Lab 12 - Measuring Enthalpy Changes

Lab 12 - Measuring Enthalpy Changes - Lab 12 M easur ing...

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4/15/13 Lab 12 - Measuring Enthalpy Changes www.webassign.net/ebooks/wsugencheml1/lab_12/manual.html 1/7 Contents > Lab 12 - Measuring Enthalpy Changes Lab 12 - Measuring Enthalpy Changes Purpose To observe changes in enthalpy in chemical processes. Goals To identify exothermic and endothermic processes. To relate enthalpy changes and entropy changes to changes in free energy. Introduction Chemical processes occur spontaneously when they lower the free energy of the system. The free energy at constant temperature and pressure is the Gibbs free energy, Δ G , which is defined as follows: ( 1 ) Δ G = Δ H T Δ S where Δ G is the change in free energy of the system, Δ H enthalpy of the system and Δ S is the change in entropy of the system. Δ G , the change in free energy , must be less than zero for a spontaneous process. This follows from the definition of a spontaneous process; it lowers the energy of the system. Free energy, given the symbol G in honor of the American chemist, J. Willard Gibbs, is the energy available from a spontaneous process to do work. For a nonspontaneous process, it is the amount of energy that must be supplied to cause the process to occur. Δ H , the change, is the amount of heat energy absorbed by a process that occurs at constant pressure (− Δ H is the heat given off). Δ H is often proportional to Δ T , the temperature change caused by the process. The proportionality constant between Δ H and Δ T is called the specific heat (s). The specific heat is a property of the substance being heated or cooled. For example, the specific heat of water is 4.18 J/(g · °C) meaning that 4.18 joules of heat energy are required to raise the temperature of 1 gram of water by 1.0°C. Not all processes that involve a change in enthalpy are accompanied by a change in temperature. Phase changes, e.g. melting, boiling, and sublimation all absorb heat. However, the substance undergoing the phase change maintains a constant temperature. All the energy supplied to the substance is consumed in the phase change. Processes that give off heat energy ( Δ H < 0) are exothermic . Combustion is obviously an exothermic chemical reaction. Condensation of steam to liquid water is also exothermic, although water maintains a constant temperature during the process. Processes that absorb heat energy ( Δ H > 0) are endothermic . Sweat, liquid water that evaporates
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4/15/13 Lab 12 - Measuring Enthalpy Changes www.webassign.net/ebooks/wsugencheml1/lab_12/manual.html 2/7 from your skin, cools you by absorbing heat from your body. Δ S , the change in entropy , is related to the number of ways the energy of the system can be distributed. Entropy is commonly defined as disorder or randomness. Entropy is high in gases, because the molecules are free to move about in all directions, fill the space available, and adopt any orientation relative to each other. Entropy is low in solids, because the molecules cannot move much relative to each other, and their orientation is fixed. With this information, one can see that
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Lab 12 - Measuring Enthalpy Changes - Lab 12 M easur ing...

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