if an electron absorbs a higher energy? |
Absorbing a higher energy may excite an electron out of a chemicalbond causing the bond to break and the molecule to break up. This iscalled homolytic fission (photodissociation). Commonly caused byhigh energy UV in stratosphere |
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What happens if very high energy is absorbed? |
If very high energy absorbed, an electron may be excited right out of the molecule.The molecule becomes a positive ion so the process is called ionisation E.g O2(g) to O2+(g) + e- |
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Why is this process not seen in the ozone? |
process is seen in the very highest layers of the atmosphere wherethe most powerful radiation from the sun arrives |
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What is a covalent bond? |
shared pair of electrons between 2 nuclei |
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What can absorbing UV or visible radiation of sufficient energy do? |
break a bond byhomolytic fission |
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what is homolytic fission |
bond breaks evenly leaving one e- with each nucleus. photodissociation=(The breaking of bonds specifically by light |
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What is heterolytic fission |
both electrons of the shared pair go to just one nucleus when the bond breaks |
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what does a whole arrow show? |
movement of 2 electrons |
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what does an arrow with half a head show? |
indicates movement of 1 electron |
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what is a radical? |
species with one unpaired electron |
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why are radicals very reactive? |
contain atoms with unfilled outer shells. They react to gain a stable filled outer shell by removing an e-from anothermolecule or atom |
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what is a biradical? |
species with 2 unpaired electrons |
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What are the tree stages of the mechanism of a radical change reaction? |
initiation propagation termination |
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What is initiation? |
a molecule absorbs radiation of correct frequency hυ (or isheated) and a bond breaks to form 2 radicals
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What is propagation? |
a radical and a molecule react forming a molecule and a newradical which is used in the next step.These step can repeat many, many times |
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what is termination? |
The reaction ends when 2 radicals join to form a stablemolecule. |
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What bond breaks when halogens are involved |
halogen bond broken, not stronger C-H or C-C bonds |
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what type of reaction is initiation? |
endothermic (bonds onlybroken |
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what type of reaction is termination? |
termination is always exothermic (bonds only made). |
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when do radical reactions often occur and charachteristics? |
Radical reactions are often explosively fast, happen in non-polar solvents or in the gas phase andrequire radiation (or heat) to start them. |
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what is reaction kinetics? |
study of rates of reaction |
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what are the main factors affecting rates of reaction? |
concentration pressure catalyst intensity of radiation surface areaa particle size temperature |
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What is the collision theory? |
Particles must collide with a minimum kinetic energy for areaction to occur. |
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why is the kinetic energy of collision coverted to potential energy? |
as bonds start to stretch and new bonds form |
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What is activation enthalpy Ea |
the minimum kinetic energy required for a pair ofcolliding particles to react successfully |
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Why does raising concentration increase rate of reaction? |
Raising concentration increases the number of particles in a givenvolume so more successful collisions per unit time. |
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larger surface area? |
Greater/larger surface area of a solid exposes more reactant particlesto collision on the surface of the solid so more collisions per unit time |
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pressure? |
Increasing pressure means more particles per unit volume so morecollisions per unit time |
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what happens to rate of reaction (for many reactions) when temperature increases by 10oC |
For many reactions a 10oC temperature rise doubles the rate of reaction.Collision theory alone does not explain this (a 10oC temp. rise is far from a doubling ofabsolute temperature) |
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What does the Maxwell Botzmann distribution show? |
shows howenergy is shared out in alarge population of particlesat a given temperature. Avery small number of particleshave very low kinetic energy, themajority have a medium energyand a few have very highenergies |
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What does the area under the curve represent? |
total number of particles |
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What does the shaded area on the right of the line show? |
