Thermochemistry. 1. Stoichiometry # g ΔHrxn where n = moles in the reaction for the specific substance Mm n

Transcription

1 Thermochemistry Review 1. Stoichiometry # g ΔHrxn where n = moles in the reaction for the specific substance Mm n Exothermic Reaction (enthalpy is a product): Endothermic Reaction (enthalpy is a reactant): H 2 (g) + Cl 2 (g) 2 HCl (g) + ΔH 2 HBr (g) + ΔH H 2 (g) + Br 2 (g) 2. Hess s Law Given a list of reactions Flip reaction = flip the sign of ΔH Multiply or Divide reaction coefficients = Multiply or Divide ΔH Add up all ΔH to get ΔHrxn 3. Standard Heats of Formation ΔHrxn = ΣH products ΣH reactants ΔSrxn = ΣS products ΣS reactants ΔGrxn = ΣG products ΣG reactants (Don t forget to multiply by # of moles) 4. Bond Enthalpies Given the energies to break bonds Draw out all molecules (including the number of molecules) (+) Energy for breaking bonds: Need energy to break bonds (-) Energy for forming bonds: Release energy when forming bonds Add up all energies of all bonds to get ΔHrxn 5. Entropy and Gibbs Free Energy ΔG = ΔH TΔS Symbol Description Negative Value (-) Positive Value (+) ΔH Enthalpy Exothermic (release) Endothermic (Absorb) ΔS Entropy Less disorder More disorder ΔG Gibbs Free Energy Spontaneous Non-spontaneous

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3 Thermochemistry Problem Set Stoichiometry 1. ZnCO 3 (s) ZnO (s) + CO 2 (g) The decomposition of zinc carbonate, as shown in the reaction above, at constant pressure requires the addition of 71.5 kj of heat per mole of ZnCO 3. a. Include the heat in the reaction above to show a balanced thermochemical equation for this reaction. b. Draw an enthalpy diagram for the reaction. 2. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) ΔH = -891 kj Consider the reaction above: a. Calculate the heat released when 8.0 g of methane is burned in excess oxygen gas. b. Calculate the heat released when 10.0 L of methane gas is burned at 1.2 atm and 25 C in the presence of excess oxygen gas. c. Calculate the heat released when 16.0 g of methane and 32.0 g of oxygen gas are reacted? 3. CH 3 OH (g) CO (g) + 2 H 2 (g) ΔH = kj Consider the reaction above: a. Is heat absorbed or evolved in the course of this reaction? b. Calculate the amount of heat transferred when 64.0 g of CH 3 OH is decomposed at constant pressure. c. For a given sample of CH 3 OH, the enthalpy change on the reaction is kj. How many grams of hydrogen gas are produced? d. What is the value of ΔH for the reverse of the previous reaction? How many kilojoules of heat are released when 42.0 g of CO (g) reacts completely with H 2 (g) to form CH 3 OH (g) at constant pressure? 4. H 2 (g) + F 2 (g) 2 HF (g) Gaseous hydrogen and fluorine combine in the reaction above to form hydrogen fluoride with an enthalpy change of -540 kj. What is the value of the heat of formation of one mole of HF (g)? (A) kj/mol (B) -540 kj/mol (C) -270 kj/mol (D) +270 kj/mol (E) +540 kj/mol 5. 2 H 2 (g) + O 2 (g) 2 H 2 O (l) ΔH = kj How much heat is released when 8.0 g of hydrogen gas is reacted with excess oxygen gas? (A) -286 kj (B) -572 kj (C) kj (D) kj (E) kj

