Big Idea 5: The laws of thermodynamics describe the essential role of energy and explain and predict the direction of changes in matterConcepts – THERMODYNAMICS – contained in Big Idea 5

1. ∆H0 rxn = ∑ ∆ Hf 0Products - ∑∆ Hf0 Reactants

= ∑ Bond Energy Reactants - ∑ Bond energy Products

∆Hrxn - exothermic ∆Hrxn + endothermic

2. ∆S0 rxn = ∑ Sf0 Products - ∑ Sf

0 Reactants

∆S0rxn - ordered ∆S0

rxn + disordered

3. ∆G0 rxn = ∆H0rxn - T ∆S 0

rxn

∆G0rxn - thermodynamically favored

∆G0rxn + thermodynamically unfavored

4. ∆G0rxn = - RT ln Q Q = Keq free energy and equilibrium

5. ∆G0 rxn = - nF E0 free energy and electrochemistry

F = 96,500 coulombs / mole electrons Faraday’s constant

6. Phase diagrams

7. ∆H rxn = q = m ( c ) ( ∆T )

Free Response Questions

1998 # 3 C6H5OH(s) + 7 O2(g) → 6 CO2(g) + 3H2O(l)

When a 2.000-gram sample of pure phenol, C6H5OH(s), is completely burned according to the equation above, 64.98 kilojoules of heat is released. Use the information in the table below to answer the questions that follow.

Substance Standard Heat of Formation, ΔH°f, at 25°C (kJ/mol) Absolute Entropy, S°, at 25°C (J/mol-K)

C(graphite) 0.00 5.69 CO2(g) -395.5 213.6 H2(g) 0.00 130.6 H2O(l) -285.85 69.91 O2(g) 0.00 205.0 C6H5OH(s) ? 144.0

a. Calculate the molar heat of combustion of phenol in kilojoules per mole at 25°C.

b. Calculate the standard heat of formation, ΔH°f, of phenol in kilojoules per mole at 25°C.

c. Calculate the value of the standard free-energy change, ΔG° for the combustion of phenol at 25°C.

d. If the volume of the combustion container is 10.0 liters, calculate the final pressure in the container when the temperature is changed to 110°C. (Assume no oxygen remains unreacted and that all products are gaseous.)

2010 #2 A student performs an experiment to determine the molar enthalpy of solution of urea, H2NCONH2. The student places 91.95 g of water at 25 oC into a coffee-cup calorimeter and immerses a thermometer in the water. After 50 s, the student adds 5.13 g of solid urea, also at 25 oC, to the water and measures the temperature of the solution as the urea dissolves. A plot of the temperature data is shown in the graph below.

a. Determine the change in temperature of the solution that results from the dissolution of the urea.

b. According to the data, is the dissolution of urea in water an endothermic process or an exothermic process? Justify your answer.

c. Assume that the specific heat capacity of the calorimeter is negligible and that the specific heat capacity of the solution of urea and water is 4.2 J g-1 oC-1 throughout the experiment.

i. Calculate the heat of dissolution of the urea in joules.

ii. Calculate the molar enthalpy of solution, ΔHsolno, of urea in kJ mol-1.

d. Using the information in the table below, calculate the value of the molar entropy of solution, ΔSsoln

o, of urea at 298 K. Include units with your answer.Accepted value

ΔHsolno of urea 14.0 kJ mol-1

ΔGsolno of urea -6.9 kJ mol-1

e. The student repeats the experiment and this time obtains a result for ΔHsolno of urea that is 11

percent below the accepted value. Calculate the value of ΔHsolno that the student obtained in this

second trial.

f. The student performs a third trial of the experiment but this time adds urea that has been taken directly from a refrigerator at 5 oC. What effect, in any, would using the cold urea instead of urea at 25 oC have on the experimentally obtained value of ΔHsoln

o? Justify your answer.

2009 B#5 Answer the following questions about nitrogen, hydrogen, and ammonia.

a. In the boxes below, draw the complete Lewis electron-dot diagrams for N2 and NH3.

b. Calculate the standard free energy change, ΔGo, that occurs when 12.0 g of H2(g) reacts with excess N2(g) at 298 K according to the reaction represented below.

