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Chapter 3

Chapter 3 - Chapter3: surroundings.Indeed,dU=w q .,itis...

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Marand’s Notes: Chapter 3 ‐ The Second Law of Thermodynamics 94 Chapter 3: The Second Law of Thermodynamics The first law of thermodynamics is basically a law that describes the conservation of energy for system + surroundings. Indeed, dU = δ w + δ q only applies for the system. Whatever work is done by the system, it is done on the surroundings and vice versa ( δ w surr = ‐ δ w). The same can be said about heat ( δ q surr = ‐ δ q). So we could write dU surr = ‐ ( δ w + δ q). Therefore dU + dU surr = 0 (that is dU Universe = 0 or U Universe = constant). This first law ensures that energy is not lost in a process. It however does not say whether a process occurs spontaneously (i.e. irreversibly), reversibly (i.e. under equilibrium conditions) or not. The second law of thermodynamics addresses this issue and provides us with a criterion to decide the direction in which a process occurs spontaneously. In other words, if we define two states A and B, the second law tells us whether the process occurs spontaneously from A to B, spontaneously from B to A, or whether there is an equilibrium between A and B. We say that a process occurs spontaneously if it can occur without the need for work to be done on the system during the process.

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Marand’s Notes: Chapter 3 ‐ The Second Law of Thermodynamics 95 The following processes are spontaneous: H 2(g) + 1/2 O 2(g) H 2 O (l) Ice melts at 10°C at 1 atm An object at temperature 100°C cools when placed in an environment at 25°C. A gas constrained in a small volume at pressure P is expanded to occupy a larger volume if the external pressure is less than P. A mass falls from a high altitude to a low altitude. Conversely, the following processes are not spontaneous and will take place only if work is done by the surroundings on the system. Cooling an object below ambient temperature, electrolysis of water to yield O 2 and H 2 , compressing a gas to a smaller volume. In a philosophical sense, the second law tells us about the direction of time. Any real system has two choices, either it evolves spontaneously in one direction (in which case, it cannot evolve spontaneously in the opposite direction), or it is at equilibrium (in which case, it does not evolve at all). Note that all living systems evolve in some fashion. The basic observation at the origin of the second law is that a spontaneous process is always accompanied by a dispersal of energy in a more disordered form .
Marand’s Notes: Chapter 3 ‐ The Second Law of Thermodynamics 96 To visualize this statement, consider the process of friction (an ever‐ present nuisance, which prevents perpetual motion). Every time a ball bounces, it loses energy. This energy is given to the ground in the form of heat, which leads to increased motion of the atoms and molecules in the ground. Heat is transferred to the ground because of the friction during the impact. There is a decrease in the organized motion of the ball (bouncing of the ball, where all atoms and molecules move down and up in unison) and an increase in the disorganized motion of the molecules in the ground (increase in temperature due to the local heating).

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Chapter 3 - Chapter3: surroundings.Indeed,dU=w q .,itis...

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