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Hess's Law

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Title: Experimental Applications of Hess’s Law
Objectives: 1) To study and understand the experimental method of Hess’s law. 2) To interpret experimental data by applying Hess’s law. 3) To determine the H of ammonium chloride solid to ammonia gas and hydrochloric acid gas by using Hess’s law.

Introduction: A reaction that release heat to surrounding is known as exothermic reaction. It is usually observed as temperature increases in solvent, container and other immediate surroundings. The expression of heat released from the reaction is: qreaction = qsolution + qcalorimeter ……..(1) The qcalorimeter is nearly zero because the calorimeter in this experiment is assumed to absorb little heat. So, the equation is simplified to qreaction = qsolution ……...(2) The change in temperature that caused by the addition of a given amount of heat will depend on specific heat, Csp of substance. Therefore, the heat involved is calculated by the equation Q = (Csp)(m)(T) ………(3)
Mass of substance is m, change is temperature is T (final temperature – initial temperature). The heat changes observed is equal to enthalpies of reaction, H for the reactions considered because this experiment was run at constant pressure. The enthalpy of these directly observed reactions and Hess’s law was used to calculate the enthalpy change for an additional reaction which is not directly observed. Hess’s law stated that the overall enthalpy change of a chemical reaction is independent of its pathway. The magnitude of enthalpy change of a chemical reaction depends only on the difference in the enthalpy content of the products and the reactants, and does not depend on how the reaction is completed.
Consider the below thermochemical cycle:

By Hess’s law, ΔH1 + ΔH2 = ΔH3 + ΔH4 In this experiment, the enthalpy for the

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