Exp 14 Essay

1208 WordsFeb 17, 20125 Pages
Exp. 14: Calorimetry and Hess's Law Purpose: This experiment uses calorimetry to determine enthalpy changes for two reactions that can conveniently be performed in the laboratory. Then it makes use of Hess's Law to calculate the enthalpy change for a reaction which is not directly observed. Introduction: An exothermic reaction releases heat, and that heat flows into the surroundings and is usually observed as a temperature increase in the solvent. container, and other immediate surroundings. An insulated container called a bomb calorimeter can be used to limit the heat flow with good accuracy to the solvent and bomb. Thus -qreaction = qsolution + qbomb -- Eqn. 1 where "q" is the symbol for heat flow which will be measured in Joules. The negative sign on the left of the equation says that heat is flowing out of the reaction; so the heat flow out of the reaction is equal to the heat flow into the solution and the calorimeter. It is very important to remember that whenever heat is flowing out of a substance or reaction the value of "q" will be negative. In the case of our experiment, the calorimeter absorbs so little heat that qbomb is nearly zero and thus the equation simplifies to -qreaction = qsolution -- Eqn. 2 The temperature change caused by the addition of a given amount of heat will depend on the specific heat, Csp, of the substance. The specific heat of water (and a good approximation for dilute aqueous solutions) is 4.184 J/g oC. Thus, the heat involved can be calculated by the equation q = (Csp)(m)(T) -- Eqn. 3 The mass of the substance is m, and the change in temperature is T. Change in temperature is always calculated as (final temperature) - (initial temperature). Our experiment is run at constant pressure, thus the heat changes observed will be equal to the enthalpies of reaction, (Hrxn for the reactions considered. We will obtain

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