4 Hess s Law 6. Calculate the enthalpy change for the reaction P 4 O 6 (s) + 2 O 2 (g) P 4 O 10 (s) Given the following enthalpies of reaction: P 4 (s) + 3 O 2 (g) P 4 O 6 (s) ΔH = kj P 4 (s) + 5 O 2 (g) P 4 O 10 (s) ΔH = kj 7. From the enthalpies of reaction 2 H 2 (g) + O 2 (g) 2 H 2 O (g) ΔH = kj 3 O 2 (g) 2 O 3 (g) ΔH = kj Calculate the heat of the reaction: 3 H 2 (g) + O 3 (g) 3 H 2 O (g) 8. From the enthalpies of reaction H 2 (g) + F 2 (g) 2 HF (g) ΔH = -537 kj C (s) + 2 F 2 (g) CF 4 (g) ΔH = -680 kj 2 C (s) + 2 H 2 (g) C 2 H 4 (g) ΔH = kj Calculate ΔH for the reaction of ethylene with F 2 : C 2 H 4 (g) + 6 F 2 (g) 2 CF 4 (g) + 4 HF (g) 9. Given the data N 2 (g) + O 2 (g) 2 NO (g) ΔH = kj 2 NO (g) + O 2 (g) 2 NO 2 (g) ΔH = kj 2 N 2 O (g) 2 N 2 (g) + O 2 (g) ΔH = kj Use Hess s Law to calculate ΔH for the reaction: N 2 O (g) + NO 2 (g) 3 NO (g) 10. C (s) + O 2 (g) CO 2 (g) ΔH = -390 kj/mol H 2 (g) + ½ O 2 (g) H 2 O (l) ΔH = -290 kj/mol 2 C (s) + H 2 (g) C 2 H 2 (g) ΔH = kj/mol Based on the information given above, what is ΔH for the following reaction? C 2 H 2 (g) + O 2 (g) 2 CO 2 (g) + H 2 O (l) (A) kj (B) kj (C) -840 kj (D) -780 kj (E) -680 kj 11. C (s) + 2 H 2 (g) CH 4 (g) ΔH = x C (s) + O 2 (g) CO 2 (g) ΔH = y H 2 (g) + ½ O 2 (g) H 2 O (l) ΔH = z Based on the information given above, what is ΔH for the following reaction? CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) (A) x + y + z (B) x + y z (C) z + y 2x

5 (D) 2z + y x (E) 2z + y 2x

6 Enthalpies of Formation Substance ΔH f (kj/mol) Substance ΔH f (kj/mol) Substance ΔH f (kj/mol) CH 4 (g) H 2 O (g) N 2 O 4 (g) 9.66 CO 2 (g) H 2 O (l) S 8 (g) CCl 4 (l) HCl (g) SO 2 (g) Fe 2 O 3 (s) H 2 S (g) SO 3 (g) FeCl 3 (s) -400 KOH (s) SiO 2 (s) K 2 CO 3 (s) SiCl 4 (l) Using values from the Enthalpies of Formation found above, calculate the value of ΔH for each of the following reactions: a. N 2 O 4 (g) + 4 H 2 (g) N 2 (g) + 4 H 2 O (g) b. 2 KOH (s) + CO 2 (g) K 2 CO 3 (s) + H 2 O (g) c. SO 2 (g) + 2 H 2 S (g) S 8 (g) + 2 H 2 O (g) d. Fe 2 O 3 (s) + 6 HCl (g) 2 FeCl 3 (s) + 3 H 2 O (g) 13. Using values from the Enthalpies of Formation found above, calculate the value of ΔH for each of the following reactions: a. 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) b. 2 H 2 (g) + O 2 (g) 2 H 2 O (g) c. CH 4 (g) + 4 Cl 2 (g) CCl 4 (l) + 4 HCl (g) d. SiCl 4 (l) + 2 H 2 O (l) SiO 2 (s) + 4 HCl (g) Mg (s) + O 2 (g) + H 2 (g) Mg(OH) 2 (s) 14. Calculate the standard enthalpy of formation of solid Mg(OH) (for the reaction above), given the following data: 2 Mg (s) + O 2 (g) 2 MgO (s) ΔH = kj Mg(OH) 2 (s) MgO (s) + H 2 O (l) ΔH = kj 2 H 2 (g) + O 2 (g) 2 H 2 O (l) ΔH = kj C 2 H 2 (g) C 6 H 6 (g) What is the standard enthalpy change, ΔH, for the reaction represented above? (ΔH f of C 2 H 2 (g) is 230 kj mol -1 ; ΔH f of C 6 H 6 (g) is 83 kj mol -1 ) (A) -607 kj (B) -147 kj (C) -19 kj (D) +19 kj (E) +773 kj

