The physical state of catalysts

Active sites on nickel speed up the reaction between ethene and hydrogen 

When the reactants and the catalyst are in the same physical state the catalyst is called a homogeneous catalyst e.g. concentrated sulfuric acid speeds up the reaction between ethanoic acid and ethanol to form the ester, ethyl ethanoate. Manganese(IV) oxide catalyses the thermal decomposition of potassium chlorate(V) to give oxygen. 

Adsorption occurs on the surface of a solid, whereas absorption occurs within the substance. 

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