The collision theory of reactions
The distribution of energies
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The distribution of energiesMolecules are moving at different speeds, at any given time. Some have higher energies, some medium energies, others have lower energies. Maxwell-Boltzmann distribution: distribution of kinetic energies in a gas, at a given temperature. 
As the temperature increases, more molecules move at higher speeds and have higher kinetic energies.  
 
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310 K What is the significance of this for reaction rates? Eg. the activation enthalpy for a reaction is: Ea is +50 kJ mol-1. How many collisions have MORE energy than +50 kJ mol-1? 
Only collisions with energies in this region can lead to a reaction. At higher temperatures, a significantly higher proportion of molecules will have energies above +50 kJ mol-1.
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Kinetic energy (E) About twice as many molecules have enough energy to react –so the reaction goes twice as fast. SL: What is removing the ozone? Several radicals remove ozone, in the stratosphere: Chlorine atom, Cl B
romine atom, Br Chloromethane, CH3Cl Bromomethane, CH3Br
(chlorine atoms react with ozone, forming radicals.) Reaction 7: ClO + O Cl + O2 (The ClO radical then reacts with oxygen atoms.) We now have TWO reactions, competing to remove ozone: Reaction 4 O + O3 O2 + O2 and Reaction 6 Cl + O3 ClO + O2
Add equations 6 and 7 together to produce the equation for the overall reaction caused by chlorine atoms. Comment on the result. What role are Cl atoms playing in the overall reaction?
(O + O3 O2 + O2 ) The radicals are regenerated, and so acting as catalysts. It is important for chemists to know which reaction (6 or 7) is happening fastest, to understand whether oxygen atoms or chlorine atoms are responsible for the removal of ozone. In fact, Cl atoms react 1500 faster with ozone, compared to O atoms. Even though Cl atoms have a much lower concentration in the stratosphere than O atoms, the fact that they can be regenerated in a catalytic cycle can have devastating effects. Bromine is 100 times more effective at destroying ozone than chlorine, despite being much less concentrated than chlorine. CI 10.5 How do catalysts work? Collision theory and enthalpy profiles can help us to understand how catalysts work. Bond breaking is endothermic (requires an energy input to overcome the activation enthalpy barrier). Bonds first stretch and then break, reactants are made and new bonds instantly form. If few molecules have enough energy to overcome this activation enthalpy, then the reaction will be slow. Catalysts provide an alternative reaction pathway for breaking and remaking bonds that has a lower activation enthalpy. 
Catalysts and equilibrium Catalysts only affect the RATE and not the position of equilibrium in a reversible reaction. The composition of the reaction mixture remains unchanged. Directory: org org -> Function codes attachment(s): (1) index: function codes (By Groups and Sub-Groups) (2) index: function codes org -> Organized Evil and the Atlantic Alliance: Moral Panics and the Rhetoric of Organized Crime Policing in America and Britain Michael Woodiwiss and Dick Hobbs org -> The International Commission on Mathematical Instruction icmi bulletin No. 54 June 2004 Editor org -> I. Intentional Torts: Physical and Emotional Harm Battery org -> This text was adapted by The Saylor Foundation under a Creative Commons Attribution-NonCommercial-ShareAlike 0 License org -> Title: Architecture, symbolic capital and elite mobilizations: The case of the Royal Bank of Scotland corporate campus org -> Operational Plan Report org -> Practicum project org -> Lise Meitner Prize for Nuclear Science 2004 Download 244.3 Kb. Share with your friends:
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