Hess\'s Law: Lab question is: What is the heat of reaction for the reaction: Mg9
ID: 1072793 • Letter: H
Question
Hess's Law: Lab question is: What is the heat of reaction for the reaction: Mg9s) + 1/2 O2 (g) = MgO (s)? Given: H2 (g) = 1/2 O2 (g) = H2O(l) delta H = -285.8 kJ. Use calorimetry to determine the delta H 1 value for the following ____ ?: Mg(s) = 2HCl (aq) = MgCl2 + H2 (g) / Data- mass of empty calorimter: 11.19 g, mass of HCl=25.14g, mass of HCl + calorimter:36.33g, mass of Mg: 0.171 g, temp of HCl: 24.9 degrees Celcius, temperature of HCl +Mg =51.6 degrees Celcius. Then used calorimetry to determine the delta H2 value for the following reaction: MgO (s) + 2 HCl (aq) = MgCl2 (aq) + H2O (l) delta H2 =____ ? Data: mass of empty calorimter: 12.5 g, mass of HCl +calorimter: 37.43 g, mass of Mg: 0.3 grams, temperature of HCl= 24.2 degrees Celcius, temperature of HCl +Mg= 34.0 degrees Celcius. Assuming the specific heat of the aqueous solutions to be 4.184 J/g degrees Celcius, calculate th Q values and the delta H values (kJ per mole of Mg and kJ per mole of MgO) for each reaction. Then use the results with Hess's Law to determine the delta H for the reaction: Mg (s) + 1/2 O2(g) = MgO (s). Theoretical value: delta H = -602 kJ
Explanation / Answer
2. For the reaction , MgO+2HCl-->MgCl2 + H2O
moles of Mg = 0.3/24= 0.0125 moles
Enthalpy change = (37.43-12.50+0.3)*4.18*(34-24.2)/0.0125 =82682 J/mole = 82.68 Kj/mole
Both reactions are exothermic since there is temperature rise
Hence
H2+0.5O2--->H2O, deltaH= -285.8 Kj (1)
Mg+2HCl--->MgCl2+H2, deltaH= -396.49 KJ (2)
MgO+2HCl--->MgCl2 + H2O, deltaH= -82.68 KJ (3)
Addition of Eq.1 and Eq.2 gives
Mg+0.5O2+2HCl -----> MgCl2+H2+H2O , deltaH= -285.8-396.49=-682.29 KJ (4)
Reversing Eq,3 gives MgCl2+H2O -->MgO+2HCl, deltaH= 82.68 Kj (3A)
Addition of Eq.4 and Eq.3A gives
Mg+0.5O2+H2+2HCl + MgCl2+H2O-->MgCl2+ H2+H2O+ 2HCl, deltaH= -682.29+82.68 =-599.61 KJ
Mg+0.5O2-->MgO, deltaH= -599.6nKJ