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.

The reversible process is a common conception throughout discussions of thermodynamics. One very common illustration has to do with grains of sand being removed from a piston (KhanAcademy, 15min). It is also helpful to put the relation of reversibility into context with the work as a path function. (YouTube, 1.5min).

Comprehension Questions
1. Describe what happens when you knock a complete block off a piston under pressure. Assume the piston has mass roughly equal to the block, the cylinder is infinitely tall and everything is adiabatic, the gas in the cylinder is ideal, and the open side of the cylinder is at atmospheric pressure. In particular, does the piston go monotonically to its equilibrium position or does it do something else? If not, then what causes it to approach in a different way and why does it eventually reach its equilibrium state?
2. Consider the same piston/cylinder as above but compare it to a piston/cylinder with grains of sand. Is the final height of the piston the same, lower, or higher when you remove the weight one grain at a time vs. knocking the block off all at once? Hint: write the energy balance and carefully consider the work accomplished in each case.

State functions explained (LearnChemE.com, 5min). Properties like those listed in the steam tables are functions of P, V, T, etc. They depend only on the state variables and knowing two of the variables is enough to figure out all the rest. Other functions are like heat and work; they depend on the path by which you proceed to evaluate their changes. Path function sample calculations (uakron, 9min) are useful in providing concrete illustrations of how the path matters for work and heat.

Comprehension Questions:
1. Consider a monatomic ideal gas in an insulated piston/cylinder with a vaccuum outside the piston, originally at 300K and 1bar. Suppose the volume is suddenly doubled with zero resistance against the piston. Compute the change in U and the work accomplished.
2. Consider the same ideal gas etc as above. Now suppose the volume is slowly doubled (e.g. using grains of sand). Compute the change in U and the work accomplished.
3. Continuing from #2 above, the insulation is removed and the piston/cylinder is allowed to equilibrate to its original temperature in #1 above. Compute the change in U and the work accomplished for this stage.
4. Compare the entire process from 2-3 above with the process in #1. Compute the change in U and the work accomplished overall. Also compare the final pressures. Is pressure a state function or a path function?

Constant pressure process using steam (LearnChemE.com, 5min). 2000 kJ of heat is isobarically added to steam piston/cylinder starting at 0.45MPa with 0.9kg as vapor and 0.1kg as liquid. Compute the final temperature, work, and state of the steam. Once we have our general energy balance defined, we can straigntforwardly reduce it to its simplest applicable form to solve problems. The energy balance is the same regardless of whether the process uses an ideal gas, steam, or some other working fluid. But the method of solving the problem changes quite a bit depending on the working fluid.
Hint: "steam" and H2O are the same thing. So "liquid steam" is also known as "water."

Comprehension questions:
1. Describe how you would solve this problem if the H2O was replaced with a monatomic ideal gas (MW=40). Use the same starting pressure and temperature as the steam, but obviously the entire 1.0 kg will be gas, with no liquid.
2. Describe how you would write the energy balance if the cylinder was 5m3 total, open to the atmosphere, and the pressure was suddenly reduced to 1 bar. Assume the piston has a mass of 0.1kg.

Adiabatic, Reversible Compression of an Ideal Gas in a Piston/Cylinder (LearnChemE.com, 5min). The standard formula for an adiabatic, reversible, ideal gas is derived here in the T,V form. You should be able to rearrange the given equation into the usual form:
(T2/T1) = (P2/P1)^(R/Cp) to show they are equivalent. (Hint: PV=RT and Cp=Cv+R). The interesting part of this video is during the last 10 seconds. Watch what happens!

Comprehension Questions:

1. My bicycle pump is about 50cm tall and 2.0cm diameter. When I pump it down, the pressure goes to 100psig (after pumping once or twice). What is the temperature of the air that goes into the tire at that point?

