Calorimetry and Hess's Law

Topics: Enthalpy, Energy, Thermodynamics / Pages: 6 (1778 words) / Published: Jun 8th, 2014
CALORIMETRY AND HESS’S LAW:
FINDING ΔH FOR THE COMBUSTION OF MAGNESIUM

MATERIALS: 12 oz. Styrofoam cup with lid, thermometer, 50-mL and 100-mL graduated cylinders, weighing boat,
1.0 M HCl, 1.0 M NaOH, magnesium ribbon, magnesium oxide, copper wire.
PURPOSE: The purpose of this experiment is to determine the enthalpy change for the combustion of magnesium:

Mg (s) + ½ O2 (g) → MgO (s) ΔHrxn = ΔH combustion

by determining the ΔH values for reactions which can be combined together according to Hess’ Law, yielding the ΔH for the desired reaction.
LEARNING OBJECTIVES: By the end of this experiment, the student should be able to demonstrate the following proficiencies:
1. Construct and use a calorimeter.
2. Determine the heat capacity of a calorimeter using a reaction with known ΔH.
3. Calculate the enthalpy change of a reaction from calorimetry data.
4. Apply Hess’s Law to calculate the enthalpy change associated with a reaction.
5. Use a spreadsheet program for data manipulation, graphing, and regression analysis.
DISCUSSION:
Elemental magnesium is one of the principal components of flares used to illuminate nighttime activities, or to aid in signaling one’s location to aircraft and ships. Your instructor may ignite a strip of magnesium ribbon to demonstrate the combustion of magnesium in air. It will be evident that a great deal of light energy is released from this reaction. A direct method for measuring the heat produced by this reaction would be difficult, so we shall resort to an indirect method in this experiment as discussed below.
Some chemical reactions (including the one above) are associated with the evolution of thermal energy and are called exothermic reactions. When there is absorption of energy in a chemical reaction, the process is called endothermic. The magnitude of the energy change is determined by the particular reaction as well as the amount of product(s) formed. The thermal energy transferred in a balanced chemical reaction

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