chemistry as level

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  1. particles in liquids and gases are always moving and colliding with each other . they don't react every time though - only when the conditions are right . collision theory states that
    a reaction won't take place between two particles unless they collide in the right direction and they collide with at least a certain minimum amount of kinetic energy
  2. the minimum amount of kinetic energy particles need to react is called the
    activation energy
  3. the particles need activation energy to
    break bonds to start the reaction
  4. reactions with low activation energies often
    happen pretty easily but reactions with high activation energies dont . you need to give the particles extra energy by heating them
  5. draw an enthalpy profile diagram
    • Image Uploadon the curve upwards the bonds within each particle are being stretched 
    • on the curve downwards the separate bust from each particle can't exist by themselves - so the form new bonds and release energy
  6. enthalpy change diagrams can be used to
    work out enthalpy change (ΔH) and whether its exothermic or endothermic
  7. ΔH is the
    difference between the enthalpy of the reactants and the enthalpy of the products on the diagram
  8. if the products have a lower enthalpy than the reactants the reaction is
  9. if the products have a higher enthalpy than the reactants then the reaction is
  10. some molecules don't have much kinetic energy and move slowly , others have loads of kinetic energy and whizz along . but most molecules are somewhere in between . if you plot a graph of the numbers of molecules in a gas with different kinetic energies you get a
    Maxwell-Boltzman distribution
  11. the Maxwell-Boltzmann distribution is a theoretical model that has been developed to explain scientific observations . it looks like this
    Image Upload
  12. if you increase the temperature of a gas , the  molecules will on average
    have more kinetic energy and will move faster . so a greater proportion of molecules will have at least the activation energy and be able to react . this changes the shape of the maxwell Boltzmann distribution curve - it pushes it over to the right , the total number of molecules is still the same so the area under the curve is still the same
  13. maxwell Boltzmann distribution curve for different temperatures
    Image Upload
  14. give another reason why increasing temperature makes a reaction faster hence what does this means
    • the molecules are moving faster so they'll collide more often . 
    • the fact that there are two reasons why increasing the temperature increases the rate of reaction means that small increases in temperature can lead to large increases in reaction rate
Card Set:
chemistry as level
2014-02-04 16:59:30
unit two

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