number of particles that haveenough energy to collide successfully and react |
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What happens when temperature increases by 10oC? |
The average energyof the particles isincreased so themost common kineticenergy is higher and approximately twice as many particles now have enough energy for successfulcollisions ( the shaded area to the right of the activation enthalpy line has doubled |
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Why does increasing temperature increase rate of reaction? |
Increasing temperature increases the proportion of particles at orabove activation enthalpy, meaning more successful collisions per unittime, increasing rate. |
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What is a catalyst? |
substance that alters the rate of reaction without itselfundergoing any permanent change |
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how do catalysts work? |
Catalysts work by providing an alternative/different reaction route with alower activation enthalpy |
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what are homogeneous catalysts? |
catalysts are in the same physical state as the reactantsthey catalyse |
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what are heterogeneous catalysts? |
catalysts are in a different physical state to the reactants |
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What type of enthalpy reaction does a catalysed reaction have |
enthalpy profile with doublehumped shape |
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what does the first hump represent? |
activation enthalpyrequired to break bonds toform an intermediatecompound between thecatalyst and the reactant. |
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What does the dip represent? |
new bond(s) forming between thecatalyst and reactant → intermediate (formed) |
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What does the second hump represent? |
bonds in the intermediate being broken. |
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What happens next? |
There is then a decrease in enthalpy down to products as new bondsare formed |
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What is stage 1? |
Reactants + catalyst to intermediate |
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What is stage 2? |
Intermediate to Products + catalyst. |
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eg |
stage 1 Br●(g) + O3(g) to BrO●(g) + O2(g) Stage 2 BrO●(g) +O(g) to Br●(g) + O2(g) To find overall reaction just add stages 1 and 2 together andremove any species occurring on both sides of the arrow Br●(g) + O3(g) + BrO●(g) +O(g) to BrO●(g) + O2(g) + Br●(g) + O2(g) Overall equation O(g) + O3(g) to 2O2(g) Catalyst = Br●(g), intermediate = BrO●(g) |
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What is the destruction of ozone mainly caused by? |
homogeneous catalysis |
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what are the damaging rays of sunlight caused by? |
UV radiation reaching us fromthe sun. UV radiation has enough energy to break chemical bonds. |
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What is the UV spectrum split into? |
Three sections; A, B and C |
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What happens to dammage as frequency of UV radiation increases? |
Damage increases A has a lower frequency than C... |
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What is considered to be a good sunscreen? |
a chemical whichcan be spread onthe skin and thatwill absorb as muchof the UV radiation as possible before it reaches the skin |
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Where is most of the UV radiation absorbed in the atmosphere? |
in the ozone layer in the stratosphere the ozone layer acts as a sunscreen |
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What is ozone? |
O3 is a form of oxygen that is created by theaction of high energy radiation on oxygen in thestratosphere |
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what is the equation for ozone creation? |
Firstly, oxygen molecules absorb UV radiation of the correct frequency to split the molecule apart (photodissocation) O2 to 2 Oxygen atom radicals then ozone is created when an oxygen radical reacts rapidly with an oxygen molecule O + O2 to O3 |
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What are the equations for ozone destruction? |
Ozone is highly reactive it is destroyed by reacting with radicals present in the stratospher O + O3 to 2O2 O3 + hν to O2 + O (= “sunscreen reaction”) |
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What type of state usually exists in the stratosphere? |
a steady state |
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what does a steady state mean? |
rate of ozoneproduction = rate of ozone destruction, so overall the amount of ozone in thestratosphere stays constant |
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What were ozone levels in the atm like in the 1980s? |
amount of ozone in the atmosphere found to be far less thancalculated and its concentration was decreasing severely |
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What catalyses destruction of ozone? |
catalysed by chlorine radicals which are largelyproduced by the action of high energy ultraviolet light on man-madeCFC’s |
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how do chlorine radicals act? |
radicals act in a 2 step process: 1) Cl .+ O3 ClO .+ O2 2) ClO. + O Cl. + O2 ClO. is the chlorine oxide radical |
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What happens if we "add" the two equations? (reactants and reactants) to (products+products) |
Cl .+ O3 + ClO. + O ClO .+ O2 + Cl. + O2 O3 + O 2O2 Cl radical = homogeneous catalyst. |
