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Homework Help: Autocatalytic reaction and rate of reaction

  1. Dec 7, 2009 #1
    1. The problem statement, all variables and given/known data

    Hi, Im trying to set up a mathematical model for an autocatalytic (self heating) reaction. However, I can't fully grasp the chemistry.

    2. Relevant equations

    A + X = B + 2X,
    X + Y = B + 2Y,
    A + Y = B.

    3. The attempt at a solution

    So one example I found is the all known reaction of hydrogen and oxygen, assuming the following conditions:

    Let free atoms of hydrogen or oxygen arise as primers in a mixture of O2 and H2 a chain reaction arises:

    O_2 and H_2 = OH + O,
    O + H_2 = OH + H,
    … (Basic Chain)
    Next, the hydroxyl radicals OH are formed and these take part in a subsequent reaction
    OH + H2 = H20 + H

    As a result, the concentration of free hydrogen atoms rises in a geometric progression. Free atoms and radicals are short-lived active intermediate reaction products. They quickly recombine on the walls of the vessel or bulk. The reaction of Chain termination is usually written as:

    H + wall = ½ H2
    H + O2 + M = HO2 + M

    Where M is an arbitrary third particle. As seen from the above reaction, the chain termination in the bulk does not lead to a stable molecule but o the peroxide-type radical HO_2, which still contains superfluous chemical energy and is a long-lved active intermediate. This radical can participate in slow chain propagating reactions:

    HO2 + H2 = H2O2 + H
    HO2 + H2 = H2O + OH

    This type of reaction exhibits a slow increase in the rate of the overall process with time. In order to fully describe the kinetics of the reaction, we must introduce yet another degradation of the perodixed radical on the surface of the walls of the vessel (slow chain termination)

    HO_2 + wall -> destruction

    And autocatalytic process of chain propagation:

    HO2 + H2O = H2O2 + OH
    HO2 + H2P2 = H2O + O2 + OH

    The last equation explains the phenomenon of autocatalysis in the final reaction product, i.e. Water. On one hand, the rate of the autocatalytic reaction increases markedly with pressure, and this may lead to thermal explosions, as we will investigate in detail.

    In order for my model to work, theoretically, the reaction needs to undergo self heating leading towards increased reaction rate, eventually the heat produced will achieve a critical value at which point an explosion occurs.

    Can i use the reaction above, and if so, how would I set up the arrhenius source constant,
    k = z*e^(-E/RT) where is z is the collision between atoms, and e^(-E/RT) the probability of a collision leading towards a reaction.

    Any help is greatly appreciated.
  2. jcsd
  3. Dec 7, 2009 #2


    User Avatar

    Staff: Mentor

    I am not sure if you HAVE to start with chemistry.

    I did something similar eons ago - I just assumed A -> B, k given by Arrhenius equation, amount of heat evolved proportional to amount of products, and mixture temperature calculated using simple q=mcΔT. As far as I remember I got a differential equation that yielded integral that I wasn't able to calculate, but it wasn't hard to integrate numerically.

    That was after some organic lab - I don't remember what I was doing, but it required ice bath and at some moment mixture got too hot... I had to clean whole fume hood :grumpy:

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