rate equations

• the rate of reaction is

  the change in concentration per unit time
• rate of reaction = change in concentration / change in time
• 2 ways to measure the rate of reaction

  use a colorimeter at suitable intervals if there is a colour change.

  if gas is evolved, use a gas syringe to collect volume of gas evolved or measure the change in mass of the reaction mixture
• temperature is the only thing that can affect the value of the rate constant for a given reaction
• rate = k[X]x[Y]y

  k=rate constant

  [X] and [Y] are concentrations of species X and Y and x and y are the orders of reaction
• the order of reaction is the power to which a species concentration is raised in the rate equation
• the overall order of reaction is the sum of the orders of reaction of all species
• a rate concentration graph is [A] against time, draw tangents at different values -> draw a secondary graph of rate against [A]
• a rate concentration graph for zero order reactants show a horizontal straight line
• a rate concentration graph for a first order reactant is a straight diagonal line
• a rate concentration graph for a second order reactant is a curve
• you can find the rate expression using the initial rate method by doing a series of experiments where you vary the concentrations so the concentrations of just one reactant changes every time.

  then you can plot a graph of concentration against time for each reactant and use the tangent at t=0 to find the initial rate of reaction.

  compare the rates and concs between each experiment to find order of reactants and overall rate equation
• when I2 is produced for an iodine clock reaction, you must add known moles of sodium thiosulfate and a little bit of starch so it reacts in a 1:2 ratio.

  I2 + 2S2O3 2- -> S4O6 2- + 2I-
• the starch turns a blue black colour in an iodine clock reaction because when all of the Na2S2O3 has been used up and I2 is produced, it reacts with starch
• you can calculate the rate of reaction from the data from an iodine clock reaction by recording the time taken for a colour change to occur then use rate=1/t
• arrhenius equation: k=Ae (-Ea/RT)

  k= rate constant

  A=pre exponential factor

  e= mathematical quantity

  R=gas constant

  T=temperature in kelvin

  Ea= activation energy in joules
• to plot a graph, convert arrhenius equation to

  lnk = -Ea/RT + lnA

  graph of lnK is a straighr line: gradient = -Ea/R and y intercept is lnA
• the rate determining step is the slowest step in a reaction mechanism which determines the overall rate of reaction
• the rate determining step links to the species involved in the rate expression because any species involved in the rate determining step appears in the rate expression. species only involved after the rate determining step dont appear in the rate expression