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Why is this bad, that chlorine radical is the catalyst? |
Speeds up the reaction destroying ozone by more than 1500 times Because the Cl atom is unchanged at the end of each reaction, it cango on to catalyse more ozone destruction |
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What can bromine radical also do? |
) Br can also catalyse O3 break down but weaker C-Br bonds often broken introposphere so few Br compounds reach the stratosphere |
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What do NO do? |
NO also is an ozone depleting substance. NO is produced in the stratosphere by O atomsattacking N2O a gas produced naturally but increasingly from artificial fertilisers. |
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What are CFCs? |
Chlorofluorocarbons They are made by replacing all the H atoms in a hydrocarbon with either Cl orF. e.g CCl3CF3 1,1,1-trichloro-2,2,2-trifluoroethane. |
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What are some properties of CFCs? |
Non-flammable Non-toxic Some are easily liquefied by pressure (useful in fridges). |
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Why are they unreactive? |
strong C-F and C-Cl bonds resistingnearly all forms of chemical attack |
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What are uses of CFCs? |
refrigeration, aerosol propellants, blowing agent for foam making,degreasing solvents. |
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What are the problems of CFCs? |
CFC‟s so unreactive they are not broken down chemically in thetroposphere (the lower atmosphere).They can reach the stratosphere where high energy UV radiation breaks CClbonds creating Cl• radicals which catalyse the break down of the ozonelayer. CF3Cl + hν •CF3 + Cl• |
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What can chlorine radicals be converted to? |
HCl and ClONO2 |
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What happens in the Anarctic regions? |
in Winter,ice crystals in “polar stratospheric clouds” adsorb these 2 species which react together:HCl + ClONO2 HNO3 + Cl2 |
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What happens to the chlorine molecules? |
The Cl2 gas molecules are released from the ice crystals and become concentrated at the centre ofswirling air (vortex). When sunlight returns the vortex stops swirling and Cl2 molecules are broken downby UV into Cl• radicals. Cl2 + hν Cl•+ Cl• |
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What does the high chlorine radical concentration lead to? |
rapid depletion of ozone |
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Why is there less ozone depletion over the North pole? |
conditions are usually slightly “warmer”(!) and polarstratospheric clouds are rarer- usually preventing a strong polar vortex from forming |
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Who discovered CFCs? |
ThomasMidgley, Jr in 1930s, he created |
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What did atmospheric scientists predict? |
The destruction of ozone (they were worried about the effects of pollutants from jet engines that they might have on the atmosphere NO. + O3 to NO2.+ O2 NO2. + O to NO. + O2 |
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What did Professor Sherry Rowland predict? |
that it took decades for CFCs to break down CCl3F to CCl2F + Cl. and that Chlorine radicals reacted with ozone 1000 times faster than oxygen, and could react with 100,000 ozone molecules before undergoing a termination step |
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Who was Joe Farman and what did he do? |
JoeFarman worked for the British Antarctic Survey Hemonitored ozone concentration using a uv spectroscopeHe started in 1981 and by the endof 3 years he was convinced something was seriously wrong |
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Why did NASA not spot this? |
ozone Concentrations were so low, the computor had discarded them |
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Could we have predicted the effect of CFCs? (1970s) |
Atmospheric chemistry in infancy Too far away to be of concern Instruments not sensitive enough todetect minute concentrations 1970 James Lovelock built electroncapture detector that could measure small concentrationsCFCs found in remote locations |
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Why were Concerns raised by Sherry Rowland andMario Molina in 1974 |
New techniques and instrumentsdeveloped eg satellites, air craft and balloons reactions studied in the conditions of a lab, computor modelling made predictions |
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What can we replace CFCs with? |
HCFCse.g. CH3CFCl2 HFCse.g. CH2FCF3 |
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What are the problems with these? |
both still GHG flourine is expensive |
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Where do HFCs break down? |
C-H bonds break down in the lower atmosphere (troposphere) so thecompounds do not survive to reach the stratosphere. HFC‟s have noO3 depleting effects even in the stratosphere |
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Problem with HCFCs |
HCFC‟s may still reach the ozone and produce Cl radicals.Snag = these solve the ozone problem but create severe global warmingcontributions! |
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What are the conditions of the ozone now? |
Global agreements have virtually stopped the making of CFC‟s but the existing ones willpersist for many more years and the ozone hole will not be fully repaired until at least 2060. as CFCs take decades to break down |
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What are the problems of ozone hole? |
