Define the Standard Enthalpy of Formation

1. Define the Standard Enthalpy of Formation (ΔfH°) then choose one below. always positive never positive sometimes positive a. Is it possible for Standard Enthalpy of Formation (ΔfH°) = 0? (If so, explain when.) 2. Write the chemical equation whose enthalpy change is equal exactly to the standard enthalpy of formation, ΔfH°, of solid urea. (ΔfH°(H2NCONH2(s))= -632kJ/mol). a. Draw the energy diagram representation of the reaction in first part of the problem. Hint: remember you need to identify your reactant and your products. 3. Calculate the heat of combustion of 132.0g of propane, C3H8 (l), in excess oxygen using the following approximate enthalpies of formation: ΔfH° (C3H8 (l)) = -104.7 kJ/mol, ΔfH° (CO2 (g)) = -393.5 kJ/mol, ΔfH° (H2O (g)) = -241.8 kJ/mol. Hint: Start with writing the balanced chemical reaction. Using Standard Enthalpy of Formation (ΔfH°) for calculating enthalpy of reactions ΔrH° ∆𝑟𝑥𝑛 𝐻° = ∑ 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 𝑛𝑃𝑟𝑜𝑑𝑢𝑐𝑡 (∆𝑓 𝐻°) − ∑ 𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠 𝑛𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡 (∆𝑓 𝐻°) = ∆𝐻𝑓𝑖𝑛𝑎𝑙 − ∆𝐻𝑖𝑛𝑖𝑡𝑖𝑎𝑙 = products − reactants a. Calculate the heat of combustion if reaction only preceded 75%. Hint: Heat (q) equals ΔrxnH° times the number of moles of reaction. 4. An atomization reaction breaks a compound apart into its gaseous atoms. Use the following atomization reactions and their enthalpies of atomization to obtain the expression for the enthalpy change of the reaction, ΔrxnH, in terms of ΔH1, ΔH2, ΔH3, and ΔH4 for the combustion of one mole of gaseous ammonia to yield gaseous water and dinitrogen pentoxide, N2O5 (g). NH3 (g)  N (g) + 3 H (g) ΔH1 H2O (g)  2 H (g) + O (g) ΔH2 O2 (g) 2 O (g) ΔH3 N2O5 (g)  2 N (g) + 5 O (g) ΔH4 Using Bond Enthalpy (ΔBH) for to calculate the ΔrH of the reaction: ∆𝑟𝑥𝑛 𝐻° = ∑ ∆𝐵 𝐻(𝑏𝑟𝑜𝑘𝑒𝑛) − ∑ ∆𝐵 𝐻(𝑓𝑜𝑟𝑚𝑒𝑑) 5. Define the Bond Enthalpy (∆BH) and choose one of choices below: Always positive Never positive Sometimes positive a. Draw the energy diagram representation for bond energy: 6. Ethanol, CH3CH2OH(g), and dimethyl ether, CH3OCH3(g) are structural isomers. a. Estimate the enthalpy of reaction (∆rH) of the given reaction using the provided bond enthalpies. CH3CH2OH(g) → CH3OCH3(g) Bond ∆BH(kJ/mol) O-H 460. C-H 410. C-O 350. 2 C-C 350. O=O 500. C=O 750. a. Calculate the enthalpy of reaction (∆rH) using enthalpy of formations. (Ethanol, CH3CH2OH(l); ΔHf°= -277 kJ/mol) and dimethyl ether, (CH3OCH3(g); ΔHf°= -184. kJ/mol) Compare the bond enthalpy estimate to the enthalpy of formation value. 7. An enthalpy diagram of hydrogen gas combusting to form gaseous water is shown below. a. Write down balanced chemical reaction for this process: Enthalpy b. Fill in the horizontal lines using only the following species and corresponding coefficients in the appropriate boxes: H(g), O(g), and H2O(g), H2O(l). C A 2 H2(g) + O2(g) ΔrxnH° B D c. For the labeled arrows on the diagram above, write the expression for the enthalpy change in terms of bond enthalpies and enthalpies of formation and enthalpy of vaporization. A: _____________________________________ B: _____________________________________ C: _____________________________________ D: _____________________________________ d. Estimate the value of ΔrH of the reaction above, in terms of the molar bond enthalpies ΔbH(H–H) = 440.0 kJ/mol; ΔbH(C–H) = 410.0 kJ/mol; ΔbH(O=O) = 500.0 kJ/mol; ΔbH(O–H) = 450.0 kJ/mol. 3 8. A 100. mL sample of 0.20 M HCl is mixed with 100. mL of 0.30 M NaOH in a calorimeter. Both solutions start at 22.50°C and, after mixing, the temperature