Finding the heat of combustion of magnesium

[Morhas]

Homework Statement

Finding the heat of combustion of magnesium.

Homework 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

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?

Much thanks in advance.

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.

 

[Borek]

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

 

[Blueshift5]

Thank you for sharing your attempt at solving for the heat of combustion of magnesium. It is important to note that the values you have used for the heat of reaction (∆Hr) in equations 1 and 2 are not correct. The correct values for these reactions are actually 104.761KJ/mol and 374.36KJ/mol, as you have mentioned in your edit. Additionally, when combining equations 1 and 2, you need to account for the fact that MgCl2 and H2O are present on both sides of the equation and should therefore cancel out. This means that the final equation should be:

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Next, it is important to consider the units for the heat of formation (∆Hf) in equation 3. The correct unit for ∆Hf is kJ/mol, not kJ/mol * ∆HF. Therefore, the final equation should be:

Mg(s) + ½O2(g) → MgO(s)

Now, when solving for the heat of combustion, it is important to take into account the stoichiometric coefficients of the reactants and products. In this case, the stoichiometric coefficient for Mg is 1, while the stoichiometric coefficient for O2 is ½. This means that the heat of combustion for magnesium should be calculated using the following equation:

∆Hcomb = ∆Hf(MgO) - ½∆Hf(O2)

Plugging in the correct values for ∆Hf(MgO) and ∆Hf(O2), we get:

∆Hcomb = -601.8 kJ/mol

This value is closer to the accepted value of 600 kJ/mol, which suggests that the value you have found may have been affected by experimental errors or inaccuracies. It is important to carefully check the units and values used in your calculations to ensure accuracy. Additionally, it may be helpful to repeat the experiment multiple times and take an average of the values to reduce the impact of any errors. Overall, it is important to carefully review and analyze your results to ensure accuracy and draw meaningful conclusions.