Name: ______

Thermodynamics (Entropy, Enthalpy, and Gibbs)

Spontaneity

  1. True or false? All exothermic reactions are spontaneous?
  2. Which variable can tell you if a process is spontaneous without exception?

a. DHrxn b. DSrxn c. DHsurr d. DSsurr e. DStot

Enthalpy and Entropy

  1. Circle the correct words: Nature tends towards (higher or lower) energy and more (order or disorder)?

4. Which of the following reactions will have the most positive ΔHo ? (Hint: Draw Lewis structures.)

a. N2(g) → 2 N(g)

b. F2(g) → 2 F(g)

c. O2(g) → 2 O(g)

d. These reactions would all have the same ΔHo .

5.  Which state of matter has the highest entropy? ______

6.  Predict whether the entropy change will be positive or negative for the following:

a.  H2O (g) g H2O (l) DS_____

b.  C6H12O6(s) g 2C2H5OH(l) + 2CO2(g) DS_____

  1. 2NH3(g) + CO2(g) g H2O(l) + NH2CONH2(aq) DS_____
  2. NaCl(s) g NaCl(aq) DS____
  3. Cu(s) (100oC) g Cu(s) (25oC) DS____
  4. 2NH3(g) D N2(g) + 3H2(g) DS____

7.  If a process is endothermic and the process creates more order than existed before, the process is:

a. always spontaneous b. never spontaneous c. spontaneous at high T d. spontaneous at low T

8. Which of the following substances has the greatest entropy per mole?

a. O2(g)

b. N2(g)

c. CO(g)

d. CO2(g)

e. C4H10(g)

9. Which of the following reactions will have an increase in entropy? Choose all that apply.

a. SO3(g) → 2SO2(g) + O2(g)

b. H2O(l) → H2O(s)

c. Br2(l) → Br2(g)

d. H2O2(l) → H2O(l) + ½ O2(g)

10. Without consulting entropy tables, predict the sign of ΔS for the following process and choose the correct reasoning for your prediction: The mass of nitrogen remains constant.

N2(g, 10 atm) → N2(g, 1 atm)

a. positive; there is an increase in the number of gas molecules

b. positive; the gas expands into a larger volume

c. negative; the gas is compressed into a smaller volume

d. negative; the gas expands into a larger volume

e. negative; there is a decrease in the number of gas molecules.

11. Without consulting entropy tables, predict the sign of ΔS for the following process:

Pb(s) + Cl2(g) → PbCl2(s).

a. ΔS > 0

b. ΔS < 0

c. ΔS = 0

d. More information is needed to make a reasonable prediction.

Standard Molar Entropies

12. Which of the following compounds has the lowest entropy at 25 oC?

a. CH3OH(l)

b. CO(g)

c. MgCO3(s)

d. H2O(l)

e. H2O(g)

13. Using the data below, calculate ΔSorxn for the following reaction:

4 Cr(s) + 3 O2(g) → 2 Cr2O3(s)

Substance / ΔS, J/K·mol
Cr(s) / 23.77
O2(g) / 205.138
Cr2O3(s) / 81.2

a. 548.1 J/K

b. 147.7 J/K

c. -147.7 J/K

d. -548.1 J/K

e. None of the above.

14.  Calculate the standard entropy change, ΔS°, for the following reaction at 25 °C:

4Al(s) + 3O2(g) ® 2Al2O3(s)

S°[Al(s)] = 28.32 S°[O2(g)] = 205 S°[Al2O3(s)] = 51.0

Free Energy

15. In 1774 Joseph Priestly prepared oxygen by heating mercury(II) oxide according to the reaction HgO(l) → Hg(l) + ½O2, for which ΔHo = 90.84 kJ/mol and ΔSo = 108 J/K.mol. Which of the following statements is true for this reaction?

a. The reaction is spontaneous only at low temperatures.

b. The reaction is spontaneous at all temperatures.

c. ΔGo becomes less favorable as temperature increases.

d. The reaction is spontaneous only at high temperatures.

e. The reaction is at equilibrium at 25 oC and 1 atm pressure.

16. For the decomposition reaction of hydrogen peroxide:

H2O2(g) → H2O(l) + 1/2 O2(g) , ΔHo = -106 kJ/mol; ΔSo = 58 J/K

Is H2O2(g) stable?

a. Yes, under all conditions.

b. Yes, if the temperature is low enough.

c. Yes, if the rate of decomposition is low.

d. Yes, if the O-O bond energy is greater than the O-H bond energy.

e. No.

17.  a. Calculate the standard free energy change, ΔG°, for the following at 25 °C:

MgO(s) + C(graphite) ® Mg(s) + CO(g)

ΔH° = 491.18 kJ ΔS° = 197.67 J/K

b. Is this reaction spontaneous at 25 °C? If not, at what temperature can we make this reaction spontaneous?

18.  Calculate DHorxn, DSorxn and DGorxn for Mg(s) + O2(g) D MgO(s)) (Hint – balance the rxn)

ΔH°f of MgO=-601.7kJ/mol, ΔS°of MgO=26.9 J/mol, ΔS° of Mg=32.7 J/mol, ΔS° of O2=205.0 J/mol,

ΔG°f of MgO=-569.4 kJ/mol,

19.  From # 18 the results for DHorxn means (exothermic or endothermic), the results for DSorxn means (disorder or order) increased, and DGorxn means (products or reactants) are favored under standard conditions.

20.  Can you measure enthalpy (H)? entropy (S)? Gibb’s free energy (G)?

Standard Free Energy of Formation

21.  Which of the following does not have a standard enthalpy of formation of zero?

A. I2(l) B. Cl2(g) C. Au(s) D. Ne(g) E. Hg(l)

22.  The standard enthalpy of formation equation for NH3(g) is:

a) N(g) + 3H(g) ® NH3(g) b) N2(g) + 3H2(g) ® 2NH3(g)

c) N2(l) + H2(g) ® NH3(g) d) N2(g) + H2(g) ® NH3(g)

23.  Calculate DH for the following reaction at 25 °C.

4NH3(g) + 5O2(g) ® 4NO(g) + 6H2O(g)

DH[NO(g)] = 90.3 DH [H2O(g)] = -241.8 DH[NH3(g)] = -45.9

24. Use the following data to calculate ΔGo at 298 K for the combustion of propane:

C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(l)

Substance / DGof, kJ/mol
C3H8(g) / -23.0
O2(g) / 0
CO2(g) / -394.6
H2O(l) / -237.2

a. 2109.6 kJ/mol

b. 608.8 kJ/mol

c. -608.8 kJ/mol

d. -2109.6 kJ/mol

e. None of the above.

25. Calculate DGorxn at 25oC for this reaction: 2 NaHCO3(s) g Na2CO3(s) + CO2(g) + H2O(g). Given: DHorxn = 128.9 kJ/mol and DSorxn = 321 J/molK