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Instructions are Available Here
 

Investigating the Rate Determining Step in a Multi-step Reaction:

Introduction

The chemical reaction in the methanol fuel cell makes possible a very interesting study of a multi-step reaction. The speed of the various steps can be measured, and the rate determining step deduced. Section 3.2 of the Mini Fuel Cell Kit instruction manual must be read before attempting this experiment - or reading these instructions for that matter!!

The basis of the experiment is that the methanol is oxidised in three stages:-

  • First to methanal (formaldehyde), releasing 2 electrons.
  • Then to methanoic acid (formic acid), releasing 2 electrons.
  • And finally to carbon dioxide, releasing another 2 electrons.
The interesting, and quite rare, thing about this multi-step reaction is that in the fuel cell it can it can be started at any point. The two intermediates, methanal and methanoic acid, are fairly easily obtainable, and the fuel cell can be run using them.

Furthermore, it is not difficult to relate the current from the cell to the reaction rate.

This is done as follows:-

  • The charge on one electron is 1.6 x 10-19 Coulombs.
  • So, a current of one amp is the same as 1/(1.6 x 10-19)= 6.25 x 1018 electrons passing per second.
  • Thus, if an electro-chemical reaction releases 1 electron each reaction, then the number of reactions is clearly also 6.25 x 1018 per second, if the current is 1 amp.
  • By similar logic, if an electro-chemical reaction releases 2 electrons, then the number of reactions per second = 1/(2 x 1.6 x 10-19) = 3.1 x 1018. The oxidation of methanoic (formic) acid in a fuel cell is such a reaction.
  • Similarly, for a reaction, such as the oxidation of methanal (formaldehyde), which releases 4 electrons, the number of reactions per second = 1/(4 x 1.6 x 10-19) = 1.6 x 1018 for each amp.
  • Finally, for a reaction, such as the complete oxidation of methanol in a fuel cell, which releases 6 electrons, the number of reactions per second = 1/(6 x 1.6 x 10-19), which is 1.04 x 1018 reactions per second at one amp.
The three numbers in bold above are multiplied by the current (in AMPS) to give the reaction rate, in reactions per second, for methanoic acid, methanal and methanol respectively.

Instructions The procedure for the experiment is quite simple, but care must be taken, since methanol and methanal are both poisonous, and methanoic acid and the KOH electrolyte are corrosive.

Another problem is that when using methanoic acid, great care must be taken to make sure that the fuel/electrolyte mixture remains alkali. For this reason only 1ml of this fuel should be used. To keep the experiment valid, only 1ml of the other fuel should be used also. This should not present any problems.

The basic procedure is as follows:-

  • Put ~75ml of KOH solution into the cell.
  • Add 1 ml of methanol.
  • Short circuit the cell with an ammeter, wait about 1 minute, and record the current. Note that the current will never completely stabilise.
  • Multiply the current (in AMPS) by 1.04 x 1018 to give the reaction rate.
  • Repeat the first three steps for methanal, multiplying the current (in AMPS) by 1.6 x 1018 to get the reaction rate.
  • Repeat for methanoic acid, this time multiplying the current in AMPS by 3.1 x 1018 to get the reaction rate.

You should find that the reaction rate when using methanol is markedly lower than the other two fuels, showing that the step methanol ---> methanal is the rate determining step. Spectroscopic studies of methanol fuel cells, specially designed so this can be done, confirm this to be the case, with very low concentrations of methanal and methanoic acid present in a working cell, indicating that these substances are quickly oxidised.