Slide 1 / 93 Slide 2 / 93 Thermodynamics Slide 3 / 93 Chemical Thermodynamics The Golden Gate Bridge is painted regularly to slow down the inevitable rusting of the iron on the bridge. In this unit you will study how heat and temperature relate to work and energy and apply principles of thermodyamics to predict when chemical reactions will occur.
Slide 4 / 93 First Law of Thermodynamics Recall a system is a portion of the universe that has been chosen for studying the changes that take place within it in response to varying conditions. A system can be relatively simple, like a glass of water, or it can be complex, like a planet, or the entire Universe can be considered a system. Slide 5 / 93 First Law of Thermodynamics Within every system exits a property called energy. In physics we learned about kinetic and potential energy. # E = W This year, we extend that by adding another way to change the energy of a system; by the flow of Heat (q). When two objects of different temperature are in contact, heat flow results in an increase of the energy of the cooler object and an identical decrease of the energy of the hotter object. B A T = 20 C T = 10 C heat flow # E = w + q Slide 6 / 93 The First Law of Thermodynamics # E = w + q The First Law of Thermodynamics tells us that energy cannot be created or destroyed. In other words the total energy of the universe is a constant. The same is true of any closed system. The First Law allows any process in which the total energy is conserved, including those where energy changes forms. Initial . Final Internal energy, E Internal energy, E E 0 E state state Energy lost to E < E 0 E > E 0 surroundings # E < 0 ( - ) # E > 0 ( + ) Energy gained Final from Initial state state surroundings E E 0 E of system increases E of system decreases
Slide 7 / 93 First Law of Thermodynamics Most of the processes in the natural world that involve transfer of energy from one form to another don't just happen naturally. For example, gold does not rust in the same way iron does. 4Au(s) + 3O 2 (g) --> 2Au 2 O 3 (s) doesn't happen 4Fe(s) + 3O 2 (g) --> 2Fe 2 O 3 (s) does happen As reserves of fossil fuels run low, people say we have an energy crisis. But if the First law of Thermodynamics is true, energy cannot be created or destroyed, so we're not actually running out of energy. What do people really mean? Slide 8 / 93 The First Law of Thermodynamics The First Law of Thermodynamics applies to any closed system. If our system is a cup, resting on a ledge at a certain height, we know the cup has potential energy and if it falls that energy is transfered to kinetic energy and thermal energy. E 0 + W = E f In this process as our system is definited, total energy remains conserved. If the initial and final energy of the system are equal to each other, why can't the process happen in reverse? Why don't we ever see a broken cup reassemble and return back to its initial position on the ledge? Slide 9 / 93 The Second Law of Thermodynamics The Second Law is a statement about which processes occur and which do not. There are many ways to state the second law: Heat can flow spontaneously from a hot object to a cold object; but not from a cold object to a hot object. It is impossible to build a perpetual motion machine. The universe always gets more disordered with time. Your bedroom will get increasingly messy unless you keep cleaning it up.
Slide 10 / 93 2nd Law: Order to Disorder Natural processes tend to move toward a state of greater disorder. Stir sugar into coffee and you get coffee that is uniformly sweet. No amount of stirring will get the sugar back out. When a tornado hits a building, there is major damage. You never see a tornado pass through a pile of rubble and leave a building behind. You never walk past a lake on a summer day and see a puff of steam rise up, leaving a frozen lake behind. The First Law of Thermodynamics maintains that the above scenarios are possible. The Second Law maintains that they won't naturally occur. Slide 11 / 93 2nd Law: Order to Disorder The Second Law tell us which processes are naturally favorable - that is they can occur without more energy being put in than is released. Favorable doesn't mean fast, it just means that it will naturally occur if a system is left on its own. Slide 12 / 93 Thermodynamically Favorable Once the valve is opened, the gas in vessel B will effuse into vessel A and vice versa , but once the the gases are mixed, they will not spontaneously unmix . The mixing of these gases is favorable because there is much higher probability of the gases being mixed than unmixed. A thermodynamically favorable process is not reversible.
Slide 13 / 93 Favorable Processes Processes that are favorable at one temperature may be not favorable at other temperatures. FOR EXAMPLE favorable at T > 0 C favorable at T < 0 C Slide 14 / 93 A reaction that is thermodynamically favorable 1 _____. A is very rapid B will proceed without a net increase in energy C is also spontaneous in the reverse direction D has an equilibrium position that lies far to the left E is very slow Slide 15 / 93 2 Which of the following statements is true? Processes that are favorable in one direction A are not favorable in the opposite direction. Processes are favorable because they B occur at an observable rate. C Favorability can depend on the temperature. A and C are true D
Slide 16 / 93 Reversible Processes In a reversible process the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process. Surroundings Surroundings System System T- # T T+ # T T T +q Heat -q Heat Endothermic Exothermic Slide 17 / 93 Irreversible Processes Movable Piston partition work Vacuum Gas Irreversible processes cannot be undone by exactly reversing the change to the system. Thermodynamically favorable processes are irreversible. Slide 18 / 93 3 A reversible process is one that __________. can be reversed with no net change in either A system or surroundings B is thermodynamically favorable C is thermodynamically unfavorable D must be carried out at low temperature E must be carried out at high temperature
Slide 19 / 93 Entropy Entropy ( S ) is a term coined by Rudolph Clausius in the 19th century. Entropy refers to the ratio of heat to the temperature at which the heat is delivered: q S = T Slide 20 / 93 Entropy Entropy can be thought of as a measure of the randomness of a system, or as a measure of the number of ways of arranging particles. It is related to the various modes of motion in molecules. Like total energy, E , and enthalpy, H , entropy is a state function. As a result, we are interested in measuring the change in entropy # S, as opposed to the absolute entropy, S # S = S final - S initial Slide 21 / 93 Entropy For a process occurring at constant temperature, the change in entropy is equal to the heat that would be transferred if the process were reversible divided by the temperature: q rev # S = T Isothermal process
Slide 22 / 93 Second Law of Thermodynamics The entropy of the universe increases for thermodynamically favorable processes and The entropy of the universe does not change for reversible processes. Slide 23 / 93 Second Law of Thermodynamics In other words: For reversible processes: ∆S= # S system + # S surroundings = 0 For irreversible processes: # S= # S system + # S surroundings > 0 This means that the entropy of the universe constantly increases. Slide 24 / 93 4 The thermodynamic quantity that expresses the degree of disorder in a system is ______. A enthalpy B internal energy C bond energy D entropy E heat flow
Slide 25 / 93 For an isothermal (constant temperature) 5 process, # S = __________. A q B q rev / T C q rev D Tq rev E q + w Slide 26 / 93 6 Which one of the following is always positive when a thermodynamically favorable process occurs? A # S system B # S surroundings C # S universe D # H universe E # H surroundings Slide 27 / 93 7 The entropy of the universe is __________. A constant B continually decreasing C continually increasing D zero E the same as the energy, E
Slide 28 / 93 Entropy on the Molecular Scale Ludwig Boltzmann described the concept of entropy on the molecular level by using statistical analysis Slide 29 / 93 ** Statistical Interpretation of Entropy and the Second Law A macrostate of a system is specified by giving its macroscopic properties – temperature, pressure, and so on. T = 16 C P = 1 atm A microstate of a system describes the position and velocity of every particle. For every macrostate, there are one or more microstates. Slide 30 / 93 Statistical Interpretation of Entropy ** and the Second Law A simple example: tossing four coins. The macrostates describe how many heads and tails there are; the microstates list the different ways of achieving that macrostate.
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