# Top-rated ScreenCasts

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03.1 - Heat Engines and Heat Pumps: The Carnot Cycle Click here. 56.6667 6

Introduction to the Carnot cycle (Khan Academy, 21min). The Carnot cycle is an idealized conceptual process in the sense that it provides the maximum possible fractional conversion of heat into work (aka. thermal efficiency, ηθ). Note that Khan uses the absolute value when referring to quantities of heat and work so his equations may look a little different from ours. By systematically adding up the heat and work increments through all stages of the process, we can infer an approximate equation for thermal efficiency (Khan Academy, 14min) The steps are isothermal and reversible expansion, adiabatic and reversible expansion, isothermal and reversible compression, and adiabatic/reversible compression.  We know how to compute the heat and work for ideal gases of each step based on Chapter 2. In this presentation by KhanAcademy, an additional proof is required (17min) to show that the volume ratio during expansion is equal to the volume ratio during compression. (Note that the presentation by KhanAcademy uses arbitrary sign conventions for heat and work. They prefer to change the sign to minimize the use of negative numbers but it doesn't always work out.) When we put it all together, the equation we get for "Carnot efficiency" is remarkably simple: ηθ = (TH - TC)/TH, where TH is the hot temperature and Tis the cold temperature. We can use this formula to quickly estimate the thermal efficiency for many processes. We will show in Chapter 5 that this formula remains the same, even when we use working fluids other than ideal gases (e.g. steam or refrigerants).

Comprehension Questions:
1. Should we express temperature in Kelvins or Celsius when calculating the Carnot efficiency? Explain.
2. What value of TC would be necessary to achieve 100% efficiency, even for this idealized, maximally efficient process? Explain.
3. Why is it impractical to reject heat at the value of Tdiscussed in Question 2 above? What is a more practical temperature for rejecting heat? (Hint: what geographical feature is very closely located near most nuclear power plants? "Geographical features" might include mountains, desserts, large bodies of water, forests, ...)
4. What value of TH would be necessary to approach 100% efficiency, even for this idealized, maximally efficient process? What are the practical limitations on TH? Explain.
5. How can the formula for Carnot efficiency help us to calculate the "lost" work in the presence of a temperature gradient?

11.06 - Redlich-Kister and the Two-parameter Margules Models Click here. 56.6667 6

Two-parameter Margules Equation (5:05) (msu.edu)

An overview of the two parameter Margules equation and how it is fitted to a single experiment.

01.2 Molecular Nature of Temperature, Pressure, and Energy Click here. 56.25 16

Molecular Nature of Internal Energy: Configurational Energy. (uakron.edu, 19min) Making the connection between "u" and "U" requires the concept configuring the molecules such that their potentials overlap. Then it is a simple matter (conceptually) to count the number of overlaps that occur and multiply by the energy of the overlap to get the "configurational energy." Adding the configurational energy to the translational (and vibrational) energy (Uig, discussed above), gives the total "U."

Comprehension Questions:

For 1-4, assume 100 molecules are held in a cylinder with solid walls. A piston in the cylinder can move to adjust the density.
1. Suppose the range of the potential (λ) was increased. Would the configurational energy increase, decrease, or stay the same?
2. Suppose the density was decreased. Would the configurational energy increase, decrease, or stay the same?
3. Suppose the temperature was increased at constant density. Would the configurational energy increase, decrease, or stay the same?
4. Suppose the temperature was increased at constant density. Would the configurational energy characterized by (U-Uig)/RT  increase, decrease, or stay the same?
5. Molecules A and B can be represented by the square-well potential. For molecule A, σ = 0.2 nm and ε = 30e-22 J. For molecule B, σ = 0.35 nm and ε = 20e-22 J.  Sketch the potential models for the two molecules on the same pair of axes clearly indicating σ's and ε's of each species. Start your x-axis at zero and scale your drawing properly.  Make molecule A a solid line and B a dashed line. Which molecule would you expect to have the higher boiling temperature? (Hint: check out Figure 1.2.)
6. Sketch the potential and the force between two molecules vs. dimensionless distance, r/σ, according to the Lennard-Jones potential. Considering the value of r/σ when the force is equal to zero, is it greater, equal, or less than unity?

10.04 - Multicomponent VLE & Raoult's Law Calculations Click here. 55 4

This example shows how to use VLookup with the xls Solver to facilitate  multicomponent  VLE calculations for ideal solutions: bubble, dew, and isothermal flash. (15min, uakron.edu) The product xls file serves as a starting point for multicomponent VLE calculations with activity models and for adiabatic flash and stream enthalpy calculations. This video shows sample calculations for the bubble, dew, and flash of propane, isobutane, and n-butane, like Example 10.1.

