c101-lecture-6

c101-lecture-6 - Chemical Thermodynamics is the study of...

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2/11/11 1 Chemical Thermodynamics Thermodynamics is the study of the energetics and order of a system. A system is the thing we want to study—it can be a chemical reaction, a solution, an automobile, or the universe. Everything else is included in the surroundings —the system may interact with the surroundings or be isolated from the surroundings. Chemical Thermodynamics The system is described by a set of variables that represent the state of the system—these are called state variable . Common state variables are temperature, pressure, and volume. The distinguishing feature of state variables is that when a change of state occurs, the path taken in the change does not effect the final result. Chemical Thermodynamics Example : raise the temperature of a piece of metal from 298 K to 1000 K. Path 1—first put metal in liquid nitrogen and cool to 77 K, then heat in a flame to 1000 K. Path 2—directly heat with flame from 298 K to 1000 K. The final result of 1000 K does not depend on which path you take—the metal is still at 1000 K. Changes of State A physical change of state does not alter the chemical identity of the system. Examples of physical changes of state include: freezing, melting, vaporization, expansion of a gas. A chemical change of state alters the chemical identity of the system. A chemical reaction is an example. State Functions A state function is a property that describes the system and is dependent on the state variables. When a change of state occurs, the change in value of a state function depends only on the initial and final locations of the system, not on the path taken. We will study the state function: energy, enthalpy, entropy, and free energy. State Functions Example : How much energy does it take for me to go from the PS lobby to the Chem office on the 6 th floor? Path 1: I first take the elevator to my lab on the 8 th floor to talk to my students, take the stairs to the 7 th floor to pick up a package from the stockroom, and then take the stairs to the Chem office.

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2/11/11 2 State Functions Path 1 E = mgh step #1: h = 29.3 m; E = 30.2 kJ step #2: h = -3.66 m; E = -3.8 kJ step #3: h = -3.66 m; E = -3.8 kJ total energy = 30.2 – 3.8 – 3.8 = 22.6 kJ State Functions Path 2: I stop on the 3 rd floor to attend a seminar for an hour and then take the elevator directly to the 6 th floor. Path 2 step #1: h = 11.0 m; E = 11.3 kJ step #2: h = 11.0 m; E = 11.3 kJ total energy = 11.3 + 11.3 = 22.6 kJ Total energy is independent of the path taken. Energy The energy of a system is perhaps the most important state function to study. Energy determines much of the behavior of the system—the desire to be at low energy drives physical and chemical changes. A system may contain energy in the form
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c101-lecture-6 - Chemical Thermodynamics is the study of...

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