Catalysts

• ·         A catalyst is a substance that speeds up the rate of a chemical reaction without being used up, decreasing the required activation energy and providing an alternate reaction pathway.
• ·         Heterogeneous catalysts are present in a different state to the reactants (usually providing a solid surface where adsorption of a reactant and then desorption can occur) whereas homogenous catalysts are in the same state as reactants.
• ·         Heterogeneous catalyst can be made more efficient by having a large surface area and being spread onto a support medium, however they are eventually rendered unusable by poisoning of impurities and separation from the support medium.
• ·         Some important examples of heterogenous catalysts are in the Haber process and the Contact process.
• ·         In the Haber process, iron catalyses the reaction: N2 (g) + 3H2 (g) >> 2NH3 (g).
• ·         Vanadium oxide is used in the Contact process which makes sulfuric acid, being part of the key step 2SO2 + O2 ⇌ 2SO3 as shown in the two steps: SO2 + V2O5 >> SO3 + V2O4 and then 2V2O4 + O2 >> 2V2O5.
• ·         An example of homogeneous catalysis by a transition metal is the use of Fe2+ ions when peroxodisulfate ions oxidise iodide ions to iodine in the overall equation

  S2O82-(aq) + 2I-(aq) >> 2SO42-(aq) + I2(aq), with the action of the catalyst being shown by S2O82-(aq) + 2Fe2+(aq)  >>  2SO42-(aq) + 2Fe3+(aq) and then 2Fe3+(aq) + 2I-(aq) >>  2Fe2+(aq)  + I2(aq).
• ·         The Fe2+ ions reduce required activation energy because the reactions using them involve ions of opposite charges, whereas the overall reaction involves ions of the same charge, causing repulsion.
• ·         Autocatalysis is when one of the products of a reaction acts as a catalyst, shown in the reaction: 2MnO4−(aq) + 16H+(aq) + 5C2O42-(aq) >> 2Mn2+(aq) + 10CO2(g) + 8H20(l), where the Mn2+ ions cause the reactions 4Mn2+ + MnO4− + 8H+ >> 5Mn3+ + 4H2O and

  2Mn3+ + C2O42− >> 2Mn2+ + 2CO2.