N2(g) + 3 H2(g) 2 NH3(g) ΔG298o = -34 kJ mol-1

c. Given that ΔH298o for the reaction is -92.2 kJ mol-1, which is larger, the total bond dissociation

energy of the reactants or the total bond dissociation energy of the products? Explain.

d. The value of the standard entropy change, ΔS298o, for the reaction is -199 J mol-1 K-1. Explain why

the value of ΔS298o is negative.

e. Assume that ΔHo and ΔSo for the reaction are independent of temperature.

i. Explain why there is a temperature above 298 K at which the algebraic sign of the value of ΔGo changes.

ii. Theoretically, the best yields of ammonia should be achieved at low temperatures and high pressures. Explain.

2005 B#7 Answer the following questions about thermodynamics.

Substance Combustion Reaction Enthalpy of Combustion, ΔHo

comb, at 298 K (kJ mol-1)H2(g) H2(g) + ½ O2(g) H2O(l) -290C(s) C(s) + O2(g) CO2(g) -390CH3OH(l) -730

a. In the empty box in the table above, write a balanced chemical equation for the complete combustion of one mole of CH3OH(l). Assume products are in the standard states at 298 K. Coefficients do not need to be whole numbers.

b. On the basis of your answer to part a) and the information in the table, determine the enthalpy change for the reaction C(s) + H2(g) + H2O(l) CH3OH(l).

c. Write the balanced chemical equation that shows the reaction that is used to determine the enthalpy of formation of one mole of CH3OH(l).

d. Predict the sign of ΔSo for the combustion of H2(g). Explain your reasoning.

e. On the basis of bond energies, explain why the combustion of H2(g) is exothermic.

2003#7 Answer the following questions that relate to the chemistry of nitrogen.

a. Two nitrogen atoms combine to form a nitrogen molecule, as represented by the following equation

2 N(g) N2 (g)

Using the table of average bond energies below, determine the enthalpy change, ΔH, for the reaction.

Bond Average Bond Energy (kJ mol-1)N – N 160N = N 420N ≡ N 950

b. The reaction between nitrogen and hydrogen to form ammonia is represented below.

N2(g) + 3 H2(g) 2 NH3(g) ΔHo = - 92.2 kJ

Predict the sign of the standard entropy change, ΔSo, for the reaction. Justify your answer.

c. The value of ΔGo for the reaction represented in part (b) is negative at low temperatures but positive at high temperatures. Explain.

d. When N2(g) and H2(g) are placed in a sealed container at a low temperature, no measurable amount of NH3(g) is produced. Explain.

2011 B#3 Answer the following questions about glucose, C6H12O6, an important biochemical energy source.

a. Write the empirical formula of glucose.

In many organisms, glucose is oxidized to carbon dioxide and water, as represented by the following equation.

C6H12O6(s) + 6 O2(g) 6 CO2(g) + 6 H2O(l)

A 2.50 g sample of glucose and an excess of O2(g) were placed in a calorimeter. After the reaction was initiated and proceeded to completion, the total heat released by the reaction was calculated to be 39.0 kJ.

b. Calculate the value of ΔHo, in kJ mol-1, for the combustion of glucose.

c. When oxygen is not available, glucose can be oxidized by fermentation. In that process, ethanol and carbon dioxide are produced, as represented by the following equation.

C6H12O6(s) 2 C2H5OH(l) + 2 CO2(g) ΔHo = -68.0 kJ mol-1 at 298 K

The value of the equilibrium constant, KP, for the reaction at 298 K is 8.9 × 1039.

i.) Calculate the value of the standard free energy change, ΔGo, for the reaction at 298 K. Include units with your answer.

ii.) Calculate the value of the standard entropy change, ΔSo, in J K-1 mol-1, for the reaction at 298 K.

iii.) Indicate whether the equilibrium constant for the fermentation reaction increases, decreases, or remains the same if the temperature is increased. Justify your answer.

d. Using your answer for part b) and the information provided in part c), calculate the value of ΔHo for the following reaction.

