Explain why the catalyst is used as a very fine powder and larger pieces of iron are not used.

Catalyst is used as a very fine powder and larger pieces of iron are not used. This is because the surface area of catalyst needs to be large so that more of the surface is exposed to the substrate and more of the substrate is catalyzed.

=> Larger Pieces of Iron has a smaller surface area than the fine particles.

=> Larger the surface area of catalysts/enzymes , more will be the reaction rate and vice versa.

Hope this helped!

the volume increases

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