Free Energy and Whether a Reaction Is Feasible · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Feasible or Not

Science · Chemistry · ages 17-18
Name ______________________   Date ____________

  1. Delta G is negative at a given temperature. What does that mean?

    • The reaction is feasible there
    • The reaction is impossible everywhere
    • The reaction runs instantly

  2. What is the Gibbs free energy equation?

    • Delta G equals delta H minus T times delta S
    • Delta G equals delta H plus T divided by delta S
    • Delta G equals T minus delta H times delta S

  3. Enthalpy is in kilojoules per mole and entropy in joules per mole per kelvin. What first?

    • Add them directly and fix the label later
    • Convert one so both use matching units
    • Drop the temperature from the equation

  4. Delta G works out at +60 kilojoules per mole at 300 kelvin. Is the reaction feasible there?

    • Yes, any value means it goes
    • Only if the mixture is stirred
    • No, a positive value means not feasible

  5. Delta H is 120 kilojoules per mole, delta S is 0.2 kilojoules per mole per kelvin, T is 300 kelvin. What is delta G in kilojoules per mole?

    Answer: ______________

  6. Delta H is 180 kilojoules per mole and delta S is 150 joules per mole per kelvin. First convert 180 kilojoules to 180000 joules. At what temperature in kelvin does delta G reach zero?

    Answer: ______________

  7. A student says a strongly negative delta G guarantees a fast reaction. This claim is true.

    Circle one:   True   False

  8. Limestone decomposition is endothermic and releases carbon dioxide gas. What does the equation predict about heating?

    • Heating never matters for this reaction
    • Heating can flip it feasible, since the entropy gain wins at high T
    • Cooling is what makes it feasible
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Answer key

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

Feasible or Not W1-mt_o1kZgBYWoM-s1

  1. The reaction is feasible there · A negative sign means feasible at that temperature, though it may still be slow.
  2. Delta G equals delta H minus T times delta S · Free energy balances enthalpy against temperature weighted entropy with T in kelvin.
  3. Convert one so both use matching units · Mismatched units skew the answer a thousandfold, so convert before calculating.
  4. No, a positive value means not feasible · Positive delta G means the reaction is not feasible at that temperature.
  5. 60 · 120 minus 300 times 0.2 gives delta G of 60 kilojoules per mole.
  6. 1200 · Setting delta G to zero gives T as 180000 over 150, which is 1200 kelvin.
  7. False · Feasibility ignores the activation barrier, so the rate can stay tiny however negative delta G is.
  8. Heating can flip it feasible, since the entropy gain wins at high T · Endothermic but entropy driven reactions turn feasible when hot, as the T times delta S term wins.
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