7 Bond Enthalpies 16. Using the Bond Enthalpies table, draw out the molecules and calculate the ΔH for each of the following gas-phase reactions: a. C 2 H 4 + HOOH HOCH 2 CH 2 OH b. C 2 H 4 + HCN CH 3 CH 2 CN c. 2 NCl 3 N Cl Using the Bond Enthalpies table, draw out the molecules and calculate the ΔH for each of the following gas-phase reactions: a. CHBr 3 + Cl 2 CClBr 3 + HCl b. HSCH 2 CH 2 SH + 2 HBr BrCH 2 CH 2 Br + 2 H 2 S c. NH 2 NH 2 + Cl 2 2 NH 2 Cl 18. Using the Bond Enthalpies table, draw out the molecules and calculate the ΔH for each of the following gas-phase reactions: a. 2 CH 4 (g) + O 2 (g) 2 CH 3 OH (g) b. H 2 (g) + Br 2 (g) 2 HBr (g) c. 2 H 2 O 2 (g) 2 H 2 O (g) + O 2 (g) d. 3 ethene (g) cyclohexane

8 19. 2 H 2 (g) + O 2 (g) 2 H 2 O (g) Based on the following information given in the table below, what is ΔH for the above reaction? Bond Average bond energy (kj/mol) H-H 500 (A) kj (B) kj (C) -500 kj (D) kj (E) kj Entropy and Gibbs Free Energy O=O 500 O-H For each of the following pairs, indicate which substance possesses the larger standard entropy: a. 1 mol of O 2 (g) at 300 C, 0.01 atm or 1 mol of N 2 (g) at 50 C, 0.01 atm b. 1 mol of H 2 O (g) at 100 C, 1 atm or 1 mol of H 2 O (l) at 100 C, 1 atm c. 0.5 mol of CO 2 (g) at 298 K, 5-L volume or 0.5 mol C 3 H 8 (g) at 298 K, 20-L volume d. 100 g of Na 2 SO 4 (s) at 30 C or 100 g Na 2 SO 4 (aq) at 30 C 21. Predict the sign of entropy change of the system for each of the following reactions: a. 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) b. Ba(OH) 2 (s) BaO (s) + H 2 O (g) c. CO (g) + 2 H 2 (g) CH 3 OH (l) d. FeCl 2 (s) + H 2 (g) Fe (s) + 2 HCl (g) e. Molten Fe solidifies f. LiCl (s) is formed from Li (s) and Cl 2 (g) g. A precipitate is formed through two aqueous solutions Substance ΔH f (kj mol -1 ) ΔS (J mol -1 K -1 ) Substance ΔH f (kj mol -1 ) ΔS (J mol -1 K -1 ) Al (s) H 2 O (l) AlCl 3 (s) HCl (g) Cl 2 (g) MgCl 2 (s) CH 4 (g) Mg(OH) 2 (s) C 2 H 6 (g) NH 3 (g) H 2 (g) N 2 H 4 (g) Calculate ΔH, ΔS, and ΔG reaction values for the following reactions by using the thermochemical data chart above at 298 K. Also, indicate whether the reaction is spontaneous or nonspontaneous. a. N 2 H 4 (g) + H 2 (g) 2 NH 3 (g) b. 2 Al (s) + 3 Cl 2 (g) 2 AlCl 3 (s) c. Mg(OH) 2 (s) + 2 HCl (g) MgCl 2 (s) + 2 H 2 O (l) d. 2 CH 4 (g) C 2 H 6 (g) + H 2 (g)

