Q6(b). Heterogeneous catalysis; PH₃ on tungsten
What is meant by heterogeneously catalyzed reactions? Mention the consecutive steps for a reaction to occur on a surface. Using Langmuir adsorption theory, account for the experimental observation that the decomposition of PH₃ on tungsten follows first-order kinetics at low pressure and zeroth-order kinetics at high pressure.
Answer in brief
In heterogeneous catalysis the catalyst is in a different phase from the reactants and the reaction takes place on its surface; for PH₃ on W, rate = kθ = kKp/(1 + Kp): first order when Kp ≪ 1 and zero order when Kp ≫ 1.
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Coversheterogeneous catalysisLangmuir adsorption isothermPH₃ decomposition on tungstenzero-order kineticssurface coverage
All parts of Question 6
- 6(a)(i) Moles of HCl formed per joule absorbed at 480 nm10 marks
- 6(a)(ii) Molar extinction coefficient from 50 % transmission5 marks
- 6(b) Heterogeneous catalysis; PH₃ on tungsten15 marks
- 6(c)(i) The sodium pump and its role10 marks
- 6(c)(ii) Bohr effect; role of cytochrome P45010 marks