The change in enthalpy for the combustion of magnesium metal
Hess’s law of heat summation states that the value of DH for a reaction is the same whether it occurs directly or as a series of steps. This principle was used to determine the change in enthalpy for a highly exothermic reaction, the combustion of magnesium metal.
Enthalpy changes for the reactions of Mg in HCl (aq) and MgO (s) in
HCl (aq) were determined experimentally, then added to that for the combustion of hydrogen gas to arrive at a value of
–587 kJ/mol Mg. Compared with the accepted value of –601.8 kJ/mol Mg, our experimental error was 2.46%.
In this investigation the change in enthalpy will be determined from the following equation: …show more content…
- Lab apron
- Magnesium oxide MgO - 1g.
- Lab balance 100mL graduated cylinder
- Hydrochloric acid HCl - 200mL/.5M
- 2 plastic foam cups
- cover for cup
- Magnesium (Mg) ribbon
- 600mL beaker (base for calorimeter)
- 400mL beaker (transportation of acid)
We began this investigation by suiting up in lab aprons and goggles, we then gathered our materials, found a lab station and got to work.
We decided to start with the magnesium in hydrochloric acid first, we measured out 198.5 L of HCl and put it in the foam-cup calorimeter and took and initial temperature reading. We then selected a piece of magnesium ribbon and found its mass: 0.01g. This piece was placed in the calorimeter and the lid was shut immediately to prevent heat from escaping. We “swirled” the liquid mixture in the calorimeter to ensure reaction, and waited for a temperature change. After a few moments, the final temperature was recorded and DT determined. This process was then repeated. On the second trial, the Mg ribbon did not completely dissolve and the results were thrown out. The third trial
(referred to as the second in the following analysis due to the exclusion of the previous one) was successful, and