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Chem Lab - Heat of Combustion of Magnesium

By abreathofoxygen Apr 22, 2010 1231 Words
Heat of Combustion of Magnesium

Background: The students were given full instructions on how to experimentally determine the enthalpy of reaction (ΔHrxn) 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 |50.0 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 |28.9(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 = 6.4(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 1.0M HCl [pic] volume |To find mass of 1.0 M HCl : Density of 1.0M HCl [pic] volume | | |of 1.0M HCl |of 1.0M 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 |50.3 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 1.3 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 = -1.3 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 ÷ 0.50g) [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 ÷ moles of MgO | |change, (H/mol | | | | |-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 ÷ 0.0120 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) ÷ 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 – 21.8(C = 22.6(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 1.0 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 1.0M HCl [pic] volume |To find mass of 1.0 M HCl : Density of 1.0M HCl [pic] volume | | |of 1.0M HCl |of 1.0M 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] 22.9(C = 4.81 kJ |50.3 g [pic] .00418 [pic] [pic] 22.6(C = 4.75 kJ | | | | | | |% Uncertainty of Heat: [(0.5mL ÷ 50 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% | | |Uncertainty 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 ± 0.09 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] = 0.010 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 ÷ moles of Mg |Enthalpy of Mg ÷ moles of Mg | |change, (H/mol | | | | |-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 ÷ 0.0100 | | |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) ÷ 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] |

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