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Reaction Rates

Reaction Rates. 18.1 Notes. Fast or Slow?. Reactions don’t all happen at the same speed! reaction rate: change in concentration (in M) of a reactant or product per unit time average rate = ∆ quantity / ∆ time determined experimentally. Collision Theory.

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Reaction Rates

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  1. Reaction Rates 18.1 Notes

  2. Fast or Slow? Reactions don’t all happen at the same speed! • reaction rate: change in concentration (in M) of a reactant or product per unit time • average rate = ∆ quantity / ∆ time • determined experimentally

  3. Collision Theory • Reacting substances must collide in order to react • Collisions have to happen in the correct orientation • Reacting substances must have enough energy to form the ‘activation complex’ ~this amount of energy is the ‘activation energy’

  4. 1. Reacting substances (atoms, ions, molecules, etc.) must collide in order to react

  5. 2. Collisions have to happen with the correct orientation

  6. 3. Reacting substances must have enough energy to form the activation complex

  7. Collision Theory - Pictoral Summary

  8. Activation Energy Activation energy: difference between starting energy and activated complex.

  9. sToP & tHinK • Assume that this reaction is reversible, meaning that products can react to reform reactants. Which ‘direction’ would have a larger activation energy? • reactants to products OR products to reactants

  10. Factors that affect Reaction Rate • Nature of Reactants • Concentration • Surface Area • Temperature • Catalysis

  11. 1. Nature of Reactants • Some elements/substances are more reactive than others

  12. 2. Concentration • Higher reactant concentrations means more particle collisions and faster reaction rate.

  13. 3. Surface Area • Greater surface area creates a larger interface for particle collisions, so increases reaction rate.

  14. 4. Temperature • Higher temperature means higher kinetic energy, more particle collisions, and faster reaction rate.

  15. 5. Catalysis • Very different from the other factors that affect reaction rate • Instead of increasing/decreasing collisions, catalysis lower the activation energy • inhibitor - slows down or even blocks a reaction

  16. sToP & tHinK • Make a table with two columns: increases reaction rate & decreases reaction rate • List as many things in each category as you can think of, for example, increasing temperature, decreasing reactant concentration, etc. • You can use opposites (like increasing temperature & decreasing temperature) and put them in different columns.

  17. Different Energy diagrams <- single step complex reaction ->

  18. Equilibrium • reached when concentrations of reactants and products don’t change • Reversible Reactions: occur in the forward and reverse direction • most reactions don’t go ‘to completion’, instead at a certain point they appear to stop

  19. Chemical Equilibrium • rate of forward and reverse reactions are equal • Rateforward = Ratereverse • Does NOT mean concentrations are equal! They just stop changing…

  20. difference in activation energy determines which ‘side’ of the reaction is ‘favored’ (or which there is more of, reactants or products)

  21. sToP & tHinK • Your friend is doing an experiment & needs to figure out when the reaction is at equilibrium. He tells you he’s waiting for the concentration of the products to equal that of the reactants. What should he look for instead? • If the product side of a chemical reaction is ‘favored’ will there be a higher concentration of reactants or products?

  22. Le Chatelier’s Principle • If a ‘stress’ is applied to a system at equilibrium, the system will shift in the direction that offsets the stress • Possible stresses: changes in concentration, volume, pressure, temperature

  23. Concentration Example Fe+3 (pale brown) + SCN-1 FeSCN+2(red) •  [Fe +3] will cause shift to right •  [Fe +3] will cause shift to left • Shift to right = more product will be made • Shift to left = more reactant will be made • Reaction shifts to undo/offset the stress

  24. Temperature Example energy + CuSO4 + 4KBr  K2[CuBr4] + K2SO4 •  temp. will cause shift to the right •  temp. will cause shift to the left

  25. Volume, Pressure Example • number of moles of gas determine pressure in a closed system • more moles = higher pressure •  volume ( pressure) will shift to side with fewer moles •  volume ( pressure) will shift to side with more moles • CO (g) + 3 H2 (g)  CH4 (g)+ H2O (g)

  26. Fe2+ +(aq) + SCN-(aq) (colorless ions)  FeSCN2+(aq) (red) Click here to go to videos!

  27. 2 NO2 (g) (brown) <=> N2O4 (g) (colorless) + energy

  28. sToP & tHinK • heat energy + CuSO4 (aq) + 4KBr (aq)  K2[CuBr4] (aq) + K2SO4 (aq) • In which direction will each of the following stresses shift equilibrium? • Decrease in temperature • Increase in [CuSO4] • Decrease in [K2SO4] • Increase in temperature • Increase in KBr

  29. sToP & tHinK - answers • heat energy + CuSO4 (aq) + 4KBr (aq)  K2[CuBr4] (aq) + K2SO4 (aq) • In which direction will each of the following stresses shift equilibrium? • Decrease in temperature = to the left (energy is a reactant… lower temp is like lower concentration of energy!) • Increase in [CuSO4] = to the right • Decrease in [K2SO4] = to the right • Increase in temperature = to the right • Decrease in KBr = to the left

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