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/g°C)(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/g°C)(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) + ½ 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) + ½ 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