# Chemistry thermo lab, Hess's Law.

by qusai77 (student)

Hess’s Law Lab

Qusai Al Omari

Introduction:

In this lab, we will be determining the change in enthalpy for the combustion reaction of magnesium (Mg) using Hess’s law.

Procedure:

1. React about 100 mL of 1.00 M hydrochloric acid with 0.80 g of MgO. Note the change in temperature and any qualitative data.
2. React about 100 mL of 1.00 M hydrochloric acid with 0.50 g of Mg. Note the change in temperature and any qualitative data.

Raw Data:

Quantitative:

Qualitative:

• Hydrochloric acid is colorless and odorless
• Magnesium tape is shiny after cleaning it from oxidants, increasing its purity.
• In both reactions, the solution became bubbly.
• There was a strong odor from the reaction.

Data Processing:

Trial 1:

Reaction 1:

First, we have to calculate the ΔT by subtracting the final temperature by initial temperature:

Now we calculate the mass of the solution, assuming it has the density as water:

Now, we can use q=mc ΔT to calculate the energy gained by the solution:

Therefore:

Now, we have to calculate the number of moles for MgO:

We can now calculate the change in enthalpy by dividing the q of the reaction by the moles of the limiting reagent:

Now, we do reaction 2, trial 1 so we can use Hess’s law to calculate the change in enthalpy of formation, but first we are going to calculate the uncertainty in this expression:

First, we calculate the uncertainty for the

:

Now for mass:

As for the energy gained:

Now for the energy of the reaction:

•                 It is multiplied by an integer (-1) so it is the same unc.

As for the moles:

Finally, the change in enthalpy:

Reaction 2:

First, we have to calculate the ΔT by subtracting the final temperature by initial temperature:

Now we calculate the mass of ...