UNIT 8948: SAMPLE ACTIVITY 32 SAMPLE ASSESSMENT ACTIVITIES AND SCEDULES 6.161 8948 CEMISTRY Production of this activity has been made possible courtesy of funding from CEMED95 and NZASE. ENTALPY CANGES This activity assesses: Unit: 8948 Calculate the enthalpy change associated with chemical reactions (Level 3). Element: 1 Calculate the enthalpy change associated with chemical reactions. CONDITIONS: Formal activity. INSTRUCTIONS: Include the correct units in your answers. Task A 1. The balanced equation for carbon disulfide, CS 2, burning in air is: CS 2 (l) + 3O 2 (g) CO 2 (g) + 2SO 2 (g) Use the enthalpy (heat) of formation data below to calculate the enthalpy of combustion, Δ, for the above equation. Show all of your working. Δ f (SO 2 (g)) = -297 kjmol -1 Δ f (CO 2 (g)) = -393 kjmol -1 Δ f (CS 2 (l)) = +88 kjmol -1 2. The balanced equation for the formation of propane gas, C 3 8, being formed from its elements is: 3C(s) + 4 2 (g) C 3 8 (g) Use the enthalpy data for the reactions below to calculate the enthalpy of formation, Δ f, for propane. Show all of your working. C(s) + O 2 (g) CO 2 (g) Δ f = -393 kjmol -1 2 O(l) 2 (g) + 1/2O 2 (g) Δ = +285 kjmol -1 C 3 8 (g) + 5O 2 (g) 3CO 2 (g) + 4 2 O(l) Δ = -2217 kjmol -1 3. The Contact Process for the manufacture of sulfuric acid involves three successive reactions i) sulfur is burned in air S(s) + O 2 (g) SO 2 (g) ii) sulfur dioxide is further oxidised SO 2 (g) + 1/2O 2 (g) SO 3 (g) iii) sulfur trioxide is reacted with water SO 3 (g) + 2 O(l) 2 SO 4 (l) ASSESSMENT ACTIVITY: ENTALPY CANGES PAGE 1/3 NEW ZEALAND QUALIFICATIONS AUTORITY - ALL RIGTS RESERVED ASSESSMENT GUIDE: CEMISTRY - JUNE 1997
6.162 SAMPLE ASSESSMENT ACTIVITIES AND SCEDULES UNIT 8948: SAMPLE ACTIVITY 32 Task B Given that the enthalpy change for the overall reaction S(s) + 3/2O 2 (g) + 2 O(l) 2 SO 4 (l) is Δ = -529 kjmol -1 and further that Δ f (SO 2 ) = -297 kjmol -1 Δ c (SO 2 ) = -396 kjmol -1 calculate the enthalpy change for reaction (iii) above ie, the reaction for sulfur trioxide with water. Show all of your working. 1.1 The bond enthalpy - is for -(g) (g) + (g). Use average bond enthalpies from the table below to calculate approximate enthalpy changes in questions 1, 2 and 3. (All values quoted are in kjmol-1). Single bond enthalpy Multiple bond enthalpy C- 414 C=C 614 C-Cl 336 C=C 839 C-C 348 N=N 945-436 O=O 498 -Cl 431 Cl-Cl 242 N- 391 1. In the presence of a catalyst (such as Pt) alkenes react with hydrogen to form the corresponding alkane. Use the average bond enthalpies given in the table above to calculate an enthalpy change for the hydrogenation of propene, ie for the reaction C 3 6 (g) + 2 (g) C 3 8 (g) Show all of your working. 2. Use the average bond enthalpies given in the table above to calculate an enthalpy change for the reaction C 3 Cl(g) + 2Cl 2 (g) CCl 3 (g) + 2Cl(g) Show all of your working. ASSESSMENT ACTIVITY: ENTALPY CANGES PAGE 2/3 NEW ZEALAND QUALIFICATIONS AUTORITY - ALL RIGTS RESERVED ASSESSMENT GUIDE: CEMISTRY - JUNE 1997
UNIT 8948: SAMPLE ACTIVITY 32 SAMPLE ASSESSMENT ACTIVITIES AND SCEDULES 6.163 8948 CEMISTRY 3. In the aber process, ammonia gas (N 3 ) is synthesised directly by the reaction of nitrogen (N 2 ) and hydrogen ( 2 ) gases according to the balanced equation N 2 (g) + 3 2 (g) 2N 3 (g) Use the average bond enthalpies given in the table above to calculate an enthalpy change for the formation of ammonia by the aber process. Show all of your working. 1.2 ASSESSMENT SCEDULE: ENTALPY CANGES PAGE 1/2 NEW ZEALAND QUALIFICATIONS AUTORITY - ALL RIGTS RESERVED ASSESSMENT GUIDE: CEMISTRY - JUNE 1997
6.164 SAMPLE ASSESSMENT ACTIVITIES AND SCEDULES UNIT 8948: SAMPLE ACTIVITY 32 Assessment Schedule: Enthalpy changes Unit Standard 8948 Task Number Element and Performance Criteria Evidence (The answers or performance expected from the students) Judgement (A statement that defines the standard to be achieved) A 1.1 1. Δ = ΣΔ f (prod) - ΣΔ f (react) Two calculations correct. = [-393 + 2(-297)] - [88 + 3 (0)] Correct units must be included with answers. = -1075 kj 2. 3C + 3O 2 3CO 2 Δ = 3 (-393) 4 2 + 2O 2 4 2 O Δ = -4(285) 3CO 2 + 4 2 O C 3 8 + 5O 2 Δ = -(-2217) 3C + 4 2 C 3 8 Δ = -102 kj mol -1 3. S + O 2 SO 2 Δ = -297 SO 2 + 0.5O 2 SO 3 Δ = -396 SO 3 + 2 O 2 SO 4 Δ = x S + 1.5O 2 + 2 O 2 SO 4 Δ = -529-297 - 396 + x = -529 x = +164 kj ASSESSMENT SCEDULE: ENTALPY CANGES PAGE 2/2 NEW ZEALAND QUALIFICATIONS AUTORITY - ALL RIGTS RESERVED ASSESSMENT GUIDE: CEMISTRY - JUNE 1997
UNIT 8948: SAMPLE ACTIVITY 32 SAMPLE ASSESSMENT ACTIVITIES AND SCEDULES 6.165 8948 CEMISTRY Task Number Element and Performance Criteria Evidence (The answers or performance expected from the students) Judgement (A statement that defines the standard to be achieved) B 1.2 1. Two calculations correct. C C C + C C C Units must be included with answers. Δ = +1050-1176 = -126 kj = 1(-) + 1(C=C) = +(436 + 614) = +1050 kj = 2(C-) + 1(C-C) = [2(414) + 348] = -1176 kj 2. Cl C Cl + 2(Cl Cl) C Cl + 2( Cl) Cl = 2(C-) + 2(Cl-Cl) = +2(414) + 2(242) = +1312 kj = 2(C-Cl) + 2(-Cl) = -[2(336) + 2(431)] = -1534 kj Δ = +1312-1534 = -222 kj 3. N N + 3( ) 2(N ) = 1(N=N) + 3(-) = +945 + 3(436) = +2253 kj = 6(N-) = -6(391) = -2346 kj Δ = +2253-2346 = --83 kj ASSESSMENT SCEDULE: ENTALPY CANGES PAGE 1/2 NEW ZEALAND QUALIFICATIONS AUTORITY - ALL RIGTS RESERVED ASSESSMENT GUIDE: CEMISTRY - JUNE 1997