2. What is the length (cm) remaining in the pump?

Understanding Enthalpy (uakron.edu, 6min) The vocabulary just keeps on coming. Usually, it helps to picture the physical process and think about what is happening with the molecules. Then the names applied nearly always make sense as they refer to some specific part of the overall picture. This is not the case for enthalpy. Enthalpy is merely a convenient lumping of other more fundamental terms. It has purely a mathematical definition. There is nothing physical about it. Keep in mind that this kind of arbitrary definition is the exception, not the rule. The rule is: try to understand each aspect of vocabulary in terms of its physical meaning. An exception is enthalpy. Enthalpy has no physical meaning.

The complete energy balance is convenient in the sense that it provides a comprehensive list of everything you need to check to ensure that you have accounted for all energy flows, but it can appear to be a little overwhelming at first glance. Common energy balances (uakron, 14 min) of the energy balance can be used in many situations, but don't forget that the process of analyzing a system and determining its proper model equations is an important part of thermodynamics, and engineering in general. Focus on learning the process, not memorizing the final equations. Energy balance practice (uakron, 18min) with Chapter 2 systems can help you build confidence and quickly prepare for mastering all the example problems in Chapter 2. Try to push pause after each problem statement and work it out for yourself, then resume to provide a check on your analysis.

Comprehension Questions. Write the simplified energy balance for the following:

1. High pressure steam flows through an adiabatic turbine to steadily produce work. 

2. High pressure steam flows into a piston-cylinder to produce work. 

3. Steam at 200 bars and 600°C flows through a valve and out to the atmosphere. 

4. A gas is filling a rigid tank from a supply line.

5. A gas is leaking from a rigid tank into the air. 

6. A sunbather lays on a blanket. At 11:30 a.m., the sunbather begins to sweat. System: the sunbather at 12 noon.

Common Property Change Calculations (uakron, 11min). When we need to compute a change in energy or enthalpy, we may quickly resort to CvΔT or CpΔT, but you should also note that large changes can occur due to phase change. These considerations motivate careful consideration of the definitions of Cv and Cp, and the development of convenient equations for estimating heat of vaporization. To know when to apply the heat of vaporization, you need to know the saturation conditions, for which a quick estimate can be obtained from the short-cut vapor pressure (SCVP) equation. When the chemical of interest is H2O, these hand calculation methods can be compared to the properties given in the steam tables. Sample calculations of property changes (uakron, 21min) can be used to illustrate the precision of the quick estimates obtained from Eqs. 2.45, 2.47 and the back flap. These calculations provide practice with the steam tables at unusual conditions as well as validating your skills with the hand calculation formulas.

Comprehension Questions:
1. Develop an adaptation of props.xlsx that is most convenient for you personally to compute quick estimates of saturation temperature, saturation pressure, ideal gas enthalpy changes. You might want to view the props.xlsx and shortcut Antoine coefficients software tutorials.
2. Quickly estimate the change in enthalpy as CO2 goes from 350K, 1bar to 300K, 1bar.
3. Quickly estimate the change in internal energy as CO2 goes from 350K, 1bar to 300K, 1bar.
4. Quickly estimate the change in enthalpy as CO2 goes from 350K, 1bar to 300K, 100bar. Hint: the change in enthalpy to go from a saturated liquid to a compressed liquid can be computed from the adiabatic, reversible pump work.

We can streamline process calculations by defining a common reference state and computing values of enthalpy for all streams. A convenient path for tabulating properties relative to a reference state is illustrated in Figure 2.6c. It is very similar to the common calculation of DH illustrated in section 2.10. We define the common reference state to be the ideal gas at 25C (298K). Then (1) compute the change in ideal gas properties to the temperature of the stream. (2) Use Eqn. 2.47 to check Psat/Tsat in case a liquid may be forming (3) if liquid, use Eqn. 2.45 to compute the change from the ideal gas to the saturated liquid (4) if P^Liq >> P^sat, compute ΔH = V_LΔP. This process is easy to automate using a spreadsheet and you can quickly tabulate all the stream enthalpies of interest, as illustrate using sample calculations for DME (uakron, 17min). Remember to push the pause button as soon as you read the problem statement and see if you can perform the calculation on your own. Then use the screencast to catch any mistakes you might have made. The procedure for a single component can be extended to multiple components to provide a spreadsheet utility for quickly performing energy balance calculations for an entire process (uakron, 7min) Note that an entire process may involve mixtures or reactions. The extension to mixtures is presented in Section 3.5.

