Enthalpies of Solution

Enthalpies of Solution - Enthalpies of Solution1 Authors:...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Enthalpies of Solution 1 Authors: B. D. Lamp, V. M. Pultz and J. M. McCormick* Last Update: January 3, 2006 Introduction Thermochemistry investigates the relationship between chemical reactions and energy changes involving heat. It was born out of the practical problem of cannon making and today continues to play an important role in almost every facet of chemistry. Practical applications of thermochemistry include the development of alternative fuel sources, such as fuel cells, hybrid gas-electric cars or gasoline supplemented with ethanol. On a fundamental level, thermochemistry is also important because the forces holding molecules or ionic compounds together are related to the heat evolved or absorbed in a chemical reaction. Therefore, chemists are interested in the thermochemistry of every chemical reaction, whether it be the solubility of Pb in drinking water or the metabolism of glucose. The amount of heat generated or absorbed in a chemical reaction can be studied using a calorimeter. A simplified schematic of a calorimeter is shown in Fig. 1. The "system" (our chemical reaction) is placed in a well-insulated vessel surrounded by water (surroundings). A thermometer is used to measure the heat transferred to or from the system to the surroundings. Note that ideally, only the water would be the "surroundings" in the thermodynamic sense, and the vessel would not allow heat to pass. However in reality, the vessel does allow heat to pass from the water to the rest of the universe, and we will need to account for that ( vide infra ). Figure 1. Schematic representation of a calorimeter.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
There are two types of calorimeters: constant-pressure and constant-volume. 2,3 In the constant-volume calorimeter, the chemical reaction under study is allowed to take place in a heavy-walled vessel called a bomb. Because the reaction in the bomb takes place at constant volume, the heat that is generated by the reaction (mostly exothermic reactions are studied in a constant volume calorimeter) is actually the change in the internal energy ( U ) for the reaction. Although U is a useful quantity, for chemists the change enthalpy ( H ) is more relevant. However, we can convert U to H using Eqn. 1, if we know the change in the number of moles of gas ( n ) in the reaction and the temperature ( T ). H = U + n·R·T (1) In a constant-pressure calorimetry experiment, like the one that you will be performing, the energy released or absorbed is measured under atmospheric pressure (i. e., constant pressure). A constant-pressure calorimeter is simpler to assemble than a constant-volume calorimeter and a wider range of chemical reactions can be studied with it. Also, because the reaction is run at constant pressure, H is equal to the amount of heat a reaction generates or absorbs ( More Info ). The enthalpy is measured by measuring the temperature of the heat released or absorbed into the surroundings when the reactants are mixed. Constant-pressure calorimetry is normally conducted with liquids or solutions that have
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 13

Enthalpies of Solution - Enthalpies of Solution1 Authors:...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online