Chem Lab – Heat of Combustion of Magnesium

Heat of Combustion of Magnesium Background: The students were given full instructions on how to experimentally determine the enthalpy of reaction (? Hrxn) for the combustion of magnesium ribbon, using Hess’s Law. Data Collection: |Reaction 1 |Reaction 2 | | |(MgO) |(Mg) | | |Trial 1 |Trial 2 |Trial 1 |Trial 2 | |Volume of 1. 00 M HCl |50. 0 mL  ± 0. 5 mL |50. 0 mL  ± 0. 5 mL |50. mL  ± 0. 5 mL |50. 0 mL  ± 0. 5 mL | |Final temperature, t2 |28. 9(C  ± 0. 1(C |28. 8(C  ± 0. 1(C |44. 8(C  ± 0. 1(C |44. 4( C  ± 0. 1(C | |Initial temperature, t1 |22. 5(C  ± 0. 1(C |22. 3( C  ± 0. 1(C |21. 9(C  ± 0. 1(C |21. 8( C  ± 0. 1(C | |Mass of solid |0. 50 g  ± 0. 01g |0. 50 g  ± 0. 01g |0. 25 g  ± 0. 01g |0. 25 g  ± 0. 01g | Data Processing: |Reaction 1 | | |(MgO) | | |Trial 1 |Trial 2 | |Change in |final temperature – initial temperature |final temperature – initial temperature | |temperature, (t |28. (C – 22. 5(C = 6. 4(C |28. 8(C – 22. 3(C = 6. 3(C | | | | | | |Uncertainty of temperature: 0. 1(C + 0. 1(C = 0. 2(C |Uncertainty of temperature: 0. 1(C + 0. 1(C = 0. 2(C | | | | | | |Change in temperature = 6. (C  ± 0. 2(C |Change in temperature = 6. 3(C  ± 0. 2(C | |Heat of HCl |Mass of 1. 0 M HCl [pic] specific heat of HCl [pic] change in |Mass of 1. 0 M HCl [pic] specific heat of HCl [pic] change in | |solution, q |temperature |temperature | | | | | | |To find mass of 1. 0 M HCl : Density of 1. M HCl [pic] volume |To find mass of 1. 0 M HCl : Density of 1. 0M HCl [pic] volume | | |of 1. 0M HCl |of 1. 0M HCl | | |1. 005 [pic] [pic] 50. 0 mL = 50. 3 g |1. 005 [pic] [pic] 50. 0 mL = 50. 3 g | | | | | | |50. 3 g [pic] . 00418 [pic] [pic] 6. 4(C = 1. 3 kJ |50. g [pic] . 00418 [pic] [pic] 6. 3(C = 1. 3 kJ | | | | | | |% Uncertainty of Heat: [(0. 5mL ? 50 mL) + (0. 2(C ? 6. 4(C)] |% Uncertainty of Heat: [(0. 5mL ? 50 mL) + (0. 2(C ? 6. 3(C)] | | |[pic] 100% = 4% |[pic] 100% = 4% | | |Uncertainty of Heat: 4% of 1. 3 kJ = 0. 5 kJ |Uncertainty of Heat: 4% of 1. kJ = 0. 5 kJ | | | | | | |Heat of HCl = 1. 3 kJ  ± 0. 5 kJ |Heat of HCl = 1. 3 kJ  ± 0. 5 kJ | |Enthalpy change,|Enthalpy change of reaction = -heat of HCl solution |Enthalpy change of reaction = -heat of HCl solution | |(H | | | | |Enthalpy of MgO = -1. kJ  ± 0. 5 kJ |Enthalpy of MgO = -1. 3 kJ  ± 0. 5 kJ | |Moles of MgO |Mass of MgO ? molar mass of MgO |Mass of MgO ? molar mass of MgO | | | | | | |0. 50 g MgO ? 40. 30[pic] = 0. 0120 mol |0. 50 g MgO ? 40. 30[pic] = 0. 0120 mol | | |% Uncertainty of moles: (0. 01g ? 0. 0g) [pic] 100% = 2% |% Uncertainty of moles: (0. 01g ? 0. 50g) [pic] 100% = 2% | | |Uncertainty of moles: 2% of 0. 0120 mol = 0. 0002 mol |Uncertainty of moles: 2% of 0. 0120 mol = 0. 0002 mol | | | | | | |Moles of MgO = 0. 0120 mol  ± 0. 0002 mol |Moles of MgO = 0. 0120 mol  ± 0. 0002 mol | |Molar enthalpy |Enthalpy of MgO ? moles of MgO |Enthalpy of MgO ? oles of MgO | |change, (H/m ol | | | | |-1. 3 kJ ? 0. 0120 mol = -110 [pic] |-1. 3 kJ ? 0. 0120 mol = -110 [pic] | | | | | | |% Uncertainty: [(0. 5 kJ ? 1. 3 kJ) + (0. 0002 mol ? 0. 0120 mol)] |% Uncertainty: [(0. 5 kJ ? 1. 3 kJ) + (0. 0002 mol ? 0. 