Homework Help: Finding the heat of combustion of magnesium

1. Nov 9, 2008

Morhas

1. The problem statement, all variables and given/known data

Finding the heat of combustion of magnesium.

2. Relevant equations

1.) MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) ∆Hr = 1047.61KJ/mol *This value was found experimentally
2.) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) ∆Hr = 374.36KJ/mol*This value was found experimentally
3.) H2(g) + ½O2(g) → H2O(l) ∆Hr = -285.5 ∆HF =kJ/mol

3. The attempt at a solution

2 - 1 = Mg(s) + 2HCl(aq)+ MgCl2(aq) + H2O(l) → MgCl2(aq) + H2(g) + MgO(s) + 2HCl(aq)
= Mg(s) + H2O(l) → H2(g) + MgO(s)
Let 4 = Mg(s) + H2O(l) → H2(g) + MgO(s)

4+3 = Mg(s) + H2O(l) + H2(g) + ½O2(g) → H2(g) + MgO(s)+ H2O(l)
= Mg(s) + ½O2(g) → MgO(s)

2 – 1 = 374.36KJ/mol - 1047.61KJ/mol
= -673.25KJ/mol
Let 4 =-673.25KJ/mol
4 + 3 = -673.25KJ/mol + -285.5KJ/mol
= -958.75KJ/mol

Now, the accepted value is about 600kj/mol. When this lab was conducted, horribly inaccurate calorimeters were used. So wouldn't that mean the value I found should be closer to 0 than the accepted value?

EDIT

Found my first problem. ∆Hr for equation one is supposed to be 104.761KJ/mol not 1047.61KJ/mol.

Now my final value is -15kj/mol.... Bleh. Mods can delete.

Last edited: Nov 9, 2008
2. Nov 10, 2008

Staff: Mentor

I have answered your question elsewhere. For the record: is dissolution of Mg in HCl exothermic, or endothermic?