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Laboratory Techniques I, CHY 114
Chemistry Department, University of Southern Maine

Thermochemistry: Measuring Enthalpy Change in Chemical Reactions

Experiment created by the UMaine InterChemNet© Team. Adapted with permission.

Assignment

Use your classroom text to read about these subjects:

• Heat capacity and specific heat
• Calorimetry
• Enthalpy change of a reaction
• Heat of solution

(These topics are covered in sections 6.4, 6.5, and 6.7 of Chang, Chemistry, 9th edition.)

Use the index in your Laboratory Handbook to find out about care and use of thermometers

Goals

1. You will learn how to use a simple calorimeter to measure temperature changes associated with chemical processes.
2. Then you will use Excel graphing to determine accurately the endpoints of temperature changes.
3. Finally, you will learn how to use your temperature data to compute enthalpy changes for chemical processes.

Overview

The study of energy changes associated with chemical processes is called thermochemistry. and it is a vital area of chemistry. Over 90 percent of the energy used by civilization comes from chemical reactions such as the combustion of fossil fuels. As chemical bonds break and form in a chemical reaction, energy in the form of heat is either absorbed (bond breakage) or released (bond formation). Under conditions of constant pressure, the energy change of a reaction is called the heat of reaction or enthalpy change, H. The amount of heat either absorbed or released during a reaction can be measured if the reaction is carried out in a container that insulates the reaction from its surroundings. By convention, if heat is released, the reaction is termed exothermic and H is negative. If heat is absorbed, the reaction is termed endothermic and H is positive.

In this experiment, you will find the heat capacity of a calorimeter, and then use the calorimeter to determine one or more of the following:

• the heat of neutralization for an acid-base reaction;
• the heat of solution for a salt dissolving in water;
• the heat of reaction for an oxidation–reduction reaction.

Preparing for Lab

The following problems require calculations similar to those called for in the report on this experiment. Learn how to work these problems, showing your calculations with units. For calculations, answers are provided. Similar questions may appear on your prelaboratory quiz. For more guidance, look at the Report Form for this experiment.

1. The specific heat of water is 4.184 J/g-°C. During a chemical reaction between dilute reactants in 100.0 g of water in a calorimeter, the temperature rises from 22.5 °C to 31.6°C. Calculate the heat produced by the reaction. Assume that chemical reactants do not alter any properties of the water except its temperature. ANSWER: 3.8 x 10³ J.
2. If the final concentration of product in question 1 is 0.050 M, calculate H_reaction, in kilojoules per mole of product. ANSWER: 761 kJ/mol.
3. Write full, balanced chemical equations for each of the reactions you might study in this experiment. Reactants in each reaction are given at the beginning of Parts 2, 3, and 4 of the Report Form.

Procedures

Download and print the Procedure for this experiment. Bring it with you to lab

We recommend that you translate all procedures into an outline, with numbered steps to follow in lab. At the prelaboratory session, ask your lab instructor for further explanation of sections you cannot translate, or terms that you cannot find in your text or do not understand.

Report

Download and print the Report Form for this experiment. Bring it with you to lab. You will write data and observations on the form during lab, as you carry out the Procedure. After lab, you will complete the Report Form by carrying out calculations to give your final lab results. These calculations include graphing your data with Excel. If you need to use USM computers for this purpose, be sure to allow time to visit the USM computer lab for this work.

CHY114 Lab Manual