Calorimetry: Hess’s Law Objectives : To practice obtaining measurements of calorimetry and to employ the data to demonstrate Hess’s Law of combining reaction enthalpies. Materials: 1 M HCl, 1 M NaOH, solid NaOH, Styrofoam cups to use as calorimeters, thermometers, small (25-mL or 50-mL) Erlenmeyer flask or weighing bottle, stopper. Safety: One molar solutions of hydrochloric acid and sodium hydroxide are corrosive. If you spill any on your skin, wash it off immediately. Sodium hydroxide pellets should not be touched or handled. Weigh them in glass containers and keep from air. The thermometers used in this experiment are fragile and expensive and should be handled with care. Safety glasses are required at all times. Waste Disposal: all the solutions may be washed down the drain with plenty of water. INTRODUCTION Calorimetry is the study of heat flow. It is not simply temperature, but the total change in temperature for a specific mass of a specific material. That is q = S.H. × m × T where q is the heat flow (also sometimes written H), S.H. stands for specific heat , m is mass , and T = T final - T initial . If the temperature increases, T will be positive and, therefore, q calorimeter will be positive. However, the q of the calorimeter is equal in magnitude but opposite in sign to the q of the reaction. q reaction = -q calorimeter When we place a thermometer in the water, we are not really measuring the heat that came out of the reaction, but the heat that went into the calorimeter. For practical purposes, remember this: whenever heat flows out of a reaction (it gets hotter), the reaction is exothermic (out-heat) and the sign of q is negative . Conversely, whenever heat flows into a reaction (it gets colder), the reaction is said to be endothermic (in-heat) and the sign of q is positive . All of the reactions in this experiment are exothermic. A calorimeter is any device that is used to measure heat flow. Its main characteristic is that it must be well insulated so that no heat is lost to the surroundings. A simple calorimeter, which will be employed in this experiment, is the “coffee cup” calorimeter. It uses a styrofoam cup (sometimes two cups stacked together) with a styrofoam or cardboard cover. It works well enough to allow reasonable results. More elaborate types of equipment include devices modeled on the familiar Thermos bottle. One example is the bomb calorimeter. A “bomb” calorimeter is the best sealed and most accurate type of equipment for these measurements, but they are relatively expensive and somewhat tedious to operate. (Despite its name, a
“bomb” calorimeter does not explode.) Using calorimetry it is possible to test the validity of Hess’s Law
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