9 23. Which of the following processes are spontaneous, and which are nonspontaneous: a. The melting of ice cubes at -5 C and 1 atm pressure. b. The dissolution of sugar in a cup of hot coffee. c. The reaction of nitrogen atoms to form N 2 molecules at 25 C and 1 atm. d. The formation of CH 4 and O 2 molecules from CO 2 and H 2 O at room temperature and 1 atm of pressure. 24. A certain reaction has ΔH = kj and ΔS = J/K. a. Is the reaction exothermic or endothermic? b. Does the reaction lead to an increase or decrease in disorder? c. Calculate ΔG for the reaction at 298 K. d. Is the reaction spontaneous at 298K under standard conditions? 25. A particular reaction is spontaneous at 450 K. The enthalpy change for the reaction is kj. What can you conclude about the sign and magnitude of ΔS for the reaction? 26. For a particular reaction, ΔH = -32 kj and ΔS = -98 J/K. Assume that ΔH and ΔS do not vary with temperature. a. At what temperature will the reaction be at equilibrium? b. If the temperature is increased from that in part (a), will the reaction occur? 27. A certain reaction is nonspontaneous -25 C. The entropy change for the reaction is 95 J/K. What can you conclude about the sign and magnitude of ΔH? Al (s) + 3 Cl 2 (g) 2 AlCl 3 (s) The reaction above is not spontaneous under standard conditions but becomes spontaneous as the temperature decreases toward absolute zero. Which of the following is true at standard conditions? (A) ΔS and ΔH are both negative. (B) ΔS and ΔH are both positive. (C) ΔS is negative, and ΔH is positive. (D) ΔS is positive, and ΔH is negative. (E) ΔS and ΔH are both equal to zero. 29. Which of the following reactions is entropy increasing? (A) 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) (B) CO (g) + H 2 O (g) H 2 (g) + CO 2 (g) (C) H 2 (g) + Cl 2 (g) 2 HCl (g) (D) 2 NO 2 (g) 2 NO (g) + O 2 (g) (E) 2 H 2 S (g) + 3 O 2 (g) 2 H 2 O (g) + 2 SO 2 (g) 30. When solid NH 4 SCN 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? ΔG ΔH ΔS (A) (B) (C) (D) + - +

10 Free Response (E) Liquid water is formed according to the following equation below. 2 H 2 (g) + O 2 (g) 2 H 2 O (l) (a) Calculate the standard enthalpy change, ΔH, for the reaction represented in the equation above. (The molar enthalpy of formation, ΔH f, for H 2 O (l) is kj mol -1 at 298 K) (b) Calculate the amount of heat, in kj, that is released when 6.0 g of H 2 (g) is burned in air. (c) What is the algebraic sign for the entropy of the reaction represented in the equation above? Justify your answer. (d) Clearly justify why this reaction above is spontaneous using thermodynamic principles. (e) Given the molar enthalpy of vaporization, ΔH vap, for H 2 O (l) is 44.0 kj mol -1 at 298 K, what is the standard enthalpy change, ΔH, for the reaction 2 H 2 (g) + O 2 (g) 2 H 2 O (g)? (Hint: develop a reaction for the heat of vaporization of liquid water) N 2 (g) + 3 F 2 (g) 2 NF 3 (g) ΔH = -264 kj mol -1 ΔS = 278 J K -1 mol The following questions relate to the reaction represented by the chemical equation and information above. (a) Calculate the value of the standard free energy change, ΔG, for the reaction. (b) Calculate the standard enthalpy change, ΔH, that occurs when a mol sample of NF 3 (g) is formed from N 2 (g) and F 2 (g) at 1.00 atm and 298 K. (c) How many bonds are broken for N 2 and F 2, respectively, as well as how many bonds are formed in NF 3 (all according to the chemical reaction above)? (d) Use both the information above and the table below of average bond enthalpies to calculate the average enthalpy of the N N bond. Bond Average Bond Enthalpy (kj mol -1 ) N N? N-F 272 F-F 141 CO (g) + ½ O 2 (g) CO 2 (g) 33. The combustion of carbon monoxide to produce carbon dioxide is represented in the equation above. (a) Determine the value of the standard enthalpy change, ΔH rxn, using the information below. C (s) + ½ O 2 (g) CO (g) ΔH = kj mol -1 C (s) + O 2 (g) CO 2 (g) ΔH = kj mol -1 (b) Determine the value of the standard entropy change, ΔS rxn, using the information below. CO (g) S = J K -1 mol -1 CO 2 (g) S = J K -1 mol -1

11 O 2 (g) S = J K -1 mol -1 (c) Determine the standard free energy change, ΔG rxn, for the reaction at 298 K. Include units with your answer. (d) Is the reaction spontaneous under these standard conditions? Justify your answer.