Transition Metal Catalysts PDF - A Level Chemistry Past Paper

A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS HOMOGENEOUS CATALYSTS Transition metals make good catalysts as they have variable stable oxidation states. It allows them to facilitate reactions that would normally have high...

A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS HOMOGENEOUS CATALYSTS Transition metals make good catalysts as they have variable stable oxidation states. It allows them to facilitate reactions that would normally have high activation energies by offering an alternative pathway for the transfer of electrons. A homogeneous catalyst is one that exists in the same phase / state as the reactants. e.g. the (aq) reaction between peroxodisulfate ions (S2O32-) and iodide ions (I-) Normally this reaction has a very high activation energy. This is because both reactants have negative charges and naturally repel each other. Fe2+(aq) is used as a “go-between” to allowing the reaction to have a much lower activation Catalysed Reaction: 2Fe2+ ions donate an 1. S2O82-(aq) + 2Fe2+(aq) → 2SO42-(aq) + 2Fe3+(aq) electron each from S2O82- The 2Fe3+ ions produced 2. 2Fe3+(aq) + 2I-(aq) → 2Fe2+(aq) + I2(aq) then accept an electron each from 2I-. Overall: S2O82-(aq) + 2I-(aq) → 2SO42-(aq) + I2(aq) Notice how the Fe2+ is regenerated / remains chemically unchanged overall. The Eact for the 2-step catalysed reaction via the *intermediate * species, is lower than Reactants Energy that of the uncatalysed reaction. Products Reaction AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS HETEROGENEOUS CATALYSTS A heterogeneous catalyst is one that exists in a different phase / state as the reactants. They are usually solids and provide active sites for the reaction to place place on. Reactants Products CATALYST A thin coating of the solid catalyst is used on a support medium (e.g. an unreactive mesh) to maximise surface area (increased efficiency) and to minimise costs. However, over time, the active sites can be poisoned (blocked) by impurities and by-products. This reduces the efficiency of the catalyst and, ultimately, cost money to replace. THE CONTACT PROCESS Catalysed Reaction: V2O5 donates an O atom 1. 2SO2 (g) + 2V2O5 (s) → 2SO3 (g) + 2V2O4 (s) to SO2 The V2O5 then 2. 2V2O4 (s) + O2 (g) → 2V2O5 (s) regenerates by reacting with oxygen Overall: 2SO2 (g) + O2 (g) ⇌ 2SO3 (g) During this reactions, vanadium is reduced from +V oxidation state to +IV and back again. The catalysed reaction has a much lower activation energy than the uncatalysed reaction. THE HABER PROCESS Fe(s) is used as a catalyst in the Haber process. You do not need to know the catalysed equations here. N2(g) + 3H2(g) ⇌ 2NH3(g) AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS AUTOCATALYSIS This phenomenon occurs when the products of a reaction act as a catalyst for the reaction itself. As a consequence, the more product that is made, the faster the reaction proceeds! e.g. the reaction between MnO4-(aq) and C2O42+(aq) 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq) → 2Mn2+(aq) + 10CO2 (g) + 8H2O(l) Mn2+ acts as a homogeneous catalyst for the reaction between the two reactants Catalysed Reaction: 1. 4Mn2+(aq) + MnO4-(aq) + 16H+(aq) → 5Mn3+(aq) + 8H2O(l) 2. 2Mn3+(aq) + 2C2O42 → 10CO2 (g) + 2Mn2+(aq) The two-step reaction has a lower activation energy than the direct reaction. This means the reaction speeds up as it proceeds… Reaction is slow to start as the initial [Mn2+] is very low Reaction speeds up as more Mn2+ is produced. [MnO4-] Reaction slows down again as [MnO4-] reaches zero. This is due to decreased frequency of collisions. Reaction AQA www.chemistrycoach.co.uk © scidekick ltd 2024