3.1.9 - Rate Equations

• How do you work out total order? add all the individual orders together
• k is only constant at a fixed temperature.
• The value of k is independent of concentration and time
• Continuous rate method for an experiment: Follow one experiment over time recording change in concentration
• What can we conclude from a concentration-time graph?

  • rate of reaction = gradient
  • reaction is fastest at the point where the gradient is steepest
• Rats of reaction decrease with time in a concentration time graph because as reactants get used up their concentration drops.
• Use a gas syringe to measure changes in a volume of gas produced over time
• A topical gas syringe is only 100ml of gas so a reaction shouldn't produce more than this volume
• Different volumes of the same initial concentrations will have the same initial rate but end in different amounts
• What are the typical units for Activation Energy when calculated from gradient?

  • J mol-1 
  • To convert to kJ / mol divide by 1000
• What is a mechanism? A series of steps through which the reaction progresses often forming intermediate compounds.
• Rate determining step:

  • Each step has a different rate of reaction.
  • The rate determining step is the slowest step which controls the rate of reaction
• How can you determine for the order of reaction for compound A from the a series of steps in a mechanism?

  • The order of reaction is the number of molecules of that species
  • involved in or before the rate determining step.
• What does the rate equation show us about a mechanism? shows species which appear in or before the rate determining step
• What is meant by order of reaction? Power of concentration term in the rate equation
• For zero order: the concentration of a has no effect on the rate of reaction

  
• For first order: The rate of reaction is directly proportional to the concentration of a

  
• For second order: The rate of reaction is proportional to the concentration of A squared

  
• Rate of reaction: Change in concentration (of any reactant or product) per unit time.
• At a given instant, how could you calculate the rate of reaction?

  
  

  Rate of reaction = change in concentration / change in time
• You can only determine the rate constant and rate expression for a reaction experimentally
• Do species need to be in the chemical equation to be in the rate expression? yes/no

  No - species in the chemical equation may be excluded and species not in the chemical equation e.g. catalysts, may be included
• Define the term order of a reaction with respect to a given product: The power to which a species’ concentration is raised in the rate equation.
• What type of reactant is this rate concentration graph showing? zero order reactant.

  
• What type of reactant is this rate concentration graph showing? first order reactant.

  
• What is the effect of a 10K temperature increase on the rate of reaction, roughly? Doubles rate of reaction
• What is true of the half life of a first order reactant (concentration against time graph)? Half life is constant