There are many reactions for which the heat of reaction cannot be easily measured experimentally. Some of these are dangerous to perform in the lab. Others generate so much heat that simple calorimeters cannot be used. In such cases, it is practical to examine a series of reactions whose net effect is the desired reaction, but whose heats of reaction are more easily measured. Once that has been done, the required heat of reaction can be calculated using Hess's Law.
In this lab, you will conduct two such investigations. In the first one we will examine the combustion of magnesium. The burning of magnesium can be represented by the equation:
Mg(s) + ½O2(g) ----> MgO(s)
This equation can be obtained by manipulating the three reactions shown below:
(1) MgO(s) + 2 HCl(aq) ----> MgCl2(aq) + H2O
(2) Mg(s) + 2 HCl(aq) -----> MgCl2(aq) + H2(g)
(3) H2(g) + ½O2(g) ---------> H2O(l)
In the second investigation, we will examine the heat of formation of solid calcium hydroxide. The equation for the reaction is
Ca(s) + O2(g) + H2(g) ----> Ca(OH)2(s)
and it can be obtained by combining the following two reactions:
(4) Ca(s) + 2 H2O(l) -----> Ca(OH)2(s) + H2(g)
(5) H2(g) + ½O2(g) -------> H2O(l)
Apparatus and Materials:
1. Read through the procedure carefully and review the data table prepared at the end of this lab.
2. Put 100 mL of 1.0 M Hydrochloric acid solution into the styrofoam cup. Record the temperature of the solution.
3. Accurately find and record the mass of about 1 g of solid magnesium oxide.
4. Add the solid magnesium oxide to the acid solution in the cup. Stir the mixture and record the highest temperature possible.
5. Discard the solution and rinse the cup thoroughly with water.
6. Put 100 mL of 1.0 M HCl solution into the styrofoam cup. Record the temperature of the solution.
7. Accurately find and record the mass of about 1.0 grams of magnesium ribbon.
8. Add the magnesium ribbon to the acid solution. Stir the mixture and record the highest temperature reached.
Part Two: The Heat of Formation of Solid Calcium
10. Put 100 mL of distilled water into the styrofoam cup.
11. Put the lid on the cup and insert the thermometer through the hole you created. Record the temperature of the water.
12. Record the mass of about 1.0 g of calcium metal. CAUTION: Do not allow any of the calcium metal pieces to become wet.
13. Add the calcium to the water through the pre-punched hole.
14. Stir the mixture and record the highest temperature reached.
2. Show how equations (1),(2) and (3) in the introduction can be combined to give the equation for the combustion of magnesium.
3. Use your experimental values for the two reactions in the experiment, along with a value for the heat of reaction for equation (3) found in a chemistry handbook to calculate the heat of combustion for magnesium.
4. Compare your calculated value with that found in tables for the standard heat of formation of magnesium oxide. Find the experimental error.
6. Use your experimental results to calculate the molar heat of reaction for the reaction of calcium with water in kJmole-1 of calcium used. Using this value, and the value of the heat of reaction for the equation
H2 + ½O2 ----> H2O(l)
calculate the molar heat of formation of calcium hydroxide.
7. Compare your calculated value with that listed in a chemistry handbook and calculate your experimental error.
Experimental Applications of Hess's Law
Error Analysis: My experimental value is different from the
accepted value because......