Why does increasing the concentration of a catalyst reduce the rate of reaction?

The limiting factor was the concentration of the reactants(the substrate). As you increased the concentration of catalyst in the solution, the rate eventually peaked, meaning there was a maximum number of E-S(enzyme-substrate) complexes being formed. After this, when you continue to add more catalyst it would not make a difference because there isn't enough substrate to bind to the enzyme's active site.

A possible reason for the rate of reaction to decrease could be that further addition of catalyst after the maximum number of E-S complexes have been formed means that more catalyst molecules collide with catalyst molecules rather than reactant(substrate) molecules, resulting in a slight delay until the substrate binds to the active site.

Or it could all be an experimental error.

I hope I helped.

Were you using the same amount of catalyst in each reaction ?

I'd guess what's happening is that the reaction rate is the same regardless of the catalysts concentration, but the overall speed of the reaction was slower for the higher concentration because with more of the catalyst per unit volume (higher conc), you'll need more time for it all to react.

I may be wrong though, I haven't done Chemistry in years but that's what came to mind and noone else is answering!

I don't know if you understood, there is no enzyme involved :') But thanks anyway

To check out the suggestion by another.... (just seems you're all getting mixed up in it all)

What are the exact volumes and concentrations you are using?

After all the 'alternative pathways with lower activation energy' have been made for every single reactant molecule, further addition of catalyst would crowd the spaces between the 2 reactant molecules, making it take a little longer to get from one to the other with an intermediate compound.

uh.. lol :P

I'm using 1g zinc, 50cm^3 H2SO4 and 10cm^3 catalyst (which is diluted with distilled water accordingly)

Well then, as you're adding different amounts of solvent when you change your amount of catalyst...... try just changing one variable at a time