5.1.1 - How Fast

• Rate of Reaction


  The change in the amount of reactants/products per unit time.
• Order of Reaction


  How the reactant's concentration affects the rate.
• Order of Reaction - First Order

  A reaction is first order with respect to (wrt) a reactant when the rate is directly proportional to [A].

  
  

  Units: s-1

  
  

  Rate-Concentration Graphs: straight line graph, directly proportional (goes through the origin).

  
  

  Concentration-Time Graphs: curved graph, from high to low.

  
• Order of Reaction - Second Order

  A reaction is second order wrt a reactant when the rate is directly proportional to [A] squared.

  
  

  Units: mol-1 dm3 s-1

  
  

  Rate-Concentration Graphs: curved graph, low to high.

  
• Order of Reaction - Zero Order


  When the concentration of the reactant has no effect on the rate, the reaction is zero order wrt the reactant.

  
  

  Units: moldm-3 s-1

  
  

  Rate-Concentration Graphs: horizontal line.

  Concentration-Time Graphs: directly proportional line, from high to low.

  
• Rate Constant (k)

  rate = k[A]^n[B]^m(where "m" and "n" are the orders of reaction wrt reactant A and B)

  
  

  Only affected by temperature.

  
  

  Rate constant can be calculated using the orders of reactants and the rate of reaction.
• Half-Life


  The time it takes for half of the reactant to be used up.

  
  

  Can calculate the rate constant of a first order reaction using half-life.

  
  

  k = ln2/t(1/2)
• Half-Life - Concentration-Time Graphs

  The half-life of a first order reaction is independent of the concentration, so each half-life will be the same.

  
  

  This means the half-life of a first order reaction can be read from a concentration-time graph

  
  

  Units: s-1
• Initial Rate

  The instantaneous rate at the start of a reaction when t= 0
• Initial Rates Method - Iodine Clock

  A more convenient way of obtaining initial rate of a reaction, with the time, t, from the start of the reaction for a colour change to occur.

  
  

  Initial rate is then proportional to 1/t

  
  

  Iodine clock relies on the formation of iodine. Starch is usually added as it forms a blue-black colour.
• The Rate-Determining Step

  The slowest step in a multi-step reaction.
• Predicting Reaction Mechanisms

  The rate equation only includes reacting species involved in the rate-determining step.

  
  

  The order of a reactant shows the number of molecules of that reactant which are involved in the rate-determining step.
• Effect of Temperature on Rate Constants

  Increasing temperature = more kinetic energy.

  
  

  Particles collide more often and have the required activation energy.

  So, a greater proportion of collisions will result in the reaction happening.

  
  

  Changing temperature therefore changes the rate constant.
• The Arrhenius Equation

  As the activation energy increases, k gets smaller. So, a large activation energy means a slow rate.

  
  

  Plotting a graph of lnk against 1/T allows us to find the gradient (which is equal to -Ea/R) and the y-intercept = lnA.

  
  

  Once you know the gradient, you can find both the activation energy and the pre-exponential factor.

  
  

  Ea = gradient x R