Comprehension Questions:
1. Tabulate the stream enthalpies of methanol relative to the ideal gas reference state at (298K,1bar) at: (a) 300K,1bar (b) 350K,1bar.
2. Compute ΔH for going between these states (a) and (b) and compare to the similar problem in Section 2.10.

Energy Balance Around a Turbine (LearnChemE.com, 7min) performs an energy balance around a turbine accounting for flow work

Throttle Temperature Change (LearnChemE.com, 3min). Shows the temperature change of non-ideal gases through an adiabatic throttle due to Joule-Thomson expansion

Heat Removal to Condense a Vapor Mixture (LearnChemE.com, 5min) uses state functions to explain how to determine heat of vaporization for binary mixture

Energy Balance on a Heat Exchanger (LearnChemE.com, 7min) do an energy balance on a heat exchanger that has superheated steam fed to it. Use the steam tables.

Adiabatic Compression of an Ideal Gas (LearnChemE.com, 6min) calculate adiabatic temperature for compression of an ideal gas, both reversibly and irreversibly.

Comprehension Questions:
1. 10 mol/s of air is compressed adiabatically and reversibly from 298K and 1bar to 5 bar.
(a) Compute the exit temperature (K). (Hint: check out Eqn. 2.51.)
(b) Compute the power requirement (kW) for this compressor.
(c) Suppose the compressor was only 75% efficient, where efficiency≡W_rev/W_act for compression. Is the temperature of the irreversible compressor higher or lower than that of the reversible compressor? Explain.
(d) Calculate the temperature exiting a 75% efficient compressor.
2. Air is expanded adiabatically and reversibly (through a turbine) from 298K and 5 bar to 1 bar.
(a) Is the outlet temperature higher or lower than 298K?
(b) Suppose the air was expanded through a 50% efficient turbine. Would the temperature be higher or lower than the reversible turbine? Explain.

Reversible Adiabatic Compression of Ideal Gas (LearnChemE.com, 5min) calculate the final conditions for adiabatic, reversible compression of an ideal gas. Here, we derive an important equation that should be memorized: (T2/T1)=(P2/P1)^(R/Cp).

Energy Balance: Filling a Tank (LearnChemE.com, 5min) determine the final properties of a tank to which steam is added.

Comprehension Questions:

1. An empty tank is filled by opening a valve and allowing steam at 300°C and 8MPa to flow into it until the pressure equalizes. Estimate the final temperature (°C).

2. An empty tank is filled by opening a valve and allowing steam at 350°C and 8MPa to flow into it until the pressure equalizes. Estimate the final temperature (°C).

3. An empty tank is filled by opening a valve and allowing steam at 300°C and 6MPa to flow into it until the pressure equalizes. Estimate the final temperature (°C).

Evaporative Cooling Energy Balance (LearnChemE.com, 6min) apply the energy balance to liquid water that is evaporating and undergoing evaporative cooling.

Tank filling: Steam, Unevacuated (uakron.edu, 15min) Unsteady state problems are relatively easy to solve for ideal gases, especially when the tank is empty to begin. The solution process becomes more difficult using steam with some H2O already in the vessel. This demonstration shows how to adapt to the new situation. The point is to learn the manner of adapting, not so much answer for this particular problem. You may not be filling many tanks with steam in your life, but you will certainly be adapting to new situations. The steps of analysis, identifying pieces of the puzzle, and putting them together are key components of any such adaptation.

Work and Enthalpy Misunderstandings (LearnChemE.com, 3min) discusses three examples related to enthalpy and work changes that are often confusing.