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Slide 2 / 93 Chemical Thermodynamics www.njctl.org
Slide 3 / 93 Chemical Thermodynamics The Golden Gate Bridge is painted regularly to slow down the inevitable rusting of the iron on the bridge. This unit will help us understand how we determine whether or not a certain reaction will occur.
Slide 4 / 93 First Law of Thermodynamics The First Law tells us that energy cannot be created nor destroyed; the total energy of the universe is a constant. The First Law allows any process in which the total energy is conserved, including those where energy changes forms.
Slide 5 / 93 First Law of Thermodynamics However, we observe processes in the natural world that just don't happen "naturally". For example, gold does not rust in the same way that iron does. The First Law of thermodynamics doesn't help us here. 4Au(s) + 3O 2 (g) --> 2Au 2 O 3 (s) doesn't happen naturally 4Fe(s) + 3O 2 (g) --> 2Fe 2 O 3 (s) does happen naturally
Slide 6 / 93 The First Law of Thermodynamics For instance, the First Law would allow a broken cup shown below to reassemble itself, but it never will. The absence of processes like this shows that the conservation of energy is not the whole story. If it were, movies that were run backwards would look perfectly normal to us!
Slide 7 / 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 8 / 93 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 says all these could happen, the Second Law says they won't.
Slide 9 / 93 Spontaneous Processes and the Second Law 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 10 / 93 Thermodynamic Favorability 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.
Slide 11 / 93 Favorable Processes Processes that are favorable in one direction are not favorable in the reverse direction.
Slide 12 / 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 13 / 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 answer E is very slow
Slide 14 / 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. answer A and C are true D
Slide 15 / 93 Reversible Processes Surroundings System T + ΔT T Heat In a reversible process the system Exothermic changes in such a way that the system and surroundings can be put back in their original states by exactly reversing Surroundings the process. System T- ΔT T Heat Endothermic
Slide 16 / 93 Irreversible Processes Piston Movable partition work Vacuum Gas Irreversible processes cannot be undone by exactly reversing the change to the system. Thermodynamically favorable processes are irreversible.
Slide 17 / 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 answer C is thermodynamically unfavorable D must be carried out at low temperature E must be carried out at high temperature
Slide 18 / 93 Entropy Entropy ( S ) is a term coined by Rudolph Clausius in the 19th century. Clausius was convinced of the significance of the ratio of heat delivered and the temperature at which it is delivered: q S = T
Slide 19 / 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 20 / 93 Entropy For a process occurring at constant temperature (an isothermal process), the change in entropy is equal to the heat that would be transferred if the process were reversible divided by the temperature: q rev ΔS = T
Slide 21 / 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 22 / 93 Second Law of Thermodynamics In other words: For reversible processes: Δ S univ = Δ S system + Δ S surroundings = 0 For irreversible processes: Δ S univ = Δ S system + Δ S surroundings > 0 This means that the entropy of the universe constantly increases
Slide 23 / 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 answer
Slide 24 / 93 For an isothermal (constant temperature) 5 process, ΔS = __________. A q q rev / T B answer q rev C D Tq rev E q + w
Slide 25 / 93 6 Which one of the following is always positive when a thermodynamically favorable process occurs? A ΔS system answer B ΔS surroundings C ΔS universe D ΔH universe E ΔH surroundings
Slide 26 / 93 7 The entropy of the universe is __________. A constant B answer continually decreasing C continually increasing D zero E the same as the energy, E
Slide 27 / 93 Entropy on the Molecular Scale Ludwig Boltzmann described the concept of entropy on the molecular level by using statistical analysis
Slide 28 / 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 29 / 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.
Slide 30 / 93 Statistical Interpretation of Entropy [ **] and the Second Law Assume each microstate is equally probable; the probability of each macrostate then depends on how many microstates are in it. The number of microstates quickly becomes very large if we have even 100 coins instead of four.
Slide 31 / 93 Statistical Interpretation of Entropy [ **] and the Second Law probabilities of various macrostates for 100 coin tosses This table lists some of the Macrostate # of microstates probability possible outcomes heads tails (macrostates) for 100 coin 100 0 1 8.0x10 -31 tosses , how many 99 1 100 8.0x10 -29 microstates they have, and 90 10 1.7x10 13 1.0x10 -17 the relative probability that 80 20 5.4x10 20 4.0x10 -10 each macrostate will occur. 60 40 0.01 1.4x10 28 55 45 0.05 6.1x10 28 Note that the probability of 45 55 0.05 6.1x10 28 getting fewer than 20 heads 20 80 or tails is extremely small. 5.4x10 20 4.1x10 -10 10 90 1.7x10 13 1.0x10 -17 1 99 100 8.0x10 -29 0 100 1 8.0x10 -31
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