Ex3: Reaction kinetics: The Iodide Persulphate Reaction

• Using the reaction between iodide ions and persulfate ions, this experiment will investigate the effect of:

  reactant concentration on reaction rate
• Clock reaction


  The use of one reaction to study the rate of another reaction
• Iodine violet colour which is then eliminated by thiosulphate (2:1 ratio) and make soln. colorless. If thiosulphate consumed, soln. is purple again. If thiosulphate amount known; iodine amount can be known
• Starch also added to form blue complex for accuracy. reaction is yellow w/o starch
• The blue colour which indicates the end-point of the reaction trials in this experiment is produced by:

  A complex of iodine and starch
• the reactant (S2O8-2 and I-) concentration affects the rate by increasing it when concentration increases and decreasing when concentration is decreased (it is a proportional relationship)
• rate is equal to k [S2O8-2]1[I-]1 (1st order with respect to S2O8-2 and 1st order with respect to I- and 2nd order overall)
• The rate of the reaction between iodide ions and persulfate ions is strongly dependant on:

  Ionic strength
• effect on rate of reaction if... S2O3 (thiosulphate) was halved


  no change
• effect on rate of reaction if... S2O8 (persulfate) was doubled


  rate doubled
• effect on rate of reaction if... temp was decreases

  decreased rate
• effect on rate constant (k) if.... S2O3 concentration halved

  no change
• effect on rate constant (k) if.... I was doubled

  no change
• effect on rate constant (k) if.... ionic strength increased

  increase
• ionic strength definition

  measure of the total concentration of ions in solution
• Why does the addition of electrolytes keep the ionic strength constant?

  A high concentration of ions in solution diminishes the strength of repulsion between two ions, shielding the negative charge, and keeping the ionic strength constant
• units of k

  M^-1 s^-1
• ionic strength equation

  Ci = molar concentration of ion

  Zi = ionic charge

  
• calculate rate constant, K

  Rate = K[S2O8-2][I-]
• calculate Rate of reaction

  Rate = - delta S2O82 / delta T
• calculate - delta S2O82

  [S2O3-2] x 1/2
• Reaction 1 (Iodide and Persulphate) is the rate determining step
• the iodine produced by Reaction 1 is consumed by Reaction 2 as quickly as it is produced, preventing any accumulation of iodine and keeping the solution colourless.