Rate equations

• Kinetics is the study of factors which determine the rate of reaction
• A tangent can be drown on a concentration time graph. The gradient is the rate of reaction at that specific moment in time
• Rate a[X][Y] means both X and Y have an equal effect on the rate
• In the reaction X + Y à Z with the rate expression a[X], Y has no effect on the rate
• In the reaction X + Y à Z with the rate expression a[X][A], A is a catalyst
• A rate expression is:

  • Rate a [X][Y]

  The equivalent rate equation is:

  • rate = k [X][Y]
• The rate constant is specific for every different reaction
• The only factor that will change the value of k is temperature
• The value of k is essentially the rate of the reaction when all concentrations are 1 moldm-3
• If in a rate equation, [X] is to the power 1, the reaction is 1st order with respect to X
• If in a rate equation, [Y] is to the power 2, the reaction is 2nd order with respect to Y
• The overall order is the sum of the orders of the rate equation
• zero order means changing the concentration of the reactant has no effect on the rate of reaction
• 1st order means the rate is directly proportionate to the concentration
• 2nd order means the rate is proportional to the square of the change in concentration
• When a rate Vs concentration graph is a horizontal line, it means the reagent is zero order
• the initial rate method is where a series of reactions are carried out in which the concentration of only one of the reactants is changed each time
• When a rate Vs concentration graph is a straight diagonal line it means the reagent is first order
• When a rate Vs concentration graph is a line curving up, it means the reagent is 2nd order
• When the orders of the reagents have been determined, any row of data from the table can be substituted into the rate equation to find the constant k
• Catalysts cannot have an order of 0
• k is only the same when conditions are equal
• For a first order reaction, the units of k are s-1
• For a second order reaction, the units of k are mol-1 dm3 s-1
• For a third order reaction, the units of k are mol-2 dm6 s-1
• The average rate of reaction is the total change in the amount over a period of time
• The instantaneous rate of reaction is the rate of reaction at a particular point in time
• The initial rate of reaction is the rate at t= 0 and can be used to figure out the order of the reaction