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.
We used 1g of MgO and .5g of Mg for the experiment. 250ml of 1M HCL was also used.
Never mind, I figured it out!
Based on your description, it seems like your hypothesis is plausible. Let's break down the problem and the steps you took to find the enthalpy of formation.
1. The enthalpy of formation (ΔHf) of MgO(s) was determined by the reaction: MgO(s) + 2H+(aq) -> Mg2+(aq) + H2O(l) with a given value of -334.83 kJ/mol.
2. You attempted to use Hess' Law to calculate the enthalpy of formation by working with two other known reactions.
- Mg(s) + 2H+(aq) -> Mg2+(aq) + H2(g) (given ΔHrxn = -1105 kJ/mol)
- H2(g) + 1/2O2(g) -> H2O(l) (given ΔHrxn = -286 kJ/mol)
3. By combining these two reactions, you obtained the equation:
Mg(s) + 1/2O2(g) -> MgO(s) with an estimated ΔHrxn of -1056.17 kJ/mol.
Now let's examine the potential source of error you identified:
You mentioned that instead of separating the contents and doing the addition of MgO(s) to diluted HCl (which should have been done in a separate container), additional HCl and Mg(s) were added to the solution created from the first part of the experiment. This could likely have caused the error because the presence of excess HCl and Mg(s) in the solution may have affected the overall reaction, leading to inaccurate measurements.
To confirm this hypothesis, you can try repeating the experiment by ensuring that the separate containers are used for each step and no additional substances are introduced beyond the prescribed reactants. By following proper experimental procedure, you can determine if this was the reason for the significant error in your previous attempt.
Remember, in chemistry, it is crucial to control variables and follow experimental protocols accurately to obtain reliable results.