RP7 - Rate of reaction - initial rate/continuous monitoring

• An experiment is done to investigate the rate of reaction in Question 04.2.

  During the experiment the concentration of cisplatin is measured at one-minute intervals. Explain how graphical methods can be used to process the measured results, to confirm that the reaction is first order [3 marks]

  • M1 plot concentration (y-axis) against time (x-axis) and take tangents / (calculate the) gradients (to calculate rates)
  • M2 Plot rate/gradients against conc
  • M3 straight line through origin / directly proportional confirms first order
• Hydrogen peroxide solution decomposes to form water and oxygen

  A student determines the order of this reaction with respect to hydrogen peroxide. The student uses a continuous monitoring method in the experiment.

  Explain why the reaction is fastest at the start [2]

  • M1 HighER/EST concentration of / more H2O2 / particles / molecules / reactants
  • M2 More frequent successful collisions
• The rate of this reaction can be followed by preparing mixtures in which only the initial concentration of propanone is varied

  State and explain why different volumes of water are added to mixtures [2 marks]

  • To make volumes constant for all mixtures.
  • So that volume of propanone is proportional to concentration
• Each sample taken from the reaction mixtures is immediately added to an excess of sodium hydrogencarbonate solution before being titrated with sodium thiosulfate solution. Suggest the purpose of this addition. Explain your answer.

  • To stop/QUENCH the reaction at that time.
  • By removing the acid catalyst for the reaction (by neutralisation).
• Explain why the use of a large excess of H2O2 and I- means rate of reaction at a fixed temperature depends only on the concentration of H+.

  • H2O2 and/or I – concentration change is negligible / H2O2 and/or I – concentration effectively constant
  • so have a constant/no effect on the rate / so is zero order (with respect to H2O2 and I– ) / a and b are zero

  
• Samples of the reaction mixture are removed at timed intervals and titrated with alkali to determine the concentration of H+ (aq).

  State and explain what must be done to each sample before it is titrated with alkali. [2 marks]

  • Stop the reaction / quench
  • By dilution / cooling / adding a reagent to react with H2O2/I-

  
• SUGGEST why conical flask is used instead of a beaker would give more results in this experiment

  • Relatively few students scored the mark for an appreciation that the NARROW neck of a conical flask will reduce the loss of droplets of liquid in a reaction giving effervescence.
• State one way this method could be improved, other than by repeating the experiment or by improving the precision of time or volume measurements.

  Explain why the accuracy of the experiment would be improved.

  • Thermostat the mixture / constant temperature / use a water bath
  • Reaction / rate affected by temperature change
• Suggest one improvement in RP7

  • Colorimeter / uv-visible spectrometer / light sensor to monitor colour change
  • or Eliminates human error in timing / more accurate time of colour change
• How can you tell that H+ acts as a catalyst in this reaction?

  • Appears in rate equation (1)
  • does not appear in (stoichiometric / overall) equation (1)
• Iodine clock experiment - describing [6 marks]

  • Measure A, B known CONCENTRATION and VOLUME
  • Add known concentration of X (that reacts rapidly with product)
  • Add known concentration and volume of C and Start the timer
  • Record time taken for blue-black colour to appear
  • Keep temperature constant (using a water bath)
• Analysis of results in iodine clock reaction

  • Rate = 1 / time
  • Plot graph of Rate on y axis and concentration of [A] in x axis
  • Deduce the order from graph =
  • Flat line = Zero order
  • Straight line, directly proportional, passes through the origin = First order etc with second order
• Suggest why the order of this reaction with respect to propanone can be ignored in this experiment

  • Concentration of propanone is MUCH LARGER than the amount / concentration of iodine = MUST BE COMPARATIVE!
  • SO concentration of propanone is effectively constant
• Suggest why 1.0 cm3 portions of the reaction mixture are added to an excess of NaHCO3 solution

  • The sodium hydrogencarbonate solution neutralises the acid (catalyst)
  • So stops the reaction
• Explain how graph shows reaction is zero order with respect to [x]

  • The graph is a straight line / has a constant gradient
  • So the rate of reaction does not change as the concentration (of iodine) changes
  • the iodine is being used up at a constant rate.