1 E XPERIMENT A8: C ALORIMETRY Learning Outcomes Upon completion of this lab, the student will be able to: 1) Measure the heat of a reaction under constant pressure conditions. 2) Calculate the enthalpy change for a reaction using the enthalpy change of two other reactions and Hess’s law of heat summation. Introduction Enthalpy and Calorimetry Enthalpy is defined as the heat of the reaction measured under constant pressure conditions. Since most reactions conducted in a laboratory are under constant pressure (i.e. atmospheric pressure), a measurement of the heat of the reaction is also a measure of the enthalpy change for that reaction. Heat of a reaction can be measured using a calorimeter. A calorimeter requires two components: 1) an insulated container that minimizes loss of heat to the surroundings and 2) a thermometer to measure the temperature before and after a chemical reaction. A commonly used insulated container is a Styrofoam cup. Hess’s Law According to the additive properties of enthalpy change ( Δ H), Hess’s law of heat summation states that states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps. For instance, assume the enthalpy change for reactions 1 and 2 below are respectively, Δ H 1 and Δ H 2 . Reaction 1: A + B ! C + D Δ H 1 Reaction 2: C + E ! B + F Δ H 2 Then according to Hess’s law the enthalpy change for reaction 3, which is the sum of reactions 1 and 2, shown below: Reaction 3: A + E ! D + F Δ H 3 Δ H 3 is the sum of Δ H 1 and Δ H 2 Therefore: Δ H 3 = Δ H 1 + Δ H 2
2 Experimental Design In this experiment two sets of reactions will be performed. The first set of reactions involves sodium hydroxide and hydrochloric acid. The enthalpy change for the following three reactions: 1) solubilization of solid sodium hydroxide in water 2) reaction between solid sodium hydroxide and aqueous hydrochloric acid and 3) reaction between aqueous solutions of sodium hydroxide and hydrochloric acid will be measured using the calorimetry method. The molecular equations for these three reactions are shown below: 1. NaOH (s) ! NaOH (aq) Δ H 1 2. NaOH (s) + HCl (aq) ! NaCl (aq) + H 2 O (l) Δ H 2 3. NaOH (aq) + HCl (aq) ! NaCl (aq) + H 2 O (l) Δ H 3 Using the measured values of Δ H 1 and Δ H 2 , a calculated value of Δ H 3 can be obtained using Hess’s law. The calculated and measured values of Δ H 3 will then be compared with each other to confirm Hess’s law.
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