Note: This is a companion file in a series. You may wish to choose your own order for viewing them. For example, you should implement the first three videos before implementing this one. Also, you might like to see how to quickly visualize the Txy analog of the Pxy phase diagram. If you see a phase diagram like the ones in section 11.8, you might want to learn about LLE phase diagrams. The links on the software tutorial present a summary of the techniques to be implemented throughout Unit3 in a quick access format that is more compact than what is presented elsewhere. Some students may find it helpful to refer to this compact list when they find themselves "not being able to find the forest because of all the trees."

Comprehension Questions - Assume the reboiler composition for the column in Example 10.1 was zi={0.2,0.3,0.5} for n-butane, isopentane, and n-pentane, respectively.

a)  Calculate the temperature at which the boiler must operate in order to boil the bottoms product completely at 8 bars.
b)  Assuming the bottoms product liquid is in equilibrium with the liquid in the boiler, calculate the temperature of boiler and composition of the vapor in the boiler.
c)  Suppose this stream is to be boiled again and the vapor returned to the column with a ratio of 2 parts vapor to 1 product. (FYI: this is known as "boilup ratio.") Find the relevant temperature and compositions.

4. Selecting Stable Roots (1:11) (msu.edu)

Selecting stable roots is often one of the confusing aspects in working with cubic equations of state. This screencasts gives a visual picture of how the roots and stability are related to the vapor pressure and EOS humps at subcritical temperatures.

Bubble Pressure (5:25) (msu.edu)

The culmination of the activity coefficient method is application of the fitted activity coefficients to extrapolate from limited experiments in a Stage III calculation. As the easiest routine to apply, the bubble pressure method should be studied first. The recommended order of study is 1) Bubble Pressure; 2) Bubble Temperature; 3) Dew Pressure; 4) Dew Temperature. Note that an entire Pxy diagram can be generated with bubble pressure calculations; no dew calculations are required.

Connecting Microstates, Macrostates, and the Relation of Entropy to Disorder (uakron.edu, 14min). For small systems, we can count the number of ways of arranging molecules in boxes to understand how the entropy changes with increasing number of molecules. By studying the patterns, we can infer a general mathematical formula that avoids having to enumerate all the possible arrangements of 10^23 molecules (which would be impossible within several lifetimes). A surprising conclusion of this analysis is that entropy is maximized when the molecules are most evenly distributed between the boxes (meaning that their pressures are equal). Is it really so "disordered" to say that all the molecules are neatly arranged into equal numbers in each box? Maybe not in a literary world, but it is the only logical conclusion of a proper definition of "entropy." It is not necessary to watch the videos on probability before watching this one, but it may help. And it might help to re-watch the probability videos after watching this one. Moreover, you might like to see how the numbers relate to the equations through sample calculations (uakron, 15min). These sample calculations show how to compute the number of microstates and probabilities given particles in Box A, B, etc and also the change in entropy.

Comprehension Questions:

1. What is the number of total possible microstates for: (a) 2 particles in 2 boxes (b) 5 particles in 2 boxes (c) 10 particles in 3 boxes.

2. What is the probability of observing: (a) 2 particles in Box A and 3 particles in Box B? (b) 6 particles in Box A and 4 particles in Box B? (c) 6 particles in Box A and 4 particles in Box B and 5 particles in Box C?

Relating the microscopic perspective on entropy to macroscopic changes in volume (uakron.edu, 11min) Through the introduction of Stirling's approximation, we arrive at a remarkably simple conclusion for changes in entropy relative to the configurations of ideal gas molecules at constant temperature: ΔS = Rln(V2/V1). This makes it easy to compute changes in entropy for ideal gases, even for subtle changes like mixing.

Comprehension Questions:

1. Estimate ln(255!).

2. A system goes from 6 particles in Box A and 4 particles in Box B to 5 particles in each. Estimate the change in S(J/K).

3. A system goes from 6 moles in Box A and 4 moles in Box B to 5 moles in each. Estimate the change in S(J/mol-K).

Energy and Enthalpy Misunderstandings (LearnChemE.com) (3:20) Three examples related to enthalpy and work changes that are often confusing...
Comprehension Questions:
1. During one stroke of a steam engine, the pressure inside the (~frictionless) cylinder is maintained at 3MPa from a supply line at 500 C while the pressure created by the force on the cam shaft and atmosphere combined is 2.5 MPa.  The volume swept by the cylinder during one stroke is 10 L.  Compute the work achieved by this process (kJ).
2. Was there any "lost work" in the above process? If so, compute its magnitude (kJ) and explain where those kJ are now.
3. Consider N2 in a closed cylinder with a piston initially at 1 bar and 300K. The system is heated to 400K such that the piston moves to maintain constant pressure. Is it true that Q = Cp dT for this system or should it be Cv dT or something else? Explain using a detailed analysis of the energy balance.