January 18, 2017

Homework Help: chemistry

Posted by Louie on Friday, April 5, 2013 at 11:27pm.

I'm trying to figure out what went wrong during my freshman chem lab.

We were figuring out the enthalpy of formation of MgO(s).

MgO(s) + 2H+(aq) -> Mg2+(aq) + H2O(l) dHrxn = -334.83kJ/mol

Mg(s) + 2H+(aq) -> Mg2+(aq) + H2(g) dHxrn = -1105kJ/mol

H2(g)+ 1/2O2(g) -> H2O(l) dHrxn = -286kJ/mol

I used Hess' Law and got

Mg(s) + 1/2O2(g) -> MgO(s) dHrxn = -1056.17

The experiement ends up with 75% error haha. It looks like the enthalpy of reaction for Mg(s) + 1/2O2(g) -> MgO(s) is abnormally low. I think instead of separating the contents, they added more HCL and Mg(s) to the solution created from the first half of the experiment, where we figured out the temperature change by adding MgO(s) to diluted HCL, instead of doing it in a separate container.

Do you think this is a plausible explanation, since the first solution created had aqueous Mg in the product according to the following equation:
MgO(s) + 2H+(aq) -> Mg2+(aq) + H20(l)

Sorry for the lengthy question.

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