Enthalpy Lab Report
Purpose: Explore the enthalpy of the combustion of magnesium using Hess’s law.
Question: How can you use Hess’s law to determine a reaction’s enthalpy when you can’t do so using a
calorimeter?
Summary: The combustion of magnesium has the equation Mg(s) + 1/2O2(g) → MgO(s). This reaction gives
off enough heat that it could melt a coffee cup calorimeter, making it difficult to compute the enthalpy of the
reaction directly. However, you can do so indirectly using Hess’s law and the following three intermediate
reactions:
Reaction 1: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Reaction 2: MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
Reaction 3: H2(g) + 1/2O2(g) → H2O(l)
I computed the enthalpy of the Reactions 1 and 2 using a coffee cup calorimeter. The enthalpy of the third
reaction is well known (–286 kJ/mol). With these three enthalpy values, I used Hess’s law to compute the
enthalpy of magnesium combustion.
Data:
Measurement Reaction 1 (Mg + HCl) Reaction 2 (MgO + HCl)
Mass of HCl (g) 100.50 100.57
Mass of solid (g) 0.20 1.57
Total mass of reactants 100.70 102.14
Initial temperature (C) 22.5 22.7
Temperature furthest from initial 31.3 30.2
temperature (C)
Delta Temperature (C) 8.8 7.5
Heat released (J) 3700 3200
Moles of solid reactant (mol) 0.00823 0.0390
(Mg: 24.3 g/ mol) (MgO: 40.3 g/ mol)
Enthalpy of reaction (KJ/ mol) -450 -82
Reaction equation Delta H (kJ/mol)
Mg(s) + 2HCl(aq) ⟶ MgCl2(aq) + H2(g) -450
MgO(s) + 2HCl(aq) ⟶ MgCl2(aq) + H2O(l) -82
H2(g) + 1⁄2O2(g) ⟶ H2O(l) -286
Mg(s) + 1⁄2O2(g) ⟶ MgO(s) -654
Percentage error : 8.62%
Purpose: Explore the enthalpy of the combustion of magnesium using Hess’s law.
Question: How can you use Hess’s law to determine a reaction’s enthalpy when you can’t do so using a
calorimeter?
Summary: The combustion of magnesium has the equation Mg(s) + 1/2O2(g) → MgO(s). This reaction gives
off enough heat that it could melt a coffee cup calorimeter, making it difficult to compute the enthalpy of the
reaction directly. However, you can do so indirectly using Hess’s law and the following three intermediate
reactions:
Reaction 1: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Reaction 2: MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
Reaction 3: H2(g) + 1/2O2(g) → H2O(l)
I computed the enthalpy of the Reactions 1 and 2 using a coffee cup calorimeter. The enthalpy of the third
reaction is well known (–286 kJ/mol). With these three enthalpy values, I used Hess’s law to compute the
enthalpy of magnesium combustion.
Data:
Measurement Reaction 1 (Mg + HCl) Reaction 2 (MgO + HCl)
Mass of HCl (g) 100.50 100.57
Mass of solid (g) 0.20 1.57
Total mass of reactants 100.70 102.14
Initial temperature (C) 22.5 22.7
Temperature furthest from initial 31.3 30.2
temperature (C)
Delta Temperature (C) 8.8 7.5
Heat released (J) 3700 3200
Moles of solid reactant (mol) 0.00823 0.0390
(Mg: 24.3 g/ mol) (MgO: 40.3 g/ mol)
Enthalpy of reaction (KJ/ mol) -450 -82
Reaction equation Delta H (kJ/mol)
Mg(s) + 2HCl(aq) ⟶ MgCl2(aq) + H2(g) -450
MgO(s) + 2HCl(aq) ⟶ MgCl2(aq) + H2O(l) -82
H2(g) + 1⁄2O2(g) ⟶ H2O(l) -286
Mg(s) + 1⁄2O2(g) ⟶ MgO(s) -654
Percentage error : 8.62%
