Free Energy and the Direction of Spontaneous Change · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Which way will it go: free energy decides

Science · Thermal & Statistical Physics · ages 20-22
Name ______________________   Date ____________

  1. At fixed T and pressure, a closed system with only pressure-volume work has negative delta G for a proposed change. What does this mean?

    • The forward change is thermodynamically favored
    • The forward change requires an external drive
    • The forward change must happen instantaneously

  2. Gibbs free energy G combines enthalpy H and entropy S at temperature T. How?

    • H plus TS
    • H over T
    • H-TS

  3. A spontaneous process always happens fast.

    Circle one:   True   False

  4. At fixed T and pressure, the reaction Gibbs energy is zero at the system's current composition. What does this indicate?

    • A maximum possible reaction speed
    • An externally forced reaction direction
    • No net thermodynamic reaction drive

  5. Pure ice at atmospheric pressure does not melt spontaneously below its melting temperature but does above it. Which relation explains melting above that boundary?

    • The positive delta H term exceeds T delta S for melting
    • The positive T delta S term exceeds delta H for melting
    • The melting entropy change becomes negative when warmed

  6. In a stated model with constant delta H<0 and delta S>0, when is delta G=delta H-T delta S negative?

    • At every positive T in this model
    • Only at low positive T in this model
    • Only at high positive T in this model

  7. Under the Gibbs spontaneity conditions, an endothermic change has negative delta G. Which relation must hold?

    • The negative entropy change outweighs absorbed heat
    • The enthalpy change equals zero despite heat absorption
    • The positive T delta S term exceeds positive delta H

  8. Treat delta H=60 J/mol and delta S=0.2 J/(mol K) as constant over the relevant range. At what temperature does delta G=delta H-T delta S become zero?

    • 12 K
    • 300 K
    • 0.003 K
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Answer key

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Which way will it go: free energy decides W1-mt_ww43eqzupN-s1

  1. The forward change is thermodynamically favored · Negative delta G favors the forward direction under these conditions. It does not set the rate.
  2. H-TS · Gibbs free energy is defined as G=H-TS.
  3. False · Spontaneity describes only the direction. Kinetics determines the rate.
  4. No net thermodynamic reaction drive · Zero reaction Gibbs energy indicates reaction equilibrium: neither direction has a net thermodynamic drive.
  5. The positive T delta S term exceeds delta H for melting · Melting has positive delta H and delta S. Above the boundary, T delta S exceeds delta H, making delta G negative.
  6. At every positive T in this model · For positive T, delta H is negative and -T delta S is also negative. Their sum is negative.
  7. The positive T delta S term exceeds positive delta H · With delta H>0, delta G can be negative only if T delta S is positive and larger than delta H.
  8. 300 K · Set 60-0.2T=0. Then T=60/0.2=300 K.
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