Theory: When a reaction can be expressed as the algebraic sum of a sequence of two or more other reactions, the heat of the reaction is the algebraic sum of the heat of these other reactions (Hess' Law).
You have seen the reaction of magnesium burning in air. The magnesium metal burns rapidly releasing both heat and light. The balanced equation for the production of one mole of MgO from magnesium and oxygen in their standard states is the reaction for the heat of formation of magnesium oxide.
Since this reaction is very rapid the heat released is difficult to measure. However by using Hess' Law we can calculate the heat of formation indirectly by dealing with the three reactions below:
1) MgO(s) + 2 HCl(aq) ---> MgCl2(aq) + H2O(l)
2) Mg(s) + 2 HCl(aq) ---> MgCl2(aq) + H2(g)
3) H2(g) + 1/2 O2(g) ---> H2O(l)
Heats of reaction for reactions 1 and 2 are easily obtained
experimentally and the heat for reaction 3 is the heat of formation
of water which can be looked up on a table.
PROCEDURE:
Reaction #1: Measure 100 ml of a 3.0 M HCl solution into a styrofoam cup. Record the temperature of the liquid to a tenth of a degree. Weigh out about 1 g of MgO(s) on a piece of paper to at least a centigram. Add the magnesium oxide to the HCl(aq), stir with your thermometer, and record the highest temperature reached during the reaction. Wash out the cup.
Reaction #2: Measure 100 ml of 3.0 M HCl solution into the same styrofoam cup. Again record the temperature. This time weigh about 0.5 g (about 0.5 meters) of magnesium ribbon onto a piece of paper. Add the magnesium to the HCl(aq), stir with your thermometer, and record the highest temperature reached during the reaction. Wash out the cup.
CALCULATIONS: For this experiment assume that the density of all solutions is 1.00 g/ml and assume the specific heat of the resulting solutions is 4.18 joule/gøC.
1. Which reactant is in excess in reaction number 1? You must show your work to get credit.
2. Which reactant is in excess in reaction number 2? You must show your work to get credit.
3. Calculate the heat, in joules, absorbed by the water solutions in both reactions 1 and 2.
4. Calculate the energy, in kilojoules, released per mole of MgO reacted in reaction #1.
5. Calculate the energy, in kilojoules, released per mole of Mg reacted in reaction #2.
6. Combine equations 1, 2, and 3 to obtain the goal reaction.
7. Use the answers to 4 and 5 in addition to the heat of formation of water from the table in your text to calculate the heat of formation of MgO. Compare your result for the heat of formation of MgO to that on the table in the textbook. Offer possible explanations for any deviation.