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Hess Law

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EXPERIMENT 3
Title : EXPERIMENTAL APPLICATIONS OF HESS’S LAW

Objective : To determine the heat of reaction by applying Hess’s Law for reactions whose energy cannot be easily measured directly.
Introduction
An exothermic reaction releases heat into the surroundings and is usually observed as temperature increases in the solvent, container, and other immediate surroundings. The amount of heat released from the reaction can be expressed as q reaction = q solution + q calorimeter ……..(1)
In this experiment, the calorimeter is assumed to absorb so little heat that q calorimeter is nearly zero and thus the equation simplifies to q reaction = q solution ……...(2)
The temperature change caused by the addition of a given amount of heat will depend on the specific heat, Csp of the substance. Thus, the heat involved can be calculated by the equation Q = (Csp)(m)(T) ………(3)
The mass of the substance is m, and the change in temperature is T (final temperature – initial temperature)
Since this experiment is run at constant pressure, thus the heat changes observed will be equal to the enthalpies of reaction, H for the reactions considered. The enthalpies of these directly observed reactions and Hess’s law will be used to calculate the enthalpy change for an additional reaction which is not directly observed. Recall that since enthalpy is a state function, the enthalpy change for any process will depend only on the initial and final states, not on the reaction path followed. The enthalpy, H(A-B), for the reaction A  B can be calculated by adding up the enthalpy changes for a series of intermediate steps. A  C  D  B
Thus, H(A-B) = H(A-C) + H(C-D) + H(D-B). This concept is especially useful in calculating the enthalpy change for reactions which are not easy to observe or

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