Lab Discussion Formation of Magnesium Oxide

In this lab report, the molar enthalpy of formation for magnesium oxide was set out to be determined. However, this is difficult to measure directly through experiment. Instead, Hess’s Law can be used to determine the value of the molar enthalpy of formation of MgO. This was done by measuring the enthalpy changes in the following three reactions:

1. Mg(s) + 2 H+(aq) → Mg2+(aq) + H2(g) ∆H1

2. Mg2+(aq) + H2O(l) → MgO(s) + 2 H+(aq) ∆H2

3. H2(g) + ½02(g) → H2O(l) ∆H3

When these reactions are rearranged the summation of them gives the equation for the formation of magnesium oxide:

Mg(s) + ½O2(g) → MgO(S) Where ∆Hf° (MgO,s) = ∆H1 + ∆H2 + ∆H3

In this lab, reaction 1 was measured directly using classic techniques. Around 50g of Mg was mixed with about 50mL of 1.00M HCl in a calorimeter where the temperature was recorded repeatedly for a length of time to get the change of temperature. The temperature change was graphed over time and a ‘best fit’ line was estimated. Using the information about the amounts of substances used in the reaction and the change in temperature, ∆H1 was found mathematically.

Reaction 2 was found by preforming the reverse of the reaction. This is because Mg2+(aq) + H2O(l) → MgO(s) + 2 H+(aq) cannot be measured directly, therefore the reverse of that reaction was performed and then the sign of ∆H2 was changed accordingly so that it could be used in conjunction with the other reactions to find ∆Hf° (MgO,s). This reaction was measured using a temperature probe, where about 50mL of 1,0M HCl was mixed with about 0.80g of magnesium oxide in a calorimeter. The temperature probe was placed in the solution and measured the temperature change and graphed the results. Using the information about the amounts of substances used and a ‘best fit’ line on the graph, ∆Hf° (MgO,s)

The method used in part A to determine Reaction 1 has some advantages over the method used in part B. Doing things manually means that there is no way a computer or...