Ch. 17 - Chemical ThermodynamicsWorksheetSee all chapters
All Chapters
Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
Ch.6 - Thermochemistry
Ch.7 - Quantum Mechanics
Ch.8 - Periodic Properties of the Elements
Ch.9 - Bonding & Molecular Structure
Ch.10 - Molecular Shapes & Valence Bond Theory
Ch.11 - Liquids, Solids & Intermolecular Forces
Ch.12 - Solutions
Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
Ch.18 - Electrochemistry
Ch.19 - Nuclear Chemistry
Ch.20 - Organic Chemistry
Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

Gibbs Free Energy (ΔG) represents energy associated with a chemical reaction that can be used to do work.

Gibbs Free Energy and Calculations

Concept #1: Gibbs Free Energy

Example #1: The reduction of iron (III) oxide with hydrogen produces iron metal and can be written as follows: 

If our Gibbs Free Energy is less than zero then a reaction is spontaneous.  

Practice: If ∆G is small and positive which of the following statements is true?

If Gibbs Free Energy is less than zero then a reaction will be spontaneous and favor both products and the forward direction. 

Practice: Nitrogen gas combines with fluorine gas to form nitrogen trifluoride according to the reaction below at 25oC:

N2 (g)  +  3 F2 (g)  →  2 NF3 (g)               ΔHo = -249.0 kJ                       ΔSo = -278 J/K

Calculate ΔGo and state if the reaction favors reactants or products at standard conditions.

To determine if a compound will either freeze or vaporize we must first determine their normal freezing and boiling point by assuming Gibbs Free Energy is zero. 

Example #2: For mercury, ΔHvap = 58.5 kJ/mol and ΔSvap = 92.9 J/Kmol at 25°C. Does mercury boil at 350°C and 1 atm pressure?

Besides our standard equation for Gibbs Free Energy we have another that connects it to our equilibrium constant K

Example #3: The chemical reaction, 2 NO2 (g) + Br2 (g), has a ΔS°= 135 J/molK and ΔH°​=926 kJ/mol. Calculate the temperature when Keq = 4.50 x 105.

Under nonstandard conditions the pressure, temperature and molarities will be different from 1.0 atm, 25oC and 1.0 M respectively and so we use the following equation for Gibbs Free Energy:

Example #4: Calculate ΔGrxn at 25°C under the conditions shown below for the following reaction. 

3 Cl2(g) → 2 Cl3 (g)     ΔG° = +31.6 kJ

Example #5: For the reaction: N(g) + 2 O2 (g) ⇌ 2 NO2 (g), ΔG° = 75,550 J/mol at 175 K and ΔG°​ = 41,875 J/mol at 225 K. 

