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Chemical kinetics

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Rates of chemical reactions

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Chemical kinetics

  1. 1. Tuesday 14 February 2017
  2. 2.  Chemical reactions take place at very different rates – some are very fast (like explosions) and others may take months or years to proceed.
  3. 3.  In this experiment the reagents are magnesium and sulphuric acid.  The products are magnesium sulphate and hydrogen gas.  The rate of the reaction is measured by measuring how fast the hydrogen gas is produced.  The limiting reagent is the amount of magnesium. This is the first reagent to be use up.
  4. 4. Rate of reaction experiment Time (s) Volume of Hydrogen (ml) 0 0 30 16 60 25 90 31 120 34 150 35 180 36 210 36 240 36
  5. 5.  We can record the rate using this relation|:  Rate of reaction = change in recorded property time for the change  Actual rate is the gradient of the line of the product concentration versus time graph.  Average rate is the gradient of the line joining the two point in time over which the rate is being measured.
  6. 6. Rapid reaction here Reaction slows down here. Reaction stops here
  7. 7. Rate at the start of the reaction.
  8. 8. Average rate of reaction between 0s and 200s.
  9. 9.  In this experiment calcium carbonate reacts with hydrochloric acid.  The rate of reaction is measured by collecting the carbon dioxide gas produced in a gas syringe.
  10. 10.  Calculate the number of moles of hydrogen produce in experiment 1 above.  Volume of hydrogen = 36ml  1 mole of hydrogen = 24000 ml  Number of moles of hydrogen = 36 ÷ 24000  = 0.0015 moles
  11. 11.  In this experiment calcium carbonate reacts with hydrochloric acid.  The rate of reaction is measured the mass of the remaining chemicals. The loss in mass is due to the escaping carbon dioxide gas.
  12. 12.  In this experiment sodium thiosulphate reacts with hydrochloric acid to produce sulphur.  The rate of reaction is measured by timing how long it takes for a cross drawn below the beaker to disappear. The cross should disappear once the same amount of sulphur is produce in each trail.
  13. 13.  Concentration  The higher the level of concentration (or pressure in gases) the faster the reaction.  This is due to increased collisions between reacting particles.
  14. 14.  Surface area in solids  The larger the surface area of a solid the faster the reaction. Finely divided substances have much larger surface areas than large chunks of a solid.  This is due to increased collisions between reacting particles.
  15. 15.  Temperature  The higher the temperature the faster the reaction.  This is due to increased and more energetic collisions between reacting particles.  A 10o C rise in temperature often results in a doubling of the reaction rate.
  16. 16.  Only molecules with enough activation energy will react to form the products.
  17. 17.  Catalysts  Catalysts increase the rates of chemical reactions.  This is due a lowering of the activation energy for the reaction.  Catalysts are not consumed in the reaction so in theory they can be used over and over again. In practice they are often contaminated and/or some is lost in a process.
  18. 18.  Scientists assume all gases are made of particles in constant random motion. They have regular elastic collisions with other molecules.  Gas molecules collide with the walls of their container and exert pressure but do not lose energy in their collisions and do not attract other molecules.  The volume of actual gas molecules in a container is negligible and their average kinetic energy is proportional to the temperate (Kelvin).

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