Chapter 6: Thermochemistry

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Chapter 6 Thermochemistry

• CHE 123 General Chemistry I
• Dr. Jerome Williams, Ph.D.
• Saint Leo University

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Overview

• Entropy Second Law Thermodynamics
• Predicting Spontaneity
• Free Energy

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Entropy Second Law Thermodynamics

• Second Law of Thermodynamics Reactions proceed
  in the direction that increases the entropy of
  the system plus surroundings.
• A spontaneous process is one that proceeds on its
  own without any continuous external influence.
• A nonspontaneous process takes place only in the
  presence of a continuous external influence.

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Entropy Second Law Thermodynamics

• The measure of molecular disorder in a system is
  called the systems entropy this is denoted S.
• Entropy has units of J/K (Joules per Kelvin).
• DS Sfinal Sinitial
• Positive value of DS indicates increased
  disorder.
• Negative value of DS indicates decreased disorder.

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Entropy Second Law Thermodynamics
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Predicting Spontaneity

• To decide whether a process is spontaneous, both
  enthalpy and entropy changes must be considered
• Spontaneous process Decrease in enthalpy
  (DH). Increase in entropy (DS).
• Nonspontaneous process Increase in enthalpy
  (DH). Decrease in entropy (DS).

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Predicting Spontaneity

• Predict whether DS is likely to be positive or
  negative for each of the following reactions.
  Using tabulated values, calculate DS for each
• a. 2 CO(g) O2(g) ? 2 CO2(g)b. 2 NaHCO3(s) ?
  Na2CO3(s) H2O(l) CO2(g)c. C2H4(g) Br2(g) ?
  CH2BrCH2Br(l)d. 2 C2H6(g) 7 O2(g) ? 4 CO2(g)
  6 H2O(g)

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Free Energy

• Gibbs Free Energy Change (DG) Weighs the
  relative contributions of enthalpy and entropy to
  the overall spontaneity of a process.
• DG DH TDS
• DG lt 0 Process is spontaneous
• DG 0 Process is at equilibrium
• DG gt 0 Process is nonspontaneous

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Free Energy

• Situations leading to DG lt 0
• DH is negative and TDS is positive
• DH is very negative and TDS is slightly negative
• DH is slightly positive and TDS is very positive
• Situations leading to DG 0
• DH and TDS are equally negative
• DH and TDS are equally positive
• Situations leading to DG gt 0
• DH is positive and TDS is negative
• DH is slightly negative and TDS is very negative
• DH is very positive and TDS is slightly positive

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Free Energy

• Which of the following reactions are spontaneous
  under standard conditions at 25C?
• a. AgNO3(aq) NaCl(aq) ? AgCl(s) NaNO3(aq)
  DG 55.7 kJ
• b. 2 C(s) 2 H2(g) ? C2H4(g) DG 68.1 kJ
• c. N2(g) 3 H2(g) ? 2 NH3(g) DH 92 kJ
  DS 199 J/K

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Free Energy

• Equilibrium (DG 0) Estimate the temperature
  at which the following reaction will be at
  equilibrium. Is the reaction spontaneous at room
  temperature?
• N2(g) 3 H2(g) ? 2 NH3(g) DH 92.0 kJ
  DS 199 J/K
• Equilibrium is the point where DG DH TDS
  0