exp_1_thermochemistry - January 2008 Experiment#1...

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January 2008 Experiment #1 Thermochemistry: The Heat of Reaction CHEMICAL THERMODYNAMICS deals with the energy transfers which accompany chemical reactions. THERMOCHEMISTRY concerns energy transfers manifested as the heat of reaction, H rxn . A reaction in which heat is lost by the reactants to the surroundings is said to be exothermic and H is negative; one in which heat is absorbed is endothermic and H is positive. Energy changes may be manifested in forms other than heat, for example, as electrical energy. Work energy done against an external force such as atmospheric pressure, also may have to be considered. The general term, heat of reaction, H rxn , may be partially classified into a number of specific categories. A few examples are: a) the heat of formation, which is the amount of heat involved in the formation of 1 mole of a substance directly from its constituent elements in their standard states; b) the heat of combustion, which is the amount of heat evolved per mole of a combustible substance, such as methane, when it undergoes a reaction with excess oxygen; c) the heats of solution, vaporization, fusion or sublimation, where the first is concerned with the heat transferred when a substance dissolves in a solvent and the last three deal with changes in state; d) the heat of neutralization, which describes the heat transferred during an acid/base reaction. In this experiment , we shall measure the heat of neutralization of a HCl solution with a NaOH solution, the heat of solution of NaOH(s), and the heat of reaction of NaOH(s) with the HCl solution. The data should permit you to discover the generalization known as Hess’s law . That is, “The total enthalpy change for a reaction is the same whether the reaction occurs in one or several steps”. You will measure the amount of heat, q, produced by the three different reactions . Heat measurements are performed by carrying out each reaction in a metal calorimeter. The calorimeter consists of a metal casing inside of which a beaker is suspended to isolate it from the surroundings. The calorimeter is covered with a special lid, through which is bored a hole for inserting a thermometer and a second hole for inserting a stirrer. Because of this design heat is lost to the surroundings only slowly when temperature changes are measured. By using a process of graphical extrapolation to determine temperature changes, we can minimize even small errors caused by such losses. Correction must also be made for the heat absorbed by the calorimeter (see Experimental Procedure, part 1). The system is comprised of the calorimeter, and the solution of reactants and products; surroundings are the remainder of the universe. We assume that any heat generated by one part of the system is absorbed by other parts of the system. Suppose a reaction being carried out in the colorimeter is exothermic; that is, it is a reaction that gives off heat. If no heat is lost to the surroundings, all the heat generated by the reaction must be absorbed by either the solution or the calorimeter.
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