rises to 23.50°C. Assuming that the calorimeter does not absorb any heat and that the solutions have the same heat capacity and density as pure water, what is the molar enthalpy of neutralization of the acid? 9. A chemist measures the enthalpy change ∆H during the following reaction: 2Al(OH)3(s)  Al2O3(s) + 3H2O(l) ∆H = 40. kJ/mol Use this information to determine the ∆rH for the following reactions: a. Al2O3(s) + 3 H2O(l)  2 Al(OH)3(s) ΔrH = ___________________kJ/mol b. 3 Al2O3(s) + 9 H2O(l)  6 Al(OH)3(s) ΔrH = ___________________kJ/mol c. 4 moles of Al2O3(s) are reacted with 6 moles of H2O(l) with 75% yield. qP = ____________ kJ d. When 3 moles of Al(OH)3(s) reacts with 100% yield. ΔrH = ___________________kJ/mol qP = ____________ kJ 10. Consider the following reaction for the oxidation of methane (CH4) to methanol (CH3OH): 2 CH4(g) + O2(g) → 2 CH3OH(l), ΔrH = –326 kJ/mol. a. Use the following bond enthalpies, and the enthalpy of reaction above, to calculate the enthalpy of vaporization of methanol (in kJ/mol). Data: ΔbH(C–H) = 413 kJ/mol, ΔbH(C– C) = 346 kJ/mol, ΔbH(C–O) = 358 kJ/mol, ΔbH(O–H) = 463 kJ/mol, ΔbH(H–H) = 436 kJ/mol, and ΔbH(O=O) = 498 kJ/mol. Δvap𝐻(CH3OH) = _______________ kJ/mol 4 11. (As seen in lecture) The enthalpy change of formation of NaBr(g) is ΔfH = −143 kJ. Complete the enthalpy diagram on the right, by labeling the three horizontal lines with the corresponding species and by labeling the three vertical arrows with the corresponding enthalpy change. Possible species to use in labels of the horizontal lines are Na(s), Na(g), Na2(g), Br2(l), Br(g) and Br2(g). The labels of the vertical arrows consist of various enthalpies of formation, ΔfH, and bond enthalpies, ΔbH. Na2(g) + Br2(g) →2 NaBr(g) ΔrH A B C Δ rH + 2 ΔfH(NaBr(g)) 2NaBr(g) 12. Consider the enthalpy change diagram below. Label horizontal lines with the substances involved in the chemical equation, then label the remaining lines with appropriate species. Write an expression for each labeled arrow (A-D) with the enthalpy change it corresponds to. N2(g) + 3 H2(g)  2NH3(g). 1 -6ΔbH(N-H) A 2 Δ b B C D H ( N 3 a N a ) + 4 Δ b 5 H ( B r - 13. When 1.00L of 2.00M Na2SO4 solution at 30.0°C is added to 2.00L of 0.75M Ba(NO3)2 solution at 30.0°C in a calorimeter, a white solid forms and the temperature of the final solution is 45.0°C. Assuming that the specific heat capacity of the solution is 6.00 J/(K·g) and that the density of the solution is 2.00g/ml calculate the enthalpy change (in kJ/mole) of BaSO4 formed. 14. Consider the following chemical reaction: N2(g) + 3H2(g)  2 NH3(g) What is the ΔrxnH and qrxn when 0.28 g of N2 and 1.8 g of H2 react? (ΔfH°(NH3 (g)]) = -46 kJ/mol ) 15. Using the following information for: 2Al(s) + 3Cl2(g)  2AlCl3(s) 2Al(s) + 6HCl(aq)  2AlCl3(aq) + 3H2(g) HCl(g) HCl(aq) H2(g) + Cl2(g)  2HCl(g) AlCl3(s) AlCl3(aq) ΔrH°1 = –1000.kJ/mol ΔrH°2 = –80.0 kJ/mol ΔrH°3 = –200. kJ/mol ΔrH°4 = –305. kJ/mol a. Calculate ΔrH using (all values in kJ/mol) b. When 1.35g of Al(s) reacts with 0.710g of Cl2(g), 250. J of work is done. What is qv for this reaction in kJ? 16. Calculate the standard enthalpy of formation ΔfH° of Co3O4(s) from the following information. 