Chemistry 30 Thermodynamics

For each of the following pairs, select the situation which favors a spontaneous reaction:

endothermic reaction or exothermic reaction

negative value of ΔH° or positive value of ΔH°

negative value of ΔS° or positive value of ΔS°

increasing entropy or decreasing entropy

positive value of ΔG° or negative value of ΔG°

Describe the circumstances where:

ΔH_f°

S = 0

ΔG_f° = 0

ΔG = 0

Which one of the following shows and increase in entropy:

dissolving sugar in a cup of hot coffee

arranging a pack of playing cards into suits.

building a sand castle on a beach.

Using values of ΔG_f° from the table shown at the right, calculate ΔG ° for the following reaction and tell whether or not the reaction will occur spontaneously. Show your work clearly.

Use the formula ΔG = ∑ΔG products - ∑ΔG reactants

Substance	ΔG_f° (kJ/mol)

C₂H₆(g)	-32.9
Cl₂(g)	0.0
C₂H₄Cl₂(g)	-80.3
HCl (g)	-95.2

C₂H_{6 (g)} + 2Cl_{2 (g)} → C₂H₄Cl_{2 (g)} + 2 HCl _(g)

Will the reaction be spontaneous?

Calculate ΔG ° using the formula ΔG = ΔH - T ΔS

Values for ΔH and ΔS are given. All reactions take place at 25 ° C (298 K). Remember to convert ΔS values to kJ.

CH₃OH _(l) + 1½ O_{2 (g)} → CO_{2 (g)} + 2 H₂O _(g)

ΔH = -638.4 kJ ΔS = 156.9 J / K

2 NO_{2 (g)} → N₂O_{4 (g)}

ΔH = - 57.2 kJ ΔS = -175.9 J / K

Calculate ΔG ° for the following reaction using values of ΔG_f° obtained from the Table of Thermochemical Data. Will the reaction be spontaneous?

Use the formula ΔG = ∑ ΔG products - ∑ ΔG reactants

3 Fe₂O_{3 (s)} → 2 Fe₃O_{4 (s)} + ½ O_{2 (g)}

For a certain spontaneous reaction, the change in enthalpy (ΔH° ) is -92.0 kJ and ΔG° = -50.2 kJ at 25°C. Calculate ΔS.

Chemistry 30

Thermodynamics

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