© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

Example 17.1 Predicting the Sign of Entropy Change For Practice 17.1 Predict the sign of ΔS for each process: a. the boiling of water b. I2(g) → I2(s) c. CaCO3(s) → CaO(s) + CO2(g)

© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

Example 17.2 Calculating Entropy Changes in the Surroundings

For Practice 17.2 Consider the reaction between nitrogen and oxygen gas to form dinitrogen monoxide:

2 N2(g) + O2(g) → 2 N2O(g) ΔHrxn = +163.2 kJ

a. Calculate the entropy change in the surroundings associated with this reaction occurring at 25 °C. b. Determine the sign of the entropy change for the system. c. Determine the sign of the entropy change for the universe. Will the reaction be spontaneous? For More Practice 17.2 A reaction has ΔHrxn = –107 kJ and ΔSrxn = 285 J/K. At what temperature is the change in entropy for the reaction equal to the change in entropy for the surroundings?

© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

For Practice 17.3 Consider the reaction:

C2H4(g) + H2(g) → C2H6(g) ΔH = –137.5 kJ; ΔS = –120.5 J/K Calculate ΔG at 25 °C and determine whether the reaction is spontaneous. Does ΔG become more negative or more positive as the temperature increases?

Example 17.3 Computing Gibbs Free Energy Changes and Predicting Spontaneity from ΔH and ΔS

© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

Example 17.4 Calculating Standard Entropy Changes (ΔSrxn)

Calculate ΔSrxn for the balanced chemical equation:

2 H2S(g) + 3 O2(g) → 2 H2O(g) + 2 SO2(g)

For Practice 17.4

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© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

Example 17.5 Calculating the Standard Change in Free Energy for a Reaction Using ΔGrxn = ΔHrxn – TΔSrxn

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For Practice 17.5 Consider the oxidation of NO to NO2: Calculate ΔGrxn at 25 °C and determine whether the reaction is spontaneous at standard conditions. °

© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

Example 17.6 Estimating the Standard Change in Free Energy for a Reaction at a Temperature Other than 25 °C Using ΔGrxn = ΔHrxn – T ΔSrxn

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For Practice 17.6 For the reaction in For Practice 17.5, calculate the value of ΔGrxn at –55 °C. Is the reaction more spontaneous (more negative ΔGrxn) or less spontaneous (more positive ΔGrxn) at the lower temperature?

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© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

One of the reactions that occurs within a catalytic converter in the exhaust pipe of a car is the simultaneous oxidation of carbon monoxide and reduction of NO (both of which are harmful pollutants).

2 CO(g) + 2 NO(g) → 2 CO2(g) + N2(g) Use standard free energies of formation to determine ΔGrxn for this reaction at 25 °C. Is the reaction spontaneous at standard conditions?

For Practice 17.7

In For Practice 17.7, you calculated ΔGrxn for the simultaneous oxidation of carbon monoxide and reduction of NO using standard free energies of formation. Calculate ΔGrxn for that reaction again at 25 °C, only this time use ΔGrxn = ΔHrxn – TΔSrxn. How do the two values compare? Use your results to calculate ΔGrxn at 500.0 K and explain why you could not calculate ΔGrxn at 500.0 K using tabulated standard free energies of formation.

For More Practice 17.7

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Example 17.7 Calculating ΔGrxn from Standard Free Energies of Formation

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© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

Find ΔGrxn for the reaction:

N2O(g) + NO2(g) → 3 NO(g) Use the following reactions with known ΔG values:

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Example 17.8 Calculating ΔGrxn for a Stepwise Reaction °

© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

Consider the reaction at 298 K:

2 H2S(g) + SO2(g) → 3 S(s, rhombic) + 2 H2O(g) ΔGrxn = –102 kJ Calculate ΔGrxn under these conditions:

PH2S = 2.00 atm; PSO2 = 1.50 atm; PH2O = 0.0100 atm

Is the reaction more or less spontaneous under these conditions than under standard conditions?

For Practice 17.9

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Example 17.9 Calculating ΔGrxn under Nonstandard Conditions

© 2014 Pearson Education, Inc. Chemistry: A Molecular Approach, 3rd Edition Nivaldo J. Tro

Calculate ΔGrxn at 298 K for the reaction:

For Practice 17.10

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Example 17.10 The Equilibrium Constant and ΔGrxn