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Thermodynamics

Enthalpy, entropy, Gibbs energy, Hess's law

Key Concepts
  • First law: energy is conserved; ΔU = Q − W
  • Enthalpy H = U + PV; at constant pressure, ΔH = Q_p
  • Standard enthalpy of formation (ΔH°_f) of elements = 0
  • Gibbs free energy predicts spontaneity: ΔG = ΔH − TΔS
  • Hess's law: ΔH is independent of pathway
Important Formulae
First law ΔU = Q − W
Gibbs free energy ΔG = ΔH − TΔS
ΔG and equilibrium constant ΔG° = −RT ln K
Born-Haber cycle (lattice energy) ΔH_lattice from Hess's law
Kirchhoff's equation ΔH₂ = ΔH₁ + ΔC_p(T₂ − T₁)
Quick Tips
  • ΔG < 0: spontaneous; ΔG > 0: non-spontaneous; ΔG = 0: at equilibrium.
  • Exothermic reactions have ΔH < 0; endothermic have ΔH > 0.
  • Entropy increases when solids dissolve, gases are produced, or temperature increases.
Sample Practice Questions

  1. Entropy is a measure of:

    • Energy
    • Enthalpy
    • Disorder or randomness
    • Temperature
    Show answer

    Answer: Disorder or randomness

  2. Bond dissociation energy is the energy required to:

    • Form one mole of a bond
    • Break one mole of a bond in gaseous phase
    • Break all bonds in a molecule
    • Melt one mole of solid
    Show answer

    Answer: Break one mole of a bond in gaseous phase

  3. For a spontaneous process at constant T and P, ΔG must be:

    • Positive
    • Negative
    • Zero
    • Infinity
    Show answer

    Answer: Negative

  4. Resonance energy is found from:

    • Hess's law alone
    • Difference between theoretical and actual bond energy
    • ΔG calculation
    • Third law of thermodynamics
    Show answer

    Answer: Difference between theoretical and actual bond energy

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Practice Questions

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