CHEM 122 - Thermodynamics


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THREE LAWS OF THERMODYNAMICS The first of the three laws of thermodynamics states that energy can be converted from one form to another, but it cannot be created nor destroyed. The second law of thermodynamics explains why chemical processes tend to favor one direction. The third law of thermodynamics is an extension of the second law. SPONTANEOUS PROCESSES One of the main objectives in studying thermodynamics is to be able to predict whether or not a reaction will occur when reactants are brought together under a specific set of conditions. A reaction that does occur under a specific set of conditions is a spontaneous reaction . If a reaction does not occur under a certain set of conditions it is said to be non-spontaneous . A large number of exothermic reactions are spontaneous. These are reactions that release energy from the system into the surroundings. But the assumption that a spontaneous process is always exothermic (always decrease a system’s energy) is not completely correct. Exothermicity favors the spontaneity of a reaction but does not guarantee it. Just as it is possible for an endothermic reaction to be spontaneous, it is possible for an exothermic reaction to be non-spontaneous. We cannot decide whether a chemical reaction will occur spontaneously solely on the basis of energy changes in the system. To make this prediction, we must take entropy under consideration. ENTROPY Entropy is often described as a measure of how spread out or dispersed the energy of a system is among all the different possible ways that a system can contain energy. The greater the dispersal, the greater the entropy. Most processes are accompanied by a change in entropy. Microstates and Entropy Consider a simple system of four molecules distributed between two equal compartments.
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There is only one way to arrange all of the molecules into the left compartment; four ways to have three molecules in the left compartment, and one on the right; and six different ways to have two molecules in each of the compartments. The eleven possible ways of distributing the molecules are called microscopic states, or microstates. Each set of microstates is a distribution. (That is, one distribution is four on the left, another is three on the left, one on the right, and the third distribution is two in each compartment). The distribution in which there are two molecules in each compartment is in fact the most probable distribution because it has the most ways to possibly achieve it, while the distribution with four molecules in one compartment is the least probable because there is only one way to achieve it. Therefore, the probability of a particular distribution (or state) depends on the number of ways (microstates) in which the distribution can be achieved. As the number of molecules approaches a macroscopic scale, it is not difficult to see that they
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