Transition State Theory and the Eyring Equation · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Counting reactions at the barrier top

Science · Chemistry · ages 20-22
Name ______________________   Date ____________

  1. What is the activated complex?

    • The reacting pair at the top
    • The slowest reactant in the mixture
    • A catalyst that lowers the barrier

  2. Why does collision theory fail for bulky molecules?

    • Molecules stop colliding when they grow large
    • Aim and orientation are miscounted
    • Bulky molecules carry no activation energy

  3. The Eyring equation builds the rate constant from enthalpy and entropy of activation.

    Circle one:   True   False

  4. How do the Arrhenius activation energy and the activation enthalpy compare for a gas reaction?

    • They are always exactly equal
    • Arrhenius energy is a little larger
    • The activation enthalpy is far larger

  5. A reaction shows a large negative entropy of activation. What does that say about the transition state?

    • It is loose and disordered
    • It has fallen apart into free atoms
    • It is tight and ordered

  6. On an Eyring plot of the natural log of k over T against one over T, what does the slope give?

    • The entropy of activation
    • Enthalpy of activation
    • The rate constant itself

  7. Rate data give a strongly negative activation entropy. A student argues the transition state is loose. How do you answer?

    • Agree, since negative values always mean disorder
    • Argue for an ordered state, since freedom was lost
    • Ask for more data, since entropy says nothing about geometry

  8. Two reactions give Eyring lines, one much steeper than the other. What follows?

    • The steeper one has the larger enthalpy of activation
    • The steeper one has the larger entropy of activation
    • The steeper one must have the smaller rate constant at every temperature
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Counting reactions at the barrier top W1-mt_JVjDMcXds1-s1

  1. The reacting pair at the top · It is the reacting pair poised where the barrier peaks.
  2. Aim and orientation are miscounted · Counting crashes works, but judging their aim does not.
  3. True · Both activation parameters sit inside the Eyring expression.
  4. Arrhenius energy is a little larger · The experimental slope energy always runs a little above the enthalpy gap.
  5. It is tight and ordered · Lost freedom on the way up means a cramped, ordered complex.
  6. Enthalpy of activation · Slope tracks the enthalpy barrier; the intercept holds the entropy.
  7. Argue for an ordered state, since freedom was lost · The value is evidence of partners pinned into order at the barrier top.
  8. The steeper one has the larger enthalpy of activation · Slope is the enthalpy signal, so steep means a tall enthalpy barrier.
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