Hess's Law

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Experiment 2 : Experimental Applications of Hess’s law Objectives: 1. To find the enthalpy change of solid sodium chloride by using Hess’s law. 2. To apply q = m x c x ∆T in the calculations of enthalpy change for a reaction. Introduction: Calorimeter is a device which is used to determine the heat release or absorb in a chemical reaction. An exothermic reaction is one in which heat is given out into the surroundings. It results in a temperature increase in the solvent, container, and other immediate surroundings. The amount of heat evolved from the reaction can be expressed as qreaction = qsolution + qcalorimeter However, in this experiment, one assumption to be made as the calorimeter only absorb very little of heat, nearly zero. Thus, the equation simplifies to qreaction = qsolution Therefore, the temperature change caused by the addition of a given amount of heat will depend on the specific heat capacity, c, of the substance. The heat flow can be calculated by using this equation: q = m x c x ∆T * q = heat * m : mass of the substance, g * c = specific heat capacity * ∆T : change in temperature (final temperature – initial temperature) Hess’s law states that the enthalpy change for a reaction is independent of the pathway or number of steps required to complete the process (provided the initial and final states are the same). By Hess’s law, enthalpy change for route 1 = enthalpy change for route 2 ∆H1 = ∆H2 + ∆H3 This concept is especially useful in calculating the enthalpy change for reactions which are not easy to observe or measure directly. Apparatus and materials : * Dewar flask * Thermometer * Beaker * Spatula * 2.0M NH3 (aq) * 2.0M HCl (aq) * Solid NH4Cl Procedures : Experimental results : Part 1 |
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