Rates of reaction

• Reaction rates are measured by observing the changes in the quantities of reactants or products over time
• In experiments, different quantities can be measured to calculate the rate of reaction. The unites are then those of the quantity measured per unit of time
• rate = quantity reacted or produced / time
• for consistency - measure rates of reaction as the change of concentration with a change in time
• rate = change in concentration / change in time
• units are mol dm-3 s-1
• Because concentration is such a common term, use [] to mean concentration of
• Oder of reaction: changing the concentration often changes the rate of reaction. The rate of reaction is proportional to the concentration of a particular reactant raised to a power
• rate is directly proportional to [A] to the power n
• 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:

  1. any number raised to the power 0 is 1
  2. concentration does not influence the 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 one
• In a first order reaction:

  1. if the concentration of A is doubled, the reaction rate increases by a factor of 2 to the power 1 = 2
  2. if the concentration of A is triples, the reaction rate increases by a factor of 3 to the power 1 = 3
• Second order: a reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power 2
• In a second order reaction:

  1. if the concentration of A is doubled, the reaction rate increases by a factor of 2 squared = 4
  2. if the concentration of A is tripled, the reaction rate increases by a factor of 3 squared = 9
• The rate equation gives the mathematical relationship between the concentrations of the reactants and the reaction rate
• rate = k [A]m [B]n
• k = rate constant
• m = order of reaction with respect to A
• n = order of reaction with respect to b
• the rate constant is the proportionality constant. it is the number that mathematically convert between the rate of reaction and concentration and orders
• Overall order: the overall order of reaction gives the overall effect of concentrations of all reactants on the rate of reaction
• overall order = sum of orders with respect to each reactant
• the units of that rate constant depend on the number of concentration terms in the rate equation
• to determine units of K:

  1. rearrange the equation to make k the subject
  2. substitute units into the expression for k
  3. cancel common units and show final units on a single line
• Orders from experimental results: orders of a reaction must be determined experimentally by monitoring how a physical quantity changes over time
• orders cannot be found directly from a chemical equation
• when comparing the effect of different concentrations of reactants on reaction rates, it is important that the rate is always measured after the same time, ideally as those to the start of the experiment as possible
• the initial rate is the instantaneous rate at the beginning of an experiment when t=0
• Continuous monitoring of rate of reaction is important to ensure that the reaction is proceeding at the desired rate
• Concentration-time graphs can be plotted from continuous measurements taken during the course of a reaction - continuous monitoring
• continuous monitoring example - monitoring by gass collection, monitoring by mass loss
• Not all reactions produce gases, so another property is needed that can be measure with time e.g. colour change - can be estimated by eye or monitored using a colorimeter
• in 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
• Orders from shapes: the gradient of a concentration-time graph is the rate of reaction. The order with respect to a reactant can also be deducted from the shape of a concentration-time graph for zero and first order reactions
• The order with respect to a reactant can only be obtained if all other reactant concentrations remain effectively constant
• Zero order: a zero order reaction produces a straight line with a negative gradient. The reaction rate does not change at all during the course of the reaction. The value of the gradient is equal to the rate constant k.
• First order: a first order reaction produces a downwards curve with a decreasing gradient over time. As the gradient decreases with time, the reaction gradually slows down. In a first order concentration-time graph, the time for the the concentration of the reactant to halve is constant. This is called half-life and the rate constant of a first order reaction can be determined using its value
• Second order: the graph for second order is also a downwards curve, steeper at the start, but tailing off more slowly