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CHBE 2100 - Final Exam
Terms in this set (51)
The van der Waals EOS contains corrections on the __________________________ and _________________________________ of gas molecules.
volume and interactions
Valid unites of pressure:
psi, ft h2o, bar, atm
[T/F] A process can be both batch and steady state
[T/F] A process can be closed, continuous, and steady state
[T/F] Molecular balances in a steady state system with no reactions can be written as in = out
[T/F] If a liquid substance completely fills a chamber at equilibrium at some temperature, the vapor pressure of the substance at that temperature is zero
In a room containing humid air, the relative humidity will [increase | stay the same | decrease] if we increase the temperature while keeping the pressure constant.
The ideal gas approximation applies better at [high | low] pressures and [high | low] temperatures.
State variable(s): [U H Q W Ws Ep Ek ]
U, H, Ep, Ek
An endothermic reaction occurs inside an adiabatic reactor. Compared to the reactants, the products will have a [higher | lower | same | can be either] temperature and a [higher | lower | same | can be either] total enthalpy.
deltaH > 0 so products have lower temperature and higher enthalpy
The relationship yA P = xA p* is called _____________________________________________.
When analyzing adiabatic cooling processes using a psychrometric chart, the process must be on the constant _____________________________________________________ line.
wet bulb temperature
Heat is energy transfer due to ________________________________, whereas work can be of two types: - _________________________ work : associated with __________________________________ - _________________________ work : associated with __________________________________
temperature difference; work shaft-mechanical interactions; work flow-changes in P & V
The standard heat of vaporization of a substance is usually defined to be at what condition?
normal boiling point, 1 atm
The standard heat of combustion of a substance is defined to be at this condition:
25*C and 1 atm
Unit(s) of power: [Joule Watt kWh kJ/mol°C Joule/K Btu/°F horsepower Btu/day]
Watt, horsepower, Btu/day
[True | False] The internal energy of an ideal gas is independent of pressure.
A helium balloon is cooled down by dipping into an ice bath. Treating the balloon as the system, write the energy balance equation and identify which terms should be considered.
deltaU = W + Q
Wind turbines are used to obtain energy from wind. Write the energy balance equations and identify which variables are important and which are negligible.
deltaEk = Ws
Name three units of energy other than "Joule" (similar units like "kJ", "MJ" don't count).
Newton, kWh, Btu, cal, ergs
When water is contacted with humid air, will the air be humidified or dehumidified? Explain
It will be humidified if the humid air is not at its saturation point; if it is near saturation, the water will dehumidify
Briefly explain why the concept of enthalpy is useful or necessary, instead of just using internal energy.
Enthalpy is useful in energy balance for open systems, where there is flow work. We can write the energy balance using U, but then we have to write the flow work every time. Instead, we can combine the terms and just write H = U + PV.
A waterfall is used to produce electricity. Write an energy balance for the system. For each term in the balance, indicate whether the term is important or negligible.
For open system: ΔEp + ΔEk + ΔH = Q + Ws
ΔEp is important because the energy source is the large difference in height of water. ΔEk may or may not be important. If we write our system as having the water at the top as in and at the bottom as out, the velocities may be similar. ΔH and Q are likely negligible. Pressures are not likely to be different (if open to the atmosphere) and temperature is also not likely to change. There is probably not much heat transfer from or to the surroundings. Ws is important, being the term that says how much electricity is produced.
[True | False] A salt cannot be present in a solution above its saturation point because it will instantaneously precipitates out of solution.
False, won't instantaneously precipitate out
Raoult's law assumes what about the components in a mixture?
ideal gas/mixturem equilibrium and no interactions
If species A is more volatile than species B at a given temperature T and pressure P, which species has the higher boiling point at the P? Which one has the higher vapor pressure at that T?
B has higher boiling point; A has higher vapor pressure
A colleague claims that if you mix two species X and Y, the triple point of the mixture will surely be somewhere in between the triple point of X and the triple point of Y. What is triple point anyway? And what do you say about that claim?
Triple point is when all three phases coexist. This is not possible because a triple point is usually referring to a specific compound so unless they're very similar they wont have the same triple point
True or False (and explain why true or false). A vapor mixture of L and M, where L is less volatile and M is more volatile, is cooled down to a point where a small amount of liquid is formed. The liquid will be pure M.
False; it will be a mixture of M and L, but more L because it is less volatile
Between mercury (Hg) and oxygen, which one has a higher vapor pressure at 20°C? Explain your reasoning (without looking up or calculating the values)
O2 because it is a gas at this temperature
A solution that is saturated with a species contains the species at its [maximum | optimum | minimum] solubility.
