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  • Created by: jade
  • Created on: 20-04-13 13:28

Giant Molecular Structures

Diamond - Each carbon atom is covalently bonded to 4 other carbon atoms 

Tetrahedral shape which produces a  crystal lattice - vibrations can travel through the molecule easily - good thermal conductor BUT can't conduct electricity as electrons are in localised bonds

Silicon (IV) Oxide - Silicon bonded covalently to 4 oxygen atoms - found in quartz and sand - not the same shape as diamond as Oxygen can only bond to 2 silicon atomsTetrahedral shape produces a crystalline solid with a high melting point but does NOT conduct electricity. 

Carbon Dioxide - Each Carbon atom forms a double bond with 2 Oxygen atoms. Gas at room temp - only bonds are instantaneous dipole - induced dipoleDissolves in water 

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The Atmosphere

Atmosphere - 78% Nitrogen + 21% Oxygen 

Parts per Million - Gases are normally given as Percentages but smaller gases are present in tiny amounts. 

Calculation = Percentage x 10 000 = ___ ppm (This is a larger amount so is easier to work with)  

The earths atmosphere absorbs radiation 

The sun gives out electromagnetic radiation - infrared radiation is absorbed by the earths atmosphere. The earths surface also absorbs radiation from the sun and is warmed. The earth then re-emits the radiation (mostly infrared) - this is lower frequency sun radiation (it has a lower temperature) 

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The Atmosphere 2

Infrared Radiation makes bond vibrate (E = hv) 

Some molecules absorb energy this causes the covalent bonds to vibrate more - only molecules made of different molecules can absorb infrared as bond polarities change. 

Those that do absorb radiation are called GREENHOUSE GASES - they stop the re-emitted radiation escaping into space. These molecules have certain vibrational energy levels so they can only jump from one level to another 

Different molecule absorb different frequencies of radiation. 

When ultraviolet or visible light hit a molecule of gas the electrons can absorb energy and jump to the next energy level.

IF ENOUGH ENERGY IS ABSORBED BONDS BREAK - RADICALS FORM 

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Greenhouse effect

Greenhouse Effect - Positive 

  • The sun emits electromagnetic radiation 
  • Most of the UV and visible is absorbed by atmospheric gases -some is reflected back to space by clouds 
  • The energy that reaches the earths surface in mainly visible and UV - this warmed the earth 
  • The earth radiates energy back to space as infrared radiation
  • some escapes through the IR Window (the range of frequencies not absorbed by greenhouse gasses) 
  • various gasses in the troposphere absorb other infrared and re-emit in all directions This is the GREENHOUSE EFFECT Without this the earth would be cooler than it is now
  • Greenhouse gasses are water vapour, carbon dioxide and methane. The extra vibrational energy is passed on to other molecules through collisions raising the overall temperature.  
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Enhanced Greenhouse effects

Enhanced greenhouse effects - this causes Global Warming 

  • Over the past 150 years we have been burning fossil fuels at an increasing rate. 
  • This has raised the concentration of greenhouse gasses meaning that less INFARED RADIATION can escape 
  • This means that too much heat is being trapped and the earth is getting warmer. 
  • Global warming can lead to sea levels rising and climate change. 

Scientists are trying to limit Global Warming 

  • Developing alternative fuels or more fuel efficient technology
  • Carbon capture and storage 
  • Increasing Photosynthesis


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Halogenoalkanes

Halogenoalkanes are made by reacting Alkanes with Halogens - Photochemical reactions (started by ultraviolet light)

A hydrogen atom is substituted by chlorine or bromine  FREE RADICAL SUBSTITUTION REACTION  The mechanism consists of three stages:

Initiation Reaction - free radicals are produced = Sunlight provides enough energy to break the bonds (PHOTODISSOCIATION), the bond splits equally and each atom gets one electron - HOMOLYTIC FISSION - they are highly reactive as have an unpaired electron. 

Propagation Reaction - free radicals are used up and created in a chain reaction = The radical attacks the alkane, the new methyl free radical can attack other halogen molecules. This continues until all molecules are wiped out. 

Termination Reaction - free radicals are used up = Two radicals join together to make a stable molecule. 

UV radiation can break halogenoalkane bonds - depends on the halogen and bond enthalpy

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CFC's

CHLOROFLUOROALKANES (they are compounds that are made by replacing all the hydrogen's with chlorine and fluorine) 

Properties 

  • Unreactive 
  • Non-flammable 
  • Harmless 

Used in fire extinguishers, propellants in aerosols and coolants in fridges BUT alternatives had to be found 

  • Created holes in the ozone layer 

HCFC's are the ALTERNATIVE until safer products are developed 

  • broken down in 10-20 years - not as big an impact on ozone 
  • HFC's don't contain chlorine so don't affect the ozone layer unfortunately they are greenhouse gases 1000 times worse than carbon dioxide. 
  • In aerosols they use nitrogen instead as the propellant and ammonia as the coolant gas. 
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Ozone 1

The Ozone is in a layer of the atmosphere called the stratosphere. It contains ozone molecules O3. This is formed when UV Radiation from the sun hits oxygen molecules creating free radicals which in turn react with oxygen. 

  • The ozone is always being destoyed and replaced as radiation can also cause the reverse reaction
  • Ozone protects the against UV radiation by removing all the high energy UVC radiation and most the UVB radiation 
  • UVB and UVA can cause skin cancer 
  • UVA is the lowest frequency and UVC the highest
  • UV radiation however produces vitimin D and tans which protects deeper tissue from radiation 
  • The british antartic survey found the concentration over antartic of ozone is low so is the concentration over the artic. 
  • alot of evidence was needed to prove that man made emissions cause thinning in the ozone layer. 
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Ozone 2

The Ozone layer is being destroyed by Homogeneous catalysts such as CFC's 

  • Chlorine free radicals are formed when CFCs are broken down by UV radiation
  • These free radicals are catalysts, they react with ozone to form an intermediate and an oxygen molecule 
  • Do the overall reaction is 

2O3 -------- 3O

and the Chlorine radical is the catalyst Other free radicals such as nitrogen oxide radicals can destroy ozone 

Photochemical smog is caused when ozone mixes with solid particles of carbon in the troposphere. 

Ozone is TOXIC to humans - it can affect the lungs and trigger asthma attacks. 

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