Rates of reaction

• Mean rate of reaction


  quantity of Product formed / time taken
• Steeper slope


  Faster reaction
• Collision theory


  • Chemical reactions only take place when reacting particles collide
  • Collisions must have sufficient energy
• Rate of chemical reaction


  Determined by the frequency of successful collisions (number of successful collisions per sec)
• Disappearing cross section


  1. Use measuring cylinder to put 10cm of sodium thiosulfate

  2. Place conical flask onto a printed black cross

  3. Add 10cm hydrochloric acid and stir solution. Stopwatch

  4. After time, solution will turn cloudy so stop the stopwatch when you can't see the cross anymore

  5. Repeat using lower concentrations of sodium thiosulfate and calculate mean
• Reproducible measurement


  Can be repeated by another person or different technique/equipment and get same results
• Problem: Different people have different eyesights so may not get same results
• Greater concentration of a chemical


  Faster the reaction takes place
• Effect of surface area on rate

  • Particles in solution can only react with particles on the surface of the solid
  • More collisions per sec, rate increases when we increase the surface area of a solid reactant
  • Smaller sized pieces of solid reactant have greater surface area to volume ratio, increases rate of reaction
• Activation energy


  Minimum amount of energy particles need to react
• Increasing temperature


  Increases rate of reaction
• Catalysts


  • Increase rate of reactions but aren't used up during reactions
  • Allow reactions to carry out without needing to increase temperature, saves money
• How catalysts work


  • Catalysts increase rate by providing a different pathway for the reaction that has a lower activation energy
• Enzymes act as catalysts in living organisms
• Don't include catalysts in chemical equation, they are not a reactant
• Different reactions need different catalysts
• Reversible reactions


  • Same amount of energy is transferred in each case
  • If a reversible reaction is exothermic in one direction it's endothermic in the other
• Equilibrium


  At some point the forward and reverse reaction will take place at exactly the same rate
• Increase concentration of NO2

  More N2O4 formed
• Decrease concentration of N2O4

  More NO2 will react
• Increase concentration of N2O4

  More NO2 will be formed
• Increase pressure on reversible reactions at equilibrium

  Position of equilibrium shifts to the side with smaller number of molecules