Chem Kinetics Practice Problems Answers

Kinetics Practice Problems 1. Methyl isocyanide undergoes a first order isomerization to form cyanide. CH3NC(g)  CH3CN(g) The initial [CH3NC] was 0.0258 M, and after 11.4 minutes analysis showed that the concentration of the product was 0.0013 M. (a) What is the first order rate constant ? (ans. 0.00453) (b) What is the half-life of methyl isocyanide ? (ans. 153 min) (c) How long will it take for 90% of the CH3NC to be used up ? (ans. 508 min) 2. At 230oC the rate constant for methyl isocyanide isomerization is 9.24x10-4 s-1. (a) What fraction of the original isocyanide will remain after 60.0 minutes? (ans. 0.036) (b) What is the half-life of methyl isocyanide at this temperature? (ans. 12.4 min) 3. The hydrolysis of sucrose follows first order kinetics: C12H22O11 + H2O sucrose  C6H12O6 + glucose C6H12O6 fructose Where R = k[C12H22O6] and at 27oC k = 2.1x10-6 s-1. (a) Starting with the sucrose concentration of 0.10 M at 27oC, what would the concentration of sucrose be 24 hours later? (ans. 0.083M) (b) What is the half-life of sucrose? (ans. 3.3x105 s) 4. It takes 30 minutes for the concentration of a reactant in a 2nd order reaction to drop from 0.4 M to 0.30 M. (a) What is the rate constant for this reaction? (ans. 0.27 M-1min-1) (b) How long will it take for the concentration to drop from 0.40 M to 0.20 M? (ans. 92.4 min) 5. The rate constant of a first order reaction is 0.0346 s-1 at 298 K. If the activation energy is 50.2 kJ/mol, how would the magnitude of the rate constant be changed at 350 K? That is, would it be a larger or smaller number? Make reference to the Arrhenius equation in justifying your answer.