C2H5OH(l) + 3 O2(g) 2 CO2(g) + 3 H2O(l)

2013 Q22. A student is given the task of determining the enthalpy of reaction for the reaction

between HCl(aq) and NaOH(aq). The following materials are available.1.00 M HCl(aq) 1.00 M NaOH(aq) distilled water2.00 M HCl(aq) 2.00 M NaOH(aq) gogglesInsulated cups with covers gloves lab coatThermometer (± 0.1 oC) stirring rod

The student may select from the glassware listed in the table below.Glassware items Precision250 mL Erlenmeyer flasks ± 25 mL100 mL beakers ± 10 mL

100 mL graduated cylinders ± 0.1 mLa. The student selects two 100 mL beakers, uses them to measure 50 mL each of

1.00 M HCl(aq) solution and 1.00 M NaOH(aq) solution, and measures an initial temperature of 24.5 oC for each solution. Then the student pours the two solutions into an insulated cup, stirs the mixture, covers the cup, and records a maximum temperature of 29.9 oC.i. Is the experimental design sufficient to determine the enthalpy of reaction

to a precision of two significant figures? Justify your answer.ii. List two specific changes to the experiment that will allow the student to

determine the enthalpy of reaction to a precision of three significant figures. Explain.

b. A second student is given two solutions, 75.0 mL of 1.00 M HCl and 75.0 mL of 1.00 M NaOH, each at 25.0 oC. The student pours the solutions into an insulated cup, stirs the mixture, covers the cup, and records the maximum temperature of the mixture.i. The student calculates the amount of heat evolved in the experiment to be

4.1 kJ. Calculate the student’s experimental value for the enthalpy of reaction, in kJ/molrxn.

ii. The student assumes that the thermometer and the calorimeter do not absorb energy during the reaction. Does this assumption result in a calculated value of the enthalpy of reaction that is higher than, lower than, or the same as it would have been had the heat capacities of the thermometer and calorimeter been taken into account? Justify your answer.

iii. One assumption in interpreting the results of the experiment is that the reaction between HCl(aq) and NaOH(aq) goes to completion. Justify the validity of this assumption in terms of the equilibrium constant for the reaction.

c. A third student calculates a value for the enthalpy of reaction that is significantly higher than the accepted value.i. Identify a specific error in procedure made by the student that will result

in a calculated value for the enthalpy of reaction that is higher than the accepted value. (Vague statements like “human error” or “incorrect calculations” will not earn credit).

ii. Explain how the error that you identified in part c i) leads to a calculated value for the enthalpy of reaction that is higher than the accepted value.

Multiple Choice Questions

1999 #61 C2H4(g) + 3 O2(g) 2 CO2(g) + 2 H2O(g)

For the reaction of ethylene represented above, ΔH is - 1,323 kJ. What is the value of ΔH if the combustion produced liquid water H2O(l), rather than water vapor H2O(g)? (ΔH for the phase change H2O(g) H2O(l) is -44 kJ mol-1.)

A) -1,235 kJB) -1,279 kJC) -1,323 kJD) -1,367 kJE) -1,411 kJ

2002 #25 3 C2H2(g) C6H6(g)

What is the standard enthalpy change, ΔHo, for the reaction represented above? (ΔHof of C2H2(g) is 230 kJ

mol-1; ΔHof of C6H6(g) is 83 kJ mol-1.)

A) -607 kJB) -147 kJC) -19 kJD) +19 kJE) +773 kJ

1999 #22 Of the following reactions, which involves the largest decrease in entropy?

A) CaCO3(s) CaO(s) + CO2(g)B) 2 CO(g) + O2(g) 2 CO2(g)C) Pb(NO3)3 + 2 KI PbI2 + 2 KNO3

D) C3H8 + O2 3 CO2 + 4 H2OE) 4 La + 3 O2 2 La2O3

2002 #41 When solid NH4SCN is mixed with solid Ba(OH)2 in a closed container, the temperature drops and a gas is produced. Which of the following indicates the correct signs for ΔG, ΔH, and ΔS for the process?

Δ G Δ H Δ S

A) - - -B) - + -C) - + +D) + - +E) + - -

2002 #73 X(s) X(l)

Which of the following is true for any substance undergoing the process represented above at its normal melting point?

