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

Roarmaster
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I have been doing an experiment in school. The reaction of zinc with sulphuric acid. To speed up the reaction I used a Cobalt(II) Nitrate; in general it increased the rate of reaction. Something I found weird was that out of my three concentrations (0.1mol, 0.2mol and 0.5mol) as I increased the concentration the rate of reaction decreased.

Can anyone explain why this happens?
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Xyloid
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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!
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vickvam
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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.
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Roarmaster
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(Original post by vickvam)
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.
I don't know if you understood, there is no enzyme involved :') But thanks anyway
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Roarmaster
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(Original post by Xyloid)
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!
The speed of the reaction and the rate of the reaction, in this scenario, is pretty much the same thing. The only thing I can think of is that the CoNO3 acts as a poison in some way
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vickvam
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(Original post by Roarmaster)
I don't know if you understood, there is no enzyme involved :') But thanks anyway
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
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username913907
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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?
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Roarmaster
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(Original post by JMaydom)
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?
I'm using 1g zinc, 50cm^3 H2SO4 and 10cm^3 catalyst (which is diluted with distilled water accordingly)
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Roarmaster
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(Original post by vickvam)
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
That does make sense! Thanks
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username913907
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(Original post by Roarmaster)
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
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Roarmaster
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(Original post by JMaydom)
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
I am only changing one variable; the concentration of the catalyst
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