Additional Problems
Consider the reaction below, what is ΔG when the pressures of the gases are as follows at 25 °C? H2 = 0.20 atm; Cl 2 = 0.30 atm; HCl = 0.90 atm. The value for ΔG° is −190 kJ/mol                         H2(g) + Cl2(g) → 2 HCl(g) A.  −190.0 kJ/mol B.  6.4 kJ/mol C.  45.5 J/mol D.  183.5 kJ/mol E.  −183.5 kJ/mol  
Consider the reaction 2 Fe2O3(s) + 3 C(s) → 4 Fe(s) + 3 CO2(g), ΔH° = 462 kJ, ΔS° = 558 J • K -1. Calculate the equilibrium constant for this reaction at 525°C. 1. 1.9 x 106 2. 2.8 x 10-2  3. 8.07 x 10-2 4. 3.04 x 10-3 5. 5.20 x 10-7      
As O2 (l) is cooled at 1 atm, it freezes at 54.5 K to form Solid I. At a lower temperature, Solid I rearranges to Solid II, which has a different crystal structure. Thermal measurements show that ΔH for the I→II phase transition = -743.1 J/mol, and ΔS for the same transition = -17.0 J/K mol. At what temperature are Solids I and II in equilibrium? A. 2.06 K B. 43.7 K C. 31.5 K D. 53.4 K E. They can never be in equilibrium because they are both solids.
Determine the equilibrium constant for the following reaction at 298 K. Cl(g) + O3(g) → ClO(g) + O2(g)                                      ΔG° = −34.5 kJ/mol A) 0.986          B) 4.98 × 10−4           C) 5.66 × 105                      D) 1.12 × 106                     E) 8.96 × 10−7
For the reaction: A (l)  + 2 D (g) → 3 X (g) + Z (s) Having ΔG° = -2400 kJ at 25°C, the equilibrium mixture _____________.   a. will consist almost exclusively of A and D b. will consist almost exclusively of A and Z c. will consist almost exclusively of X and Z d. will consist of significant amounts of A, D, X, and Z e. Has a composition predictable only if one knows T and ΔH° and ΔS°
Water gas, a commercial fuel, is made by the reaction of hot coke with steam. C (s) + H2O (g) → CO (g) + H2 (g) When equilibrium is established at 800°C the concentrations of CO, H2, H2O are 4.00 x 10 -2, 4.00 x 10 -2, and 1.00 x 10 -2 mole/liter, respectively. What is the value of ΔG° for this reaction at 800°C? A. 109 kJ B. -43.5 kJ C. 193 kJ D. 16.3 kJ E. none of these
Calculate ΔG°rxn for the following reaction at 1000°C. 2CO(g) + 2NO(g) → N2(g) + 2CO2(g) ΔH° = -748.6 kJ; ΔS° = -197.8 J/K A) -496 kJ B) +1000 kJ C) -1000 kJ D) +496 kJ E) -551 kJ
Given the following free energies of formation calculate Kp at 298 K for: C2H2 (g) + 2 H2 (g) → C2H6 (g) C2H2 (g)          ΔG° f  = 209.2 kJ/mol C2H2 (g)          ΔG° f  = -32.9 kJ/mol   A. 9.07 x 10 -1 B. 97.2 C. 1.24 x 10 31 D. 2.74 x 10 42   
Use Hess's law to calculate ΔG°rxn using the following information. H2O(g) + C(s) → CO(g) + H2(g)          ΔG°rxn = ? H2(g) + ½ O2(g)→ H2O(g)                 ΔG°rxn = -228.6 kJ C(s)+ ½ O2(g) → CO(g)                     ΔG°rxn = -137.2 kJ A) -365.8 kJ B) +365.8 kJ C) -91.4 kJ D) +91.4 kJ E) more information is required
Which of the following reactions will have the smallest equilibrium constant (K) at 298 K? A) CaCO3(s) → CaO(s) + CO2(g)                           ΔG° =+131.1 kJ B) Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3 CO2(g)          ΔG° = -28.0 kJ C) 3 O2(g) → 2 O3(g)                                             ΔG° = +326 kJ D) 2 Hg(g) + O2(g) → 2 HgO(s)                              ΔG° = -180.8 kJ E) It is not possible to determine without more information.
The figure represents a reaction at 298 K. Which statement is true? 1. At point B, the reaction will shift to the right to reach equilibrium.  2. The reactants possess less free energy than the products. 3. K is less than 1. 4. The ∆Go  of reaction is zero at point C.
Which combination of ∆G◦ and K is possible at standard conditions? 1. ∆G◦ = 99.3 kJ, K = 1.02 2. ∆G◦ = 73.4 J, K = 1.38 × 10 7 3. ∆G◦ = −41.1 kJ, K = 0.971 4. ∆G◦ = −33.3 J, K = 1.01  5. ∆G◦ = −44.6 J, K = 5.62 × 10 −18  
Consider the reaction below. A2 (g) + 3 B2 (g) → 2 AB3 (g) A flask is allowed to come to equilibrium at 584 °C, and is found to contain 0.420 atm of A  2; 0.780 atm of B2 and 1.362 atm of AB3. What is the correct value ΔG° for this reaction? a. -156.9 kJ b. -10.8 kJ c. -10.2 kJ d. -15.9 kJ e. -6.9 kJ      
For the reaction: 2C(graphite) + H2(g) → C2H2(g) ΔG° = +209.2kJ at 25°C. If P(H2) = 100 atm, and P(C2H2) = 0.10 atm, calculate ΔG for reaction. A. +192.1 kJ B. +266.3 kJ C. -16.9 kJ D. +207.8 kJ E. +17.3 kJ    
HI has a normal boiling point of -35.4 oC and its ΔHvap is 21.16 kJ/mol. Calculate the molar entropy of vaporization (ΔSvap). 
Fill in the blanks: If ΔG°< 0, then K is _____. If ΔG° > 0, then K is _____. If ΔG° = 0, then K is ______.    (a) > 1, < 1, = 1     (b) < 1, > 1, = 1     (c) < 0, > 0, = 0 (d) > 0, < 0, = 0     (e) < 1, > 1, = 0
Which of the following has ΔG of  = 0 at 25 oC? A)  CO2 (l)    B)  H2O (l)    C)  Hg (s)    D)  O2 (g)    E)  NH3 (g)
The standard molar Gibbs free energy of formation of NO 2 (g) at 298 K is 51.30 kJ·mol−1 and that of N2O4 (g) is 97.82 kJ·mol−1. What is the equilibrium constant at 25°C for the reaction 2 NO2(g) ⇌ N2O4(g) ? 1. 6.88 2. 0.145 3. 7.01 × 10−9 4. 1.00 5. None of these 6. 1.02 × 10−10 7. 9.72 × 109 8. 0.657
Use Hess's law to calculate ΔG°rxn using the following information. ClO(g) + O3(g) → Cl(g) + 2 O2(g)    ΔG°rxn = ?   2 O3(g)→ 3 O2(g)                            ΔG°rxn = +489.6 kJ Cl(g) + O3(g) → ClO(g) + O2(g)       ΔG°rxn = -34.5 kJ a) -472.4 kJ b) -210.3 kJ c) +455.1 kJ d) +262.1 kJ e) +524.1 kJ
A certain reaction is spontaneous at 72 °C. If the enthalpy change for the reaction is 19 kJ/mol, what must be the minimum value of ΔS (in J/K-mol) for this reaction?
You calculate the value of ΔG° for a chemical reaction and get a positive value. Which would be the most accurate way to interpret this result? 1. If a mixture of reactants and products is created and left to equilibrate, the equilibrium mixture will contain more reactant than product.  2. If a mixture of reactants and products is created, we cannot say anything about its composition at equilibrium but we can say it will reach equilibrium very rapidly.  3. The reaction will not occur under any circumstances. 4. If a mixture of reactants and products is created and left to equilibrate, the equilibrium mixture will contain more product than reactant. 
Calculate ΔG° for the following reaction 3NO2(g) + H2O(l) →  2HNO3(l