Effect of amount of catalyst in a reaction

  1. Does increasing the amount of mass of the catalyst increase the rate of reaction? I'm not sure as increasing the volume of one reagents doesn't increase the rate of reaction. But for a catalyst I'm not sure if it works the same way. If the catalyst is solid then the concentration will be increased when mass is doubled but for fluid catalysts I'm not so sure. So what affects the rate of a catalysed reaction in terms of the catalyst itself? Thanks for the help! :smile:
  2. jcsd
  3. Borek

    Staff: Mentor

    In general - yes.

    Note that for solid catalysts it is usually not mass that counts, but the surface area.
  4. Hi Borek, i have some queries about the same topic too

    For example if i have 2 experiments set up, the chemicals are NaOH and HCl. In both set-ups they have the same concentration, but for the second experiment, the reagent in excess (say NaOH) has twice the volume than in the first one. In this scenario, I'm guessing that the rate of reaction will be the same as the factor that is changed is volume which doesn't affect the rate of reaction? So the gradient will more or less be exactly the same?

    Also, when we have an aqeuous catalyst, will it be better for it to have a high concentration and low volume rather than the other way? Because the water in the aqueous part will contribute to diluting the reactant particles so as a result the reaction speed might suffer a little. But with solid catalysts it does not have this negative factor as it does not affect the concentration of the reactant particles?

    say we are measuring the reaction speed of HCl+Zn. In both cases the volume and concentration of the HCl is the same but the mass of the zinc in the first experiment is 0.2g and the other is 0.1g. What will be the effect of this decrease in mass? I'm guessing the factor that affects the rates is the surface area. So the 0.1g has a smaller surface area, thus smaller reaction speed?

    Lastly, does the concentration of H2O2 when it decomposes affect the rate of reaction? Since now there isn't any particle collision to begin with..

    Thanks for the help Borek!
    Last edited: May 9, 2012
  5. Borek

    Staff: Mentor

    Please elaborate what do you mean by "same concentration but twice the volume". Are you going to mix the reagents solution? If so, the final concentration is dictated by dilution, so it can't be the same. Are you going to select initial concentration in such a way that you have to use twice the volume to get the same concentration in the final mix? If so initial concentration doesn't matter, only final matters - and it is the same. And if don't plan on mixing them there will be no reaction at all, so neither concentration nor volume matters.

    I am confused about what you are trying to say.
  6. oh i'm sorry about that.
    I mean for the first experiment you have 10cm^3 of 1mol/dm^3 of NaOH and HCl. In the second experiment you have 20cm^3 of NaOH at 1 mol/dm^3 and 10cm^3 of HCl at also 1mol/dm^3. So i was wondering how the rate of reaction of this experiment would go. I'm guessing it will be the same as each other
  7. Borek

    Staff: Mentor

    This is not a good example, as neutralization is so fast it will be limited by the mixing and diffusion, not by the reaction speed. But in general you should start calculating concentrations after mixing. They will be different, so the reaction speed should be different as well.
  8. I did the mole calculation. End the end the mole ratio of one is increased and the other is decreased. So I'm not sure what to assume previously each concentration is 0.5mol/dm3 with the added volume, the concentration of NaOH is 0.67mol/dm3 and HCL has a concentration of 0.33mol/dm3. Yea, so I'm not sure about the reaction speed..

    Also, how does increasing the concentration of H2O2 increase the rate of reaction? Since there is no collisions to begin with (decomposition). I read on chemguide that more particles have enough energy to decompose at any one time. However, won't increasing the volume also have the same effect?

    Thanks for the help Borek! :smile:
    Last edited: May 9, 2012
  9. Borek

    Staff: Mentor

    I wonder if you are not confusing intensive and extensive properties. Reaction speed is intensive, so it doesn't depend on the volume, amount of product is extensive, so it depends on the volume.
  10. Hi I guess I'm not very sure about the effects if concentration and volume on concentration. Could you please explain them to me? I can't seem to find any proper information relating them. Worst of all is volume I can't find any sources of information that is relevant... thanks for the help Borek!
  11. Borek

    Staff: Mentor

    Effects of volume and concentration on concentration? Your confusion is confusing.
  12. Sorry about that, I think my mind slipped. I'm not sure how volume affects the rate of reaction because in my example, with a larger volume of same concentration. One of the reagents has an increased concentration and the other has a reduced concentration. So I'm not sure how to determine the rate of reaction when dealing with situations like this. Thanks!
  13. Borek

    Staff: Mentor

    Use volume only to calculate concentration. Once you have concentrations forget about volume, it doesn't matter as long as the question is "what is the reaction speed".
  14. What if my original concentrations are 0.5mol/dm^3 for both, but after adding then my concentration of one of the reagents increased to 0.67 while the other decreases to 0.33? Then how do I tell the rates of reaction? Thanks for the help! :smile:
  15. Borek

    Staff: Mentor

    First things first - how do you calculate the reaction rate when concentrations were 0.5M?
  16. Er, I have not learnt how to calculate them, all I've learnt is the factors affecting it.. but it doesn't encompass this part.. thanks for the help Borek!
  17. Hi Borek, so with the two changed concentrations will there be a different rate of reaction? Thanks!
  18. Borek

    Staff: Mentor

    In most cases yes, but not necessarily - for example for the second order reaction if you change both concentrations in such a way their product stays the same, reaction rate doesn't change.
  19. Meaning in the example i gave the rates of reaction will change? But if the volume is changed such that the products is the same then the rates will be the same?
  20. Borek

    Staff: Mentor

    Which example?

    Assuming you have a reaction

    A + B -> C

    [tex]v = k [A] [/tex]


    so if you have both concentrations of 0.5M or one of 0.25M and the other of 1M, initial speed will be identical in both cases.

  21. Hi Borek, I meant this example:What if my original concentrations are 0.5mol/dm^3 for both, but after adding then my concentration of one of the reagents increased to 0.67 while the other decreases to 0.33? Then how do I tell the rates of reaction? Thanks for the help!

    Thanks for the help! BTW what is the name of that formula? Thanks!
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