Increased risk of skin cancers Increase in cataracts and other eye problems.Effects on ocean plankton could disrupt food chains. as less UV radiation absorbed |
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What is the Greenhouse effect? |
UV/Visiblelight fromthe sunreachesthe Earth. Some isreflectedback intospace andsome isabsorbedby theEarth. AbsorbingUV/visibleradiationwarms up the surface. The Earth re-emits a lot of this heat energy as infra-red radiation. Most emitted IR radiation escapes into space but some is absorbed bygases in the troposphere (the lowest layer of the atmosphere) |
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What is the result of the green house effect? |
steady state where the loss of energy from the sun is balanced by the gain ofenergy from the sun- the average global temperature is near constan |
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Explain |
If gas molecules absorb IR radiation their bonds will vibrate more. This increase in vibrational energy spreads to other molecules throughcollisions. This causes the overall kinetic energy of the gas to rise. rise in kinetic energy of a gas = rise in the temperature of thetroposphere. Some molecules re-emit IR back to the ground causing morewarming. |
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What is global warming potential? |
The effectiveness of different types of gas at absorbing IR |
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What type of GWP do Nitrogen, oxygen and argon which make up the bulk of the troposphere have |
almost no GWP |
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What type of GWP does CO2 and water vapour have? |
CO2 has a global warming potential of 1, water vapour has a factor of 0.1 |
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What does this mean? |
currently most of the greenhouse effect is coming from CO2(and water vapour because there are large amounts of it in the atmosphere |
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Why is methane of increasing concern? |
produced from anaerobic decay(landfill sites), melting tundra regions and farm animals. It has a globalwarming potential of 25 and has increased by 2½ times since the industrialrevolution |
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Why is it difficult to predict effects of increased concentrations of GHG? |
Climate is affected by many variables so is difficult to predict but computer models areimproving all the time |
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What do models show? |
very strong correlation between rising greenhouse gasconcentration and a rise in measured global temperatures.The link has not been absolutely proved but most scientists accept it |
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What could the role of water vapour in the atm do? |
–hotter climates could mean more evaporation(with more H2Og leading to further global warming) but more water vapour in the air could also reflectmore radiation back into space and cause cooling. |
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What are the problems of man made gases such as CFCs? |
huge global warming potentials plusabsorb parts of the IR spectrum that waterand CO2 do not! (in the window) causing vast amounts of energy to beabsorbed by the troposphere and giving disastrousglobal warming |
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What is the "window" |
The regions of the IR spectrum that are notabsorbed by water and CO2 |
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What is dynamic equilibrium? |
when the rate of the forward and reverse reactionsare equal and concentrations stay constant in a closed system. |
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why is it dynamic? |
forwardand reverse reactions are occurring at the same rate |
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Why is it equilibrium |
unchangingamounts of the reactants and products |
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When is the “position of equilibrium” saidto lie to the right (favours products) |
when equilibrium is reached the concentration of reactants is very low andthe concentration of products is very high |
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When is the position of equilibrium said to lie to the left (favours reactants)? |
equilibrium reactant concentration is very high and product concentrationis low |
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What is La Chatelliers principle? |
A system at equilibrium will respond to counteract any changes made to theconditions”. |
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What happens when reactant concentration increases? |
to oppose the change in reactants theposition of equilibrium will move to the right (turning reactants into products).Removing product will cause the position to shift to the right as morereactants are turned into products to make up for the loss. |
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What reactions are affected by pressure? |
reactions involving gases changing pressure can affect equilibrium |
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What happens if you increase pressure? |
Increasing the pressure will move the position of equilibrium to the side of theequation with the fewest moles of gas. (The reaction is trying to reduce thepressure increase by reducing the volume of gas present). |
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What happens if you decrease pressure? |
cause the equilibrium to shift to the side of theequation with the most moles of gas (making more gas will push the pressureback up) |
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What is the effect of using a catalyst? |
Using a catalyst changes the rate of a reaction but has no effect on theposition of equilibrium as the forward and reverse reactions are speeded upby the same amount |