2CoO(s)  2 Co(s) + O2(g) ΔrH°rxn1= 500. kJ/mol 6 CoO(s) + O2(g)  2 Co3O4(s) ΔrH°rxn2 = −300. kJ/mol 17. Give the expression for the enthalpy of formation, ΔfH, of CO2(g) in terms of H1, H2, and H3. H2O(l)  H2(g) + ½ O2(g) rH = H1 C2H6(g)  2C(s) + 3H2(g) rH = H2 2CO2(g) + 3H2O(l)  C2H6(g) + 7/2 O2(g) rH = H3 18. The enthalpy of neutralization for the reaction of strong acid with strong base is −60. kJ/mol water produced. How much energy (in kJ) will be released when 200.0 mL of 0.400M HCl is mixed with and 150.0 mL of 0.500M NaOH? 19. Consider the following generic chemical reactions. A + 2 B  C + 3 D ΔrH°rxn1= 150. kJ/mol 2C + 6 D  E ΔrH°rxn2= 100. kJ/mol Determine the value of ΔH°rxn for the following related reactions. a. A + 2B  ½ E b. 3 C + 9 D  3 A + 6 B c. ½ C + 3⁄2 D  ½ A + B 6 20. A chemist measures the enthalpy change ∆H during the following reaction Hint: look at the states of the reaction ∆vapH(H2O) =41kJ/mol. 2Al(OH)3(s)  Al2O3(s) + 3H2O(l) ∆rxnH = 40. kJ/mol Use this information to determine the ∆rxnH for the following reactions: a. Al2O3(s) + 3 H2O(g)  2 Al(OH)3(s) ΔrH = ________kJ/mol b. 3 Al2O3(s) + 9 H2O(g)  6 Al(OH)3(s) ΔrH = ________kJ/mol c. 4 moles of Al2O3(s) are reacted with 6 moles of H2O(g) with 68% yield. qP = _____kJ 21. Fill in the horizontal lines using only the following species and their corresponding coefficients: N2(g), N(g), O(g), O2(g), N2O(g). For the labeled arrows on the diagram, write the expression for the enthalpy change in terms of bond enthalpies and enthalpies of formation. N2(l) + 1/2 O2(g) → N2O(g) D A C B N2(l) + ½ O2(g) ∆r H 22. Alcohols are proposed as fuel sources for the future, and currently there are even cars that run on ethanol. The balanced chemical equations for the complete combustion of a standard alkane fuel and an alcohol are: CH3CH2CH3 (propane) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) CH3CH2CH2OH (propanol) + O2(g) → 3 CO2(g) + 4 H2O(g) 7 The result of combustion of one mole of propane is the same as combustion of one mole of propanol: three moles of carbon dioxide and four moles of water. This means the difference in the energy content of the two fuels is due to the bond enthalpies of the reactants. Which one propane or propanol makes a better fuel? Hint: Use bond enthalpies calculate the enthalpy difference, ΔHpropanol - ΔHpropane, using the Lewis Structures above. Bond ∆BH(kJ/mol) O-H 460. C-H 410. C-O 350. C-C 350. O=O 500. C=O 750. Remember, bond enthalpies give only approximate results for gas phase reactions. The actual enthalpies of combustion for propane and propanol are respectively: -2226 kJ/mol and -2020 kJ/mol. Seeing as you have to carry these fuels around with you, circle which fuel is more efficient on a per kilogram basis Numerical Answers: Line 2: [Reactants (standard states)] N2(g) + 3 H2(g) Line 3: [Product] 2NH3(g) Line 4: [Product] 2NH3(l) A: ΔbH(N≡N) + 3∙ΔbH(H-H) B: ΔrxnH or 2∙ΔfH(NH3(g)) C: 2∙ΔfH(NH3(l)) D: 2ΔvapH 13. -360. kJ/mol 14. -92 kJ/mol; -0.92kJ 15. a. ΔHrxn = -1470 kJ/mol; b. qv = -4650 J 16. -900. kJ/mol 17. 3/2 ΔH1 – ½ ΔH2 – 1/2 ΔH3 = ΔHf 18. -4.5 kJ 19. a. 200. kJ/mol b. -450. kJ/mol c. -75.0 kJ/mol 20. a. -160 kJ/mol b. -490 kJ/mol c. -220 kJ 21. a. 2 N(g) + O(g) A: ΔHB(N≡N) + ½ ΔHB(O=O) b. N2(g) + ½ O2(g) B: - ΔHf(N2(l)) c. N2(l) + ½ O2(g) C: ΔHf(N2O(g)) d. N2O(g) D: - ΔHB(N≡N)- ΔHB(N-O) 22. Propane is better both by bond enthalpy and by enthalpies of combustion 1. Sometimes positive a. Yes 2. C(graphite) + ½ O2(g) + 2H2(g) + N2(g)  H2NCONH2(s) ΔrH°= ΔfH°( H2NCONH2(s)) 3. C3H8 (l) +5O2(g)3CO2(g) +4H2O(g); 100% = -6116 kJ a. 75% = -4587 kJ 4. ΔHrxn= ΔH1 - 3/2 ΔH2 + 2 ΔH3 – ½ ΔH4 5. Always positive 6. a. 50. kJ/mol b. 93 kJ/mol 7. a. 2H2(g) + O2(g)  2 H2O(g) b. top: 4H(g) + 2 O(g) middle: 2H2O(g) bottom: 2H2O(l) c. A: 2ΔbH(H–H)+ ΔbH(O=O) B: −4ΔbH(O–H) C: −4ΔbH(O–H) − ΔvapH D: 2ΔfH(H2O(l)) d. −420.0 kJ/mol 8. -42 kJ/mol 9. a. -40. kJ/mol b. -120 kJ/mol c. -60. kJ d. 40. kJ/mol; 60. kJ 10. 4 kJ/mol 11. A: ΔbH(Na−Na)+ ΔbH(Br−Br) B: 2*(−ΔbH(Na−Br)) C: −ΔfH(Na2(g)) −ΔfH(Br2(g)) 12. Line 1: [Separated atoms] 2N(g)+6H(g) 8