A) ΔS < 0B) ΔH = 0C) ΔH = TΔGD) TΔS = 0E) ΔH = TΔS

2013

8.The graph above shows the speed distribution of molecules in a sample of a gas at a certain temperature. Which of the following graphs shows the speed distribution of the same molecules at a lower temperature (as a dashed curve)?

9. A 100 g sample of a metal was heated to 100 oC and then quickly transferred to an insulated container holding 100 g of water at 22 oC. The temperature of the water rose to reach a final temperature of 35 oC. Which of the following can be concluded?a. The metal temperature changed more than the water temperature did; therefore

the metal lost more thermal energy than the water gained.b. The metal temperature changed more than the water temperature did; but the

metal lost the same amount of thermal energy as the water gained.c. The metal temperature changed more than the water temperature did; therefore

the heat capacity of the metal must be greater than the heat capacity of the water.d. The final temperature is less than the average starting temperature of the metal

and the water; therefore the total energy of the metal and water decreased.

12. Which of the following reactions is not thermodynamically favored at low temperatures but becomes favored as the temperature increases?

Reaction ΔHo (kJ / molrxn) ΔSo (J / (molrxn K)a. 2 CO(g) + O2(g) 2 CO2(g) -566 -173b. 2 H2O(g) 2 H2(g) + O2(g) 484 90.0c. 2 N2O(g) 2 N2(g) + O2(g) -164 149d. PbCl2(s) Pb2+(aq) + 2Cl-(aq) 23.4 -12.5

Questions 18-20 refer to three gases in identical rigid containers under the conditions given in the table below.

Container A B CGas Methane Ethane ButaneFormula CH4 C2H6 C4H10

Molar mass (g mol-1) 16 30 58Temperature (oC) 27 27 27Pressure (atm) 2.0 4.0 2.0

18. The average kinetic energy of the gas molecules isa. greatest in container Ab. greatest in container Bc. greatest in container Cd. the same in all three containers

27.

The dissolution of an ionic solute in a polar solvent can be imagined as occurring in three steps, as shown in the figure above. In step 1, the separation between ions in the solute is greatly increased, just as will occur when the solute dissolves in the polar solvent. In step 2, the polar solvent is expanded to make spaces that the ions will occupy. In the last step, the ions are inserted into the spaces in the polar solvent. Which of the following best describes the enthalpy change, ΔH, for each step?a. All three steps are exothermic.b. All three steps are endothermic.c. Steps 1 and 2 are exothermic, and the final step is endothermic.d. Steps 1 and 2 are endothermic, and the final step is exothermic.

Questions 34-38K(s) + ½ Cl2(g) KCl(s) ΔHo = -437 kJ/molrxn

The elements K and Cl react directly to form the compound KCl according to the equation above. Refer to the information above and the table below to answer the questions that follow.

Process ΔHo (kJ/molrxn)K(s) K(g) vK(g) K+(g) + e- wCl2(g) 2 Cl(g) xCl(g) + e- Cl-(g) yK+(g) + Cl-(g) KCl(s) z

34. How much heat is released or absorbed when 0.050 mol of Cl2(g) is formed from KCl(s)?a. 87.4 kJ is releasedb. 43.7 kJ is releasedc. 43.7 kJ is absorbedd. 87.4 kJ is absorbed

36. Which of the values of ΔHo for a process in the table is (are) less than zero (i.e., indicate(s) an exothermic process??

a. z onlyb. y and z onlyc. x, y, and z onlyd. w, x, y, and z

37. It is observed that the reaction producing KCl from its elements goes essentially to completion. Which of the following is a true statement about the thermodynamic favorability of the reaction?a. The reaction is favorable and driven by an enthalpy change only.b. The reaction is unfavorable and driven by an entropy change only.c. The reaction is favorable and driven by both enthalpy and entropy changes.d. The reaction is unfavorable due to both enthalpy and entropy changes.

38. Cl2(g) + 2 e- 2 Cl-(g)Which of the following expressions is equivalent to ΔHo for the reaction represented

above?a. x + yb. x – yc. x + 2yd. (x/2) – y