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What is the effect on increasing temperature? |
increases rate of reaction and position of equilibrium will shift to remove theextra heat so the reaction favours the endothermic direction (ΔH positive) |
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What does the enthalpy change written always refer to? |
the forwards reaction (eg -92) so forwards enthalpy change=92 and so backwards =+92 position of equilibrium will shift left |
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How are simple molecular molecules held together? |
by weak intermolecular bonds little energy needed to overcome so low melting point and bp |
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do they dissolve in organic solvents? |
yes, often some such as CO2 orethanol may also dissolve in water |
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How are Giant covalent structures held together? |
These contain vast lattices of atoms allheld together by covalent bonds |
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structure of SiO2 |
network structure based upon each Si having asingle bond to 4oxygens and each oxygen having single bonds to 2 Siatoms. |
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What type of shape is SiO2? |
a 3-dimensional structure (lattice) builds up. The shape about each silicon is tetrahedral. |
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Why do giant covalent networks have high mp and bp? |
To melt or boil, large numbers ofcovalent bonds will have to be broken needing large amounts of energy |
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Why are the insoluble in all solvents? |
No solvent can replace the strong covalentbonds between atoms with stronger forces |
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Why do most not conduct electricity? |
outer electrons are firmly localised in (locked into) covalentbonds. |
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What are the 3 allotropes (forms) of carbon |
diamond graphite fullerenes |
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What is the structure of diamond? |
has each C atom forming 4 covalent bonds to aneighbouring C. The shape around each C is tetrahedral |
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What are the properties of diamond? |
The material is very hard and veryhard to melt due to the need to breaklarge numbers of covalent bonds toscratch or melt/boil it.It does not conduct as all electronsare locked up in covalent bonds |
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What is the structure of graphite? |
Each C in graphite forms 3 covalent bonds. This givesrise to flat sheets of carbon atoms. On each Cthere is a non-bonding electron. |
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What are the properties of graphite? |
conduct electricity as of delocalised electrons sheets held together by weak intermolecular bonds so sheets can peel off easily making thematerial soft and slippery. (e.g pencils and as a lubricant) The strong covalent bonds within sheets mean very high boiling and meltingpoints and zero solubility. |
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What is the simplest fullerene |
Buckinster fullerene (C60) (ball shaped) |
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properties of buckmister fullerene |
C60 shows many typical simple molecular properties includingsolubility in organic solvents and turning into a gas when thesolid is heated strongly (527oC). |
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Possible (chemical) uses? |
cancer treatments, superconductorsand optical applications |
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What is more than half of GW due to? |
rise in CO2(g) caused by our useof fossil fuels. |
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What is IR spectroscopy used for? |
measure the amount of of CO2 in the air. (predict doubleby end of century, concs before industrial revolution) |
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Why does not all carbon dioxide produce stay in the atmosphere? |
There is an equilibrium set up between CO2 in the air and CO2 dissolved inwater |
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eg |
Phytoplankton in the sea remove CO2(aq) quicklyencouraging CO2(g) to dissolve.a second equilibrium occurs but this lies far to the left so very little acidity isproduced by dissolved CO2. [CO2(aq)] is much higher than [H+(aq)]. |
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What has happened as atomospher carbon dioxide lead to? |
more hasdissolved in the oceans causing a measurable increase in acidity. |
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What is the result on temperature of increased CO2 concs in the atm? |
3oC temp rise by end of 21st C |
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What would this cause? |
thawing permafrost and reduced snow cover-melting of polar ice raising sea levels-increase in heat waves, increased rainfall in higher latitudes but less rainfallin the tropics.-more intense hurricanes/typhoons in the tropics. |
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How could you redce CO2 concs? |
reducing fossil fuel use, and converting to renewablesources of energy such as solar, tidal, wind or geothermal.Planting more trees to increase photosynthesis could help to lower levels. |
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CCS? |
Trap the CO2 from power stations (using a solvent). Liquefy CO2 and pump itinto storage areas such the floor of very deep oceans or in disused oil / gaswells.The technology needed means this will be a very costly process and wouldalmost double the cost of generating electricity. |