120 mol)] | | |[pic] 100% = 40% |[pic] 100% = 40% | | |Uncertainty: 40% of -110 [pic] = 40 [pic] |Uncertainty: 40% of -110 [pic] = 40 [pic] | | | | | | |Molar enthalpy change = -110 [pic]  ± 40 [pic] |Molar enthalpy change = -110 [pic]  ± 40 [pic] | |Average molar |(molar enthalpy change of Trial 1 + molar enthalpy change of Trial 2) ? | |enthalpy change |( -110 [pic] + -110 [pic] ) ? 2 = -110 [pic] | | | | | |Uncertainty: (40 [pic] + 40 [pic] ) ? 2 = 40 [pic] | | | | | |Average molar enthalpy change = -110 [pic]  ± 40 [pic] | |Reaction 2 | | |(Mg) | | |Trial 1 |Trial 2 | |Change in |final temperature – initial temperature |final temperature – initial temperature | |temperature, (t |44. 8(C – 21. 9(C = 22. 9(C |44. 4(C  œ 21. 8(C = 22. (C | | | | | | |Uncertainty of temperature: 0. 1(C + 0. 1(C = 0. 2(C |Uncertainty of temperature: 0. 1(C + 0. 1(C = 0. 2(C | | | | | | |Change in temperature = 22. 9(C  ± 0. 2(C |Change in temperature = 22. 6(C  ± 0. 2(C | |Heat of HCl |Mass of 1. M HCl [pic] specific heat of HCl [pic] change in |Mass of 1. 0 M HCl [pic] specific heat of HCl [pic] change in | |solution, q |temperature |temperature | | | | | | |To find mass of 1. 0 M HCl : Density of 1. 0M HCl [pic] volume |To find mass of 1. 0 M HCl : Density of 1. 0M HCl [pic] volume | | |of 1. 0M HCl |of 1. M HCl | | |1. 005 [pic] [pic] 50. 0 mL = 50. 3 g |1. 005 [pic] [pic] 50. 0 mL = 50. 3 g | | | | | | |50. 3 g [pic] . 00418 [pic] [pic] 22. 9(C = 4. 81 kJ |50. 3 g [pic] . 00418 [pic] [pic] 22. 6(C = 4. 75 kJ | | | | | | |% Uncertainty of Heat: [(0. 5mL ? 0 mL) + (0. 2(C ? 22. 9(C)] |% Uncertainty of Heat: [(0. 5mL ? 50 mL) + (0. 2(C ? 22. 6(C)] | | |[pic] 100% = 1. 9% |[pic] 100% = 1. 9% | | |Unc ertainty of Heat: 1. 9% of 4. 81 kJ = 0. 09 kJ |Uncertainty of Heat: 1. 9% of 4. 75 kJ = 0. 09 kJ | | | | | | |Heat of HCl = 4. 81 kJ  ± 0. 09 kJ |Heat of HCl = 4. 75 kJ  ± 0. 9 kJ | |Enthalpy change,|Enthalpy change of reaction = -heat of HCl solution |Enthalpy change of reaction = -heat of HCl solution | |(H | | | | |Enthalpy of Mg = -4. 81 kJ  ± 0. 09 kJ |Enthalpy of Mg = -4. 75 kJ  ± 0. 09 kJ | |Moles of Mg |Mass of Mg ? molar mass of Mg |Mass of Mg ? molar mass of Mg | | | | | | |0. 25 g ? 24. 30[pic] = 0. 010 mol |0. 25 g ? 24. 30[pic] = 0. 10 mol | | |% Uncertainty of moles: (0. 01g ? 0. 25g) [pic] 100% = 4% |% Uncertainty of moles: (0. 01g ? 0. 25g) [pic] 100% = 4% | | |Uncertainty of moles: 4% of 0. 0100 mol = 0. 0004 mol |Uncertainty of moles: 4% of 0. 0100 mol = 0. 0004 mol | | | | | | |Moles of Mg = 0. 0100  ± 0. 0004 mol |Moles of Mg = 0. 0100  ± 0. 0004 mol | |Molar enthalpy |Enthalpy of Mg ? oles of Mg |Enthalpy of Mg ? moles of Mg | |change, (H/mo l | | | | |-4. 81kJ ? 0. 0100 mol = -481 [pic] |-4. 75 kJ ? 0. 0100 mol = – 475[pic] | | | | | | |% Uncertainty: [(0. 09 kJ ? 4. 81 kJ) + (0. 0004 mol ? 0. 0100 |% Uncertainty: [(0. 09 kJ ? 4. 75 kJ) + (0. 0004 mol ? 0. 100 | | |mol)] [pic] 100% = 6% |mol)] [pic] 100% = 6% | | |Uncertainty: 6% of -481 [pic] = 29 [pic] |Uncertainty: 6% of -475 [pic] = 29 [pic] | | | | | | |Molar enthalpy change = -481 [pic]  ± 29 [pic] |Molar enthalpy change = -475 [pic]  ± 29 [pic] | |Average molar |(molar enthalpy change of Trial 1 + molar enthalpy change of Trial 2) ? | |enthalpy change |(-481 [pic] + -475 [pic] ) ? 2 = -478 [pic] | | | | | |Uncertainty: (29 [pic] + 29 [pic] ) ? 2 = 29 [pic] | | | | | |Average molar enthalpy change = -478 [pic]  ± 29 [pic] |