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Catalyst lowers Activation Energy, but the reaction goes slower. Watch

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    Hi guys.

    I'm doing my A2 Chem CW and I'm investigating the kinetics of the reaction between Iodide ions & Peroxodisulphate ions.

    I've found that for the uncatalysed reaction, the order wrt both reactants is 1. I've found that the activation energy is about 50 KJ/mol. I've found that Fe 2+ Ions act as a catalyst and reduce the activation energy to about 35KJ/mol

    Now I've hit a bit of a wall. Basically I looked at which D-block ions electrode potentials fell between the two half equations in the reaction and the only two that did were Fe2+ ---> Fe3+ and Co3+ ----> Co2+. As said above, the Iron one worked.

    When I was running my experiments for the Cobalt catalyst I found that the reactions were running a lot slower than they would without a catalyst so I assumed that it didn't work as a catalyst and was actually slowing down the reaction.

    However, now I've come to analyse the data I've found that, using the Arrhenius equation, the activation energy with Cobalt 2+ ions is actually lower then both the uncatalysed reaction and with the Iron catalyst. About 34KJ/mol.

    Is this possible? How can a catalyst make the reaction go slower whilst lowering the activation energy? The only thing I can come up with is that the Pre exponential factor has somehow decreased but I don't know why.

    Any Ideas?
    Thanks
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    Hey! Im on a year out lol but I remember some of advanced H chemistry. Is it possibly something to do with the fact you have lower energy within the particles and therefore less collisions within a given time period? Hope thats of some help, can't really think of anything else.
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    are you taking into account that adding another component to the mixture will dilute your reactants and thus give you a lower concentration and so rate of reaction?
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    (Original post by gingerbreadman85)
    are you taking into account that adding another component to the mixture will dilute your reactants and thus give you a lower concentration and so rate of reaction?
    Yeah I am. I'm not adding much anyway but I have taken it into account.

    I've had a look online and the catalyst definately shouldnt work, but I just can't understand why my results show it to have a lower activation energy.
 
 
 
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