Author: Dr. Anne M. Kelley Last Revised on 8/14/18 by MMR Revised by Dr. Jess C. Vickery 10 – 1 Experiment#10 – Thermochemistry II: Reaction Enthalpy and Hess’s Law OBJECTIVES In successfully completing this lab you will: investigate the quantity of energy transferred by heat ( q p ) for chemical reactions; quantitatively determine the enthalpy change for several chemical reactions; and evaluate the validity of Hess’s Law of constant heat summation. INTRODUCTION Measurements of the energy transferred through heat are typically performed by carrying out chemical reactions in a constant-pressure calorimeter. A calorimeter is simply a device that thermally insulates the reacting system and its immediate surroundings from the outside world. In this experiment, the reactions are carried out in aqueous solution and the calorimeter consists of two nestled Styrofoam cups with an accompanying lid. The energy released or absorbed through heat by a reaction results in a change in the temperature of the surrounding solution, which is measured with a thermometer. The energy transferred by heat for an aqueous solution is: (1) where q is the quantity of energy transferred by heat (in Joules), m is the mass of the solution (in grams), C s is the constant-pressure specific heat capacity of the solution (in J/g°C), and Δ T is the temperature change (in °C). Therefore, by measuring the temperature change of a given mass of the solution with a known specific heat capacity, the amount of energy produced or consumed by the reaction through heat can be determined. Moreover, if the number of moles of the limiting reactant is known, the reaction enthalpy, Δ H rxn , in kJ/mol can also be determined. In this experiment, the enthalpies for the following reactions will be measured: 1. Neutralization of 1.0 M HCl (aq) by 1.0 M NaOH (aq). 2. Dissolution of solid NaOH in water to yield a 1.0 M NaOH (aq) solution. 3. Reaction of solid NaOH with 1.0 M HCl (aq). Comparison of the Δ H rxn values for reactions 1, 2, and 3 will allow you to demonstrate Hess’s Law of constant heat summation. Considering the balanced equations for these reactions, it is clear that the sum of reactions 1 and 2 are identical to reaction 3:
Experiment#10 – Enthalpy & Hess’s Law 10 – 2 Equipment and Supplies: two thermometers with 0.1 °C divisions; two calorimeter assemblies (labeled A & B) with lids; hotplate; plastic pipets. Chemicals: 1.0 M hydrochloric acid; 1.0 M sodium hydroxide; solid sodium hydroxide. SAFETY PRECAUTIONS Always wear safety goggles: most reagents in the lab are very corrosive, especially to your eyes. If any reagent gets in your eyes, flush them with water at the eyewash station for at least 15 minutes. Wear gloves and rinse-off any residual solutions: sodium hydroxide (NaOH) and hydrochloric acid (HCl) can cause chemical burns if left in contact with skin.
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