lab manual experiment 3

lab manual experiment 3 - EXPERIMENT 3 Thermochemistry...

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Experiment 3- Page 1 EXPERIMENT 3 Thermochemistry INTRODUCTION Heat exchange is one of the most common processes explored in the chemical laboratory. The examination of the relationships between heat energy and chemical changes is called thermochemistry. If heat energy is released by a reaction, the change is termed exothermic. If, on the other hand, heat energy is absorbed, the change is said to be endothermic. The magnitude of heat energy which flows as a reaction proceeds is measured using a device called a calorimeter and the measurement of this heat flow is called calorimetry. Specific heat, heat of fusion and heat of reaction are a few of the heat exchange processes you will be exploring in this experiment. The amount of heat energy ( q ) required to raise the temperature of a substance by T depends both upon the mass ( m ) and the identity (chemical composition) of the substance. The average heat capacity of a system is the amount of heat energy required, per degree of temperature rise, between any two given temperature points. The heat capacity per unit mass ( m ) of a substance is the specific heat capacity or specific heat ( c ), a physical property of the substance: c Tm q or q (m)(c)( T) If moles ( n ) are used in place of mass ( m ), the molar heat capacity ( C P ) is obtained: C P Tn q or q (n)(C P )( T) Cooling is the reverse of heating and either process can be used to obtain the heat capacity. The specific heat of water is calculated to be 4.182 J/K g and its molar heat capacity is 75.36 J/K mol at 20°C. Table I indicates that the specific heat of an unknown metal might be used to help in identifying it. The molar heat capacity, of course, cannot be calculated without knowing the molar mass. Note that T has the same value in the Celsius and Kelvin scale. As illustrated in Table I, the molar heat capacities of the metallic elements are about equal, with an average approximate value of 26 J/K mol. This discovery is known as the Law of Dulong and Petit, after the men who made this discovery in the early 1800s. They did not understand why this was so, but they used the observation to estimate the atomic masses of metals. You will also apply this law to estimate the atomic mass of an unknown metal from its measured specific heat. The law applies to monatomic substances in which the individual atoms are allowed to vibrate around their equilibrium positions in all three directions. Such conditions are met by the metallic elements in the solid state. Chemists can now predict that the molar heat capacities (under these circumstances) should be close to three times the value of the ideal gas constant ( R = 8.314 J/K mol). This is, indeed, the value observed by Dulong and Petit. TABLE I.
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This note was uploaded on 07/24/2010 for the course CHEM CHEM 6BL taught by Professor Berniolles during the Summer '08 term at UCSD.

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lab manual experiment 3 - EXPERIMENT 3 Thermochemistry...

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