The heat of a reaction is generally determined by

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each reactant equals the number of moles in the balanced equation. The heat of a reaction is generally determined by measuring the temperature change of the reaction solution. Calculating the heat of reaction requires knowledge of the specific heat, C , of the solvent and the mass, w , of the solvent. A reaction that generates 10 kJ of heat will raise the temperature of 100 g of solvent more than that same amount of energy will raise the temperature of 200 g of solvent. Initially, the heat generated during a reaction raises the temperature, T, of the reaction solution. However, heat may also be transferred to the outside, through the walls of the reaction flask, changing the temperature of the surrounding air. If this heat transfer is rapid, we cannot accurately measure the heat of reaction. Therefore, special precautions are made to minimize undesirable heat transfer. The most direct method of measuring the enthalpy change of a reaction is by “ calorimetry ”. In calorimetry, the amount of heat associated with a reaction is determined by measuring the change in temperature, D T , of a reaction mixture (solution) in an insulating standard container. A calorimeter makes this measurement possible by thermally isolating the reaction mixture and container from the surroundings. Ideally, the vessel containing the reaction mixture is completely isolated from the surroundings, such that all of the reaction heat is contained within the reaction solution and vessel. In this experiment, a Thermos ® Food Jar is used to isolate the reaction from the surroundings. Even if the reaction mixture is completely isolated from its surroundings, the solution is still in contact with the inside walls of the reaction flask. The inner walls absorb (or
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Chemistry 132 Lab Manual Page 19 release) heat depending on the temperature difference between the solution and the walls. Heating (or cooling) of the walls must be considered in the calculations. The raw data should be corrected for this effect using a "calorimeter" constant, which is equivalent to a heat capacity and accounts for the heat energy absorbed by the reaction vessel walls per degree of temperature increase. This calorimeter constant has to be determined separately. Reactions In the first measurement, the heat of reaction is measured for the neutralization of two weak acids by the strong base NaOH . The two acids to be studied are acetic acid and chloroacetic acid. The reactions can be written as CH COOH NaOH CH COO Na H O - + + ® + + 3 3 2 (2.4) ClCH COOH NaOH ClCH COO Na H O - + + ® + + 2 2 2 (2.5) A shorthand notation may be used for acetic acid to give the equation - + + ® + + HAc NaOH Ac Na H O 2 (2.6) Acetic acid is a weak acid and only partially dissociates before the neutralization reaction, whereas the strong base is essentially completely dissociated. The heat that is measured, when base is added to the acidic solution, is a combination of the heat of dissociation of the acid and the heat of neutralization. The heat of neutralization for the reaction of H 3 O + and OH is –56.9 kJ/mol as shown in Equation 2.7: + - + ® D = - H O (aq) OH (aq) H O( ) H . kJ / mol 0 3 2 2 56 9 l (2.7) This reaction can be used along with the heat from reaction (2.6), and
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