The decomposition of phosphorus pentachloride in the presence of moisture takes place by the following mechanism. The rate equation for this reaction will be:
Correct answer: B. Rate = k [PCl5][H2O]
- A. Rate = k [POCl3][H2O]3
- B. Rate = k [PCl5][H2O]
- C. Rate = k [PCl5][H2O]4
- D. Rate = [PCl5][H2O]
Explanation
For the rate equation, the reactants of ONLY the rate-determining step, which is the slow step, are taken into account in a multi-step reaction, NOT the overall equation or the fast step. Since the slow step here consists of one mole of PCL5 and one mole of H2O in the reactants, the rate equation will contain these two substances, each raised to the power 1, which is reflected in option B only. Option A takes into account the fast step only. Option C takes into account the overall reaction. Option D does not have the rate constant k mentioned in the question.
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About Reaction Kinetics
Reaction kinetics relates reaction rate to concentration, temperature, surface area and catalysts. Questions cover rate laws, reaction order, rate constants, activation energy and the activated complex, including how a catalyst lowers the activation energy without changing the overall energy change or equilibrium position.
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