Write the three thermochemical equations needed to calculate the delta H for the below reaction.?
2Mg(s) + O2(g) --------> 2 MgO(s) delta H rxn = ?
(Include delta H values in kJ for each reaction)
To calculate the ΔH (enthalpy change) for the given reaction, we need to use three thermochemical equations and their respective ΔH values. Here are the equations and the steps to calculate the overall ΔH:
1. Equation 1: Formation of magnesium oxide (MgO):
Mg(s) + 1/2 O2(g) → MgO(s) ΔH1 = ΔHf(MgO)
This equation represents the formation of one mole of MgO from its elements in their standard states. The enthalpy change, ΔH1, for this reaction is the standard enthalpy of formation of MgO, denoted as ΔHf(MgO).
2. Equation 2: Combustion of magnesium (Mg):
Mg(s) + 1/2 O2(g) → MgO(s) ΔH2 = -ΔHf(Mg)
This equation represents the combustion of one mole of Mg to form MgO. The enthalpy change, ΔH2, for this reaction is the negative of the standard enthalpy of formation of Mg, denoted as -ΔHf(Mg).
3. Equation 3: Formation of oxygen gas (O2):
1/2 O2(g) → O2(g) ΔH3 = 0 kJ
This equation represents the formation of one mole of O2 from its element in its standard state. The enthalpy change, ΔH3, for this reaction is zero since O2 is in its standard state.
Now, we have the necessary equations to calculate the ΔH for the overall reaction. The overall ΔH can be determined using the Hess's Law, which states that the ΔH for a reaction is the sum of the ΔH values of the individual reactions as long as the stoichiometry is properly adjusted.
ΔH rxn = Σ(ΔH products) - Σ(ΔH reactants)
= (2 * ΔH1) - (ΔH2 + O3)
= 2ΔHf(MgO) - (-ΔHf(Mg))
= 2ΔHf(MgO) + ΔHf(Mg)
The overall ΔH is the sum of twice the standard enthalpy of formation of MgO and the standard enthalpy of formation of Mg.
Please note: The values of ΔHf(MgO) and ΔHf(Mg) need to be looked up in a thermochemical table since they are experimentally determined values. Once you find these values (in kJ/mol), you can substitute them into the equation to calculate ΔH rxn.