Thermodynamics Slide 3 / 93 Slide 4 / 93 First Law of - PDF document

Slide 1 / 93 Slide 2 / 93 Thermodynamics Slide 3 / 93 Slide 4 / 93 First Law of Thermodynamics Chemical Thermodynamics Recall a system is a portion of the universe that has been The Golden Gate Bridge is painted regularly to slow chosen for studying the changes that take place within it in response to varying conditions. down the inevitable rusting of the iron on the bridge. 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. 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 5 / 93 Slide 6 / 93 The First Law of Thermodynamics First Law of Thermodynamics # E = w + q Within every system exits a property called energy. The First Law of Thermodynamics tells us that energy cannot be created or destroyed. In other words the total energy of the In physics we learned about kinetic and potential energy. universe is a constant. The same is true of any closed system. # E = W The First Law allows any process in which the total energy is This year, we extend that by adding another way to change conserved, including those where energy changes forms. the energy of a system; by the flow of Heat (q). Initial . Final When two objects of different temperature are in contact, heat flow Internal energy, E Internal energy, E E 0 E state state results in an increase of the energy of the cooler object and an Energy identical decrease of the energy of the hotter object. lost to E < E 0 E > E 0 surroundings # E < 0 ( - ) # E > 0 ( + ) B A T = 20 C T = 10 C Energy gained Final Initial from state state surroundings heat flow E E 0 E of system increases E of system decreases # E = w + q

Slide 7 / 93 Slide 8 / 93 First Law of Thermodynamics The First Law of Thermodynamics The First Law of Thermodynamics applies to any closed system. Most of the processes in the natural world that involve transfer of If our system is a cup, resting on a ledge at a certain height, we energy from one form to another don't just happen naturally. know the cup has potential energy and if it falls that energy is transfered to kinetic energy and thermal energy. For example, gold does not rust in the same way iron does. E 0 + W = E f 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. In this process as our system is definited, total energy remains What do people really mean? 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 Slide 10 / 93 The Second Law of Thermodynamics 2nd Law: Order to Disorder Natural processes tend to move toward a state of greater disorder. The Second Law is a statement about which processes occur and which do not. There are many ways to state the second law: Stir sugar into coffee and you get coffee that is uniformly sweet. No amount of stirring will get the sugar back out. Heat can flow spontaneously from a hot object to a cold object; but not from a cold object to a hot object. When a tornado hits a building, there is major damage. You never see a tornado pass through a pile of rubble and It is impossible to build a perpetual motion machine. leave a building behind. You never walk past a lake on a summer day and see a The universe always gets more disordered with time. puff of steam rise up, leaving a frozen lake behind. Your bedroom will get increasingly messy unless you keep The First Law of Thermodynamics maintains that the above cleaning it up. scenarios are possible. The Second Law maintains that they won't naturally occur. Slide 11 / 93 Slide 12 / 93 Thermodynamically Favorable 2nd Law: Order to Disorder Once the valve is opened, the gas The Second Law tell us which processes are naturally favorable - in vessel B will effuse into vessel A that is they can occur without more energy being put in than is and vice versa , but once the the released. gases are mixed, they will not spontaneously unmix . Favorable doesn't mean fast, it just means that it will naturally occur if a system is left on its own. 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 Slide 14 / 93 Favorable Processes A reaction that is thermodynamically favorable 1 _____. Processes that are favorable at one temperature may be not favorable at other temperatures. A is very rapid FOR EXAMPLE B will proceed without a net increase in energy C is also spontaneous in the reverse direction favorable at T > 0 C D has an equilibrium position that lies far to the left E is very slow favorable at T < 0 C Slide 15 / 93 Slide 16 / 93 Reversible Processes 2 Which of the following statements is true? In a reversible process the system changes in such a way that Processes that are favorable in one direction the system and surroundings can be put back in their original A are not favorable in the opposite direction. states by exactly reversing the process. Processes are favorable because they B Surroundings occur at an observable rate. Surroundings System System C Favorability can depend on the temperature. T- # T T+ # T T T A and C are true D +q Heat -q Heat Endothermic Exothermic Slide 17 / 93 Slide 18 / 93 3 A reversible process is one that Irreversible Processes __________. Movable can be reversed with no net change in either Piston partition A system or surroundings B is thermodynamically favorable C is thermodynamically unfavorable work Vacuum Gas D must be carried out at low temperature Irreversible processes cannot be undone by exactly E must be carried out at high temperature reversing the change to the system. Thermodynamically favorable processes are irreversible.

Slide 19 / 93 Slide 20 / 93 Entropy Entropy Entropy can be thought of as a measure of the randomness of a system, or as a measure of the number Entropy ( S ) is a term coined of ways of arranging particles. by Rudolph Clausius in the 19th century. It is related to the various modes of motion in molecules. Entropy refers to the ratio of Like total energy, E , and enthalpy, H , entropy is a state heat to the temperature at function. As a result, we are interested in measuring the which the heat is delivered: change in entropy # S, as opposed to the absolute entropy, S q S = # S = S final - S initial T Slide 21 / 93 Slide 22 / 93 Entropy Second Law of Thermodynamics 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: The entropy of the universe increases for thermodynamically favorable processes q rev and # S = T The entropy of the universe does not change for reversible processes. Isothermal process Slide 23 / 93 Slide 24 / 93 4 The thermodynamic quantity that expresses the Second Law of Thermodynamics degree of disorder in a system is ______. In other words: A enthalpy For reversible processes: B internal energy ∆S= # S system + # S surroundings = 0 C bond energy D entropy For irreversible processes: E heat flow # S= # S system + # S surroundings > 0 This means that the entropy of the universe constantly increases.

Slide 25 / 93 Slide 26 / 93 6 Which one of the following is always positive when 5 For an isothermal (constant temperature) a thermodynamically favorable process occurs? process, # S = __________. A q A # S system q rev / T B B q rev # S surroundings C D Tq rev C # S universe E q + w D # H universe E # H surroundings Slide 27 / 93 Slide 28 / 93 7 The entropy of the universe is __________. Entropy on the Molecular Scale A constant Ludwig Boltzmann B continually decreasing described the concept of entropy C continually increasing on the molecular level by using D zero statistical analysis E the same as the energy, E Slide 29 / 93 Slide 30 / 93 Statistical Interpretation of Entropy ** ** Statistical Interpretation of Entropy and the Second Law and the Second Law A simple example: tossing four coins. The macrostates describe A macrostate of a system is specified by giving its macroscopic how many heads and tails there are; the microstates list the properties – temperature, pressure, and so on. different ways of achieving that macrostate. 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.