chapter 18

• rate = quantity reacted or produced / time
• rate = change in concentration / change in time
• for each reactant, the power is the order of reaction for that reactant, in a reaction different reactants can have different orders and each may affect the rate in different ways
• zero order

  when the concentration of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant, in a zero order reaction:

  • any number raised to the power of zero is 1
  • concentration does not influence rate

  
• first order

  a reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of 1, a first order reaction:

  • if the concentration of A is doubled, the reaction rate increases by a factor of 2
  • if the concentration of A is tripled, the reaction rate increases by a factor of 3

  
• second order

  a reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of 2, in a second order reaction:

  • if the concentration of A is doubled, the reaction rate increases by a factor of 4
  • if the concentration of A is tripled, the reaction rate increases by a factor of 9

  
• the rate equation gives the relationship between the concentrations of the reactants and the reaction rate

  
• rate constant k

  proportionality constant, number that converts between the rate of reaction and concentration and orders
• overall order

  gives the overall effect of the concentrations of all reactants on the rate of reaction, sum of orders with respect to each reactant

  
• units of rate constant k

  depend on the number of concentration terms in the rate equation
• initial rate

  instantaneous rate at the beginning of an experiment when t=0
• continuous monitoring

  concentration time graphs can be plotted from continuous measurements taken during the course of a reaction, can monitor a reaction by:

  • monitoring gas collection
  • monitoring by mass loss

  not all reactions produce gases so another property is needed that can be measured with time, such as a colour change which can be estimated by eye or using a colorimeter
• monitoring rate with a colorimeter

  the wavelength of the light passing through a coloured solution is controlled using a filter, the amount of light absorbed by a solution is measured

  
• zero order line on a concentration time graph


  graph

  
• zero order line on a rate concentration graph


  graph

  
• first order line on a concentration time graph


  graph

  
• first order line on a rate concentration graph


  graph

  
• second order line on a rate concentration graph


  graph

  
• second order line on a concentration time graph


  graph

  
• in a first order concentration time graph, the time for the concentration of the reactant to halve is constant, this is the half life, and the rate is constant of a first order reaction can be determined using the half life
• half life

  time for the concentration of the reactant to halve, will

  
• rate concentration graphs

  for zero, first and second

  
• initial rate can be found by measuring the gradient of a tangent drawn at t=0 on a concentration time graph
• clock reaction

  more convenient way of obtaining the initial rate of a reaction by taking a single measurement, time from t from the start of an experiment is measured for a visual change to be observed often a colour change or a precipitate
• in a second order reaction, the length of half life increases with a decreasing concentration
• in a zero order reaction, the half life decreases with increasing concentration
• rate equation

  Rate = k[A][B][C]
• rate equation rearranged

  k = Rate / [A][B][C]
• the larger k is, the faster the reaction
• k is constant if temperature is constant too
• units for an overall 0 order reaction

  moldm-3 s-1
• units for an overall 1st order reaction

  s-1
• units for an overall 2nd order reaction

  mol-1 dm3 s-1
• units for an overall 3rd order reaction

  mol-2 dm6 s-1
• initial rate

  rate at time 0s
• rate determining step

  slowest step in a reaction mechanism
• increasing the temperature shifts the boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy Ea
• as the temperature increases, particles move faster and collide more frequently
• to react particles must also collide with the correct orientation
• change in rate is mainly determined by activation energy because with increasing temperature, the increased frequency of collisions is comparatively small compared with the increase in the proportion of molecules that exceed Ea from the shift in boltzmann distribution