1.  Safety
2.
Objectives/Overview
3.
Procedures
4. Observations
5.
Equipment
6.

View Techniques
A. Calibration of Temperature Probe
B. Enthalpy of Combustion of Magnesium
C. Temperature Extrapolation from Graph

7.

Waste Disposal/Clean-up

8. Calculations/Set-ups
9. Conclusions
10. Grading Scale
11.
Review Prelab Questions
12.
Review Postlab Questions

Experiment 8
Enthalpy of Combustion

CALCULATIONS/SET-UPS

ENTHALPY OF REACTION: MgO(s) + 2 HCl(aq) ---> MgCl2(aq) + H2O(l)

Mass of MgO
mMgO = mass of MgO and weigh boat - mass of weigh boat

Temperature change

Note: Tf and Ti are both determined from the graph of temperature versus time for this reaction

DT = Tf - Ti
Mass of HCl solution
mHCl = (DHCl)(VHCl) = (1.00 g/mL)(VHCl)
Heat gained by solution
qsoln = (mHCl)(sHCl)(DT) = (mHCl)(4.184 J/gC)(DT)
Heat gained by calorimeter

Note: for Ccal use value determined in experiment 6

qcal = (Ccal)DT
Total heat produced by the reaction
qrxn = (- qsoln ) + (- qcal )
Moles of MgO used
MMMgO = molar mass of MgO
n = mMgO / (MMMgO)
Enthalpy of reaction
DH(MgO) = qrxn / n

ENTHALPY OF REACTION: Mg(s) + 2 HCl(aq) ---> MgCl2(aq) + H2(g)

Mass of Mg
mMg = mass of Mg and weigh boat - mass of weigh boat

Temperature change

Note: Tf and Ti are both determined from the graph of temperature versus time for this reaction

DT = Tf - Ti
Mass of HCl solution
mHCl = (DHCl)(VHCl) = (1.00 g/mL)(VHCl)
Heat gained by solution
qsoln = (mHCl)(sHCl)(DT) = (mHCl)(4.184 J/gC)(DT)
Heat gained by calorimeter

Note: for Ccal use value determined in experiment 6

qcal = (Ccal) DT
Total heat produced by the reaction
qrxn = (- qsoln ) + (- qcal )
Moles of Mg used
AMMg = atomic mass of Mg
n = mMg / (AMMg)
Enthalpy of reaction
DH(Mg) = qrxn / n

COMBUSTION OF Mg: DHcombustion Mg(s) + 1/2 O2(g) ---> MgO(s)

Note: for the reactions and DH values

H2(g) + O2(g) ---> H2O(l), DHf(H2O) = -286 kJ/mol
MgO(s) + 2 HCl(aq) ---> MgCl2(aq) + H2O(l), DH(MgO) = (see above)
Mg(s) + 2 HCl(aq) ---> MgCl2(aq) + H2(g), DH(Mg) = (see above)

Note: The values of each of the factors a, b, and c in the below equation are determined from the Hess' Law rules for combining the three reactions that correspond to the three DH values to produced the desired net reaction (Mg(s) + O2(g) ---> MgO(s) ). An equation that is combined with no changes has a factor of +1. An equation that is reversed before being combined has a factor of -1. If all the coefficients of the reaction equation are multiplied by a number, the factor is the same as that number.

Experimental DHcombustion of Mg (J/mol)
DHcombustion = (a)DH(Mg) + (b)DH(MgO) + (c)DHf(H2O)
Theoretical DHcombustion of Mg (J/mol)
DHcombustion = S(nprod x DHf(products)) - S(nreact x DHf(reactants))
% error in experimental DHcombustion of Mg
% error = ((Theoretical DHcombustion - Experimental DHcombustion) / Theoretical DHcombustion) x 100

 
 
 
 
 

 

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