FLASHBACKWhat is internal energy anyway ? The sum of all the microscopic forms of energy, the kinetic and potential energies of the molecules of a system. It does not include the translation or rotation of the system as a whole.  Cannot measure E directly. Can only measure the change in it l∆EFLASHBACK Open System Can exchange matter and energy with surroundings. Closed System Can exchange energy with surroundings Isolated System Cannot exchange matter or energy with surroundingsChapter 19: Thermochemistry II: Entropy and free Energy Introduction In Chapter 5, Thermochemistry, we introduced the First Law of Thermodynamics and saw how to use enthalpy to understand energy transfers for heating & cooling, phase changes, and chemical reactions. In this chapter we look more deeply into the laws of thermodynamics to see how they help us to predict the actual direction of change. What does the word Thermodynamics bring to mind ? Thermo ‐ heat Dynamics ‐ motion or change The science of Thermodynamics was developed around the study of steam engines and how to efficiently turn fuel into useful work. So statements of the laws of thermodynamics will often contain this kind of language. First Law: For a closed system the change in the internal energy of the system is equal to the sum of the energy in the form of heat exchanged between system and surroundings (q) and the work done on the system (w). ΔEsystem = q + w But the Laws of Thermodynamics are far more profound than simply describing the efficiency of steam engines. They constitute the fundamental principles that govern all energy changes. We will see how the concepts of heat and work generalize to the two principle ways that energy can be exchanged between system and surroundings. This will be clearer after we develop the ideas embodied in the Second Law but first, here are other statements of the First Law. First Law: The internal energy of an isolated system is constant. Eisolated = constant Energy is conserved. The total energy of the universe is constant. Euniverse = constant The change in the energy of the universe is zero. ΔEuniverse = 0 The First Law postulates the concept of internal energy, E, and tells us that while energy may change forms, it is not created or destroyed. Notice that the First Law does not tell us what changes are allowed. To illustrate this, take a look at the following "film clip": QuickTime™ and a decompressorare needed to see this picture.1 2 3 What's wrong with this picture ? If you read left to right you notice that it is backwards. That is, the natural process occurs right to left, a diver leaps off the board into the water. We never observe the opposite happening, the water spontaneously gathering together and the diver flying backward up onto the board. It is no violation of the First Law for this to happen. Either way we view the "film clip", energy is conserved, but we know that there is an allowed direction and that we never observe the opposite. If the Laws of Thermodynamics tell us the fundamental principles that govern all energy changes, there must be another law that tells us the allowed direction of change. The Second Law of Thermodynamics summarizes this idea and is one of the most profound laws in nature. Here is a statement of the second law: Second Law All physical and chemical changes occur such that at least some energy disperses: the total concentrated or organized energy of the universe decreases, the total diffuse or disorganized energy of the universe increases. The Second Law has to do with moving from concentrated energy to dispersed energy in the form of random thermal motion. Think about this statement in light of the "film clip". The random, disorganized energy of the water and diver (frame 1) won't spontaneously organize itself and propel the diver backwa rd onto the board (frame 3). The natural direction of change is for the concentrated energy of the diver to be changed into a leap with the water absorbing this energy and dissipating it into the pool. The First Law is about the amount of energy. The Second Law is about the quality of the energy. The First Law postulates the thermodynamic variable E, the internal energy. The Second Law postulates a new thermodynamic variable S, the Entropy, a measure of the dissipated energy within a system at each temperature that is unavailable to do work. Entropy has units of: ∆Energy per degree = (Joule / kelvin) There are many different terms used to describe entropy ‐ disorganized, chaotic, random, dispersed, diffuse, unconcentrated, dissipated, delocalized ... In all cases they are referring to the random molecular motion within a system that cannot be harnessed to do work. Increasing the temperature increases the random motion and thus increases the entropy. Entropy tells us the dire ction of spontaneous change in nature. Change occurs in the direction of increasing total entropy. An isolated system, one that cannot exchange matter or energy with the surroundings (think thermos jar), will never spontaneously decrease in entropy. It will remain unchanged or move to higher entropy. The direction of change is that in which the energy goes from a more concentrated state to a more diffuse state. Entropy is about the dispersal of energy, like the spreading out of water poured onto sand. OWL Example Problem 19.3a Active Figure: Matter and Energy Dispersal 19.3b Active Figure: Entropy and Dissolving 19.3c Active Figure: Dissolution of NH4NO3 Here are some statements of the Second Law using the concept of Entropy: Second Law All physical and chemical changes occur such that the total entropy the universe increases. Suniverse = increases or ΔSuniverse > 0 The entropy of an isolated system never spontaneously decreases. ΔSisolated ≥ 0 Time's Arrow and Microstates You may hear the statement "Entropy is time's arrow". In light of the "film clip" this should make sense, the time axis points to the left, not in the opposite direction. Why is the direction of change toward dissipated energy ? Why doesn't the energy dispersed throughout the water spontaneously gather together ? One way to think about this in terms of probability. There are a huge number of ways that the total energy can be distributed into the random motions of the water