Heat of Reaction for the Combustion of Magnesium (Obj. 3)

EXPERIMENT 19B

Heat of Reaction for the Combustion of Magnesium (obj. 3)

When a reaction can be expressed as the algebraic sum of a sequence of two or more simpler reactions, the heat of reaction is the algebraic sum of the heats of these simpler reactions. This generalization has been found to be true for all REACTIONS that have been tested!!!!!

In this experiment you will use this generalization to determine the DH for a reaction that is difficult to measure directly using a Styrofoam calorimeter. Magnesium metal burns very rapidly, as you have observed in the demonstration leading into this lab, releasing much heat and light. The reaction is represented by the equation

Mg(s) + .5 O2(g) ----> MgO(s) (1)

Equation #1 can be obtained using Hess’s law to combine equations (2),(3), and(4).

MgO(s) + 2 HCl (aq) ----> MgCl2(aq) + H2O(l) (2)

Mg(s) + 2HCl(aq) ----> MgCl2(aq) + H2(g) (3)

H2(g) + .5 O2(g) ----> H2O(l) (4)

Combine these three equations to obtain equation #1 BEFORE doing the experiment. See item 1 under Processing the Data.

Heats of reaction for equations (2) and (3) can easily be determined. The DH for reaction (4) can be

obtained from a table of values for previously measured reactions, found on page 884 in your textbook.

PROCEDURE

For both of these reactions, measure the HCl solution into a Styrofoam calorimeter. Record the temperature of the solution and add either MgO or Mg to the solution. Stir the mixture and record the HIGHEST temperature reached during each reaction. Measure temperature readings to the nearest 0.1˚C and mass to the nearest .01 g.

REACTION (2)

Measure the change in temperature that occurs as about 1.00 g of

Magnesium oxide, MgO, is added to 100 +/- 1 ml of 1.00 M HCl.

REACTION (3)

Measure the change in temperature that occurs as about 0.50 g of magnesium metal, Mg, is reacted with 100 +/- 1 ml of 1.00 M HCl.

PROCESSING THE DATA

1. Combine equations (2),(3), and (4) to obtain equation (1).

2. Calculate the change in temperature, Dt, for each reaction, (2) and (3).

3. Calculate the kj of energy released for each reaction. Assume that 4.184 joules of energy is required to change the temperature of one ml of solution one degree Celsius.

4. Calculate the kj/mole of Mg and kj/mole of MgO.

5. Determine the DH for reaction #1. Report DH in kj/mole of MgO.

6. Determine the % error using:

accepted value - experimental value x 100 = % error

accepted value