Heat of Combustion: Magnesium
In this experiment, you will use Hess’s law as you determine a heat of a reaction that would be
difficult to obtain by direct measurement—the heat of combustion of magnesium ribbon. The
reaction is represented by the equation:
Mg(s) + 1/2 O2(g) → MgO(s) This is the target equation.
This equation can be obtained by combining equations (1), (2), and (3):
(1) MgO(s) + 2 HCl(aq) → MgCl2(aq) + H2O(l)
(2) Mg (s) + 2 HCl(aq) → MgCl2(aq) + H2 (g)
(3) 2H2(g) + O2(g) → 2H2O(l) ΔH for this reaction -571.6 kJ.
In this experiment heats of reaction for equations (1) and (2) will be determined.
Pre-lab exercise
In the space provided below, “rewrite” and combine equations (1), (2), and (3) to obtain the desired
target equation.
(1) MgCl2(aq) + H2O(l) → MgO(s) + 2 HCl(aq)
(2) Mg (s) + 2 HCl(aq) → MgCl2(aq) + H2 (g)
(3) H2(g) + 1/2 O2(g) → H2O(l)
Target: Mg(s) + 1/2 O2(g) → MgO(s)
Procedure: Bullet points or numbered list. Concise. Including amounts of chemicals used.
Graphs imported from logger pro: BELOW
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Heat of Combustion: Magnesium
With Magnesium Oxide:
With Magnesium:
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Heat of Combustion: Magnesium
Materials available
Go-Temp Probe 1.00 M HCl
styrofoam cup/calorimeter magnesium oxide, MgO no more than 2 g
utility clamp and stand etc Mg ribbon, no more than 1.0 g
graduated cylinders stirring rod
beakers weighing cup(s)
massing balance laptop computer
Data Colection: measured values only
Reaction 1
(MgO)
Reaction 2
(Mg)
Mass of Sample (Mg or
MgO)
1.49g 0.72g
Volume of HCl used 100mL 100mL
Mass of Styrofoam Cup 7.92g 7.92g
Mass of Cup + HCl 108.30 107.61
Accounting for Energy Loss:
● Real Temperature Change = slope * 0.5 * ^Time
○ Time it took to get to highest temperature
Data procesing:
a. Temperature correction: explanation and calculations
The temperature must be corrected to account for the heat lost because the styrofome cup
was open. In other words, the actual temperatures are actually higher than they appear
because part of the heat is lost in the surroundings.
Magnesium Oxide:
Time to Reach Maximum Temperature (Plateau): 120 Seconds
Temperature Change (from LoggerPro) = 28.2oC - 20.4oC = 7.8o C
Slope (from LoggerPro) = -0.00396
Real Temperature Change:
RTC = (0.00396 Degrees C/Sec)(0.5)(120)
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Heat of Combustion: Magnesium
= 0.24o C
Adjusted Temperature Change = 7.8o C + 0.24o C = 8.04oC
Magnesium:
Time to Reach Maximum Temperature (Plateau) = 143.5 Seconds
Temperature Change (from LoggerPro) = 50.8oC - 20.8oC = 30oC
Slope (from LoggerPro) = -0.01811 Degrees C/Sec
Real Temperature Change:
RTC = (0.01811 Degrees C/Sec)(0.5)(143.5)
= 1.30o C
Adjusted Temperature Change = 30o C + 1.30o C = 31.3oC
b. Calculations to determine ΔH values in kJ/mole for reaction 1 and 2
Reaction 1 (MgO):
Q = mC ΔT
= (1.49g)(4.18 J/Kg*C)(8.04OC)
= 24.2 J
= 0.0242 KJ
Per Mole = 0.0242KJ/(1.49g/40.3g per mol) = 0.655 kJ/mol
Reaction 2 (Mg):
Q = mC ΔT
= (0.72g)(4.18 J/Kg*C)(31.3oC)
= 195 J
= 0.195 KJ
Per Mole = 0.195KJ/(0.72g/24.3g per mol) = 6.58 kJ/mol
c. Calculation to determine ΔH values in kJ/mole for heat of combustion of Mg
Q = mCΔT
(1) MgCl2(aq) + H2O(l) → MgO(s) + 2 HCl(aq) -- Enthalpy = -0.655 kJ/mol
(2) Mg (s) + 2 HCl(aq) → MgCl2(aq) + H2 (g) -- Enthalpy = 6.58 kJ/mol
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Heat of Combustion: Magnesium
(3) H2(g) + 1/2 O2(g) → H2O(l) -- Enthalpy = -571.6 kJ/mol / 2 = 285.8 kJ/mol
d. Percentage error in ΔHcomb of Mg : Look up the accepted value. Cite your source!
The accepted value of ΔHcomb of Mg is -601.8kJ, which is the same as the ΔHform of
MgO.
e. Error explanation. What reasons (Do not write human error!) contributed to your error. Be
mathematical.
HTTP://NSHS-SCIENCE.NET/CHEMISTRY/COMMON/PDF/R-STANDARD_ENTHALPY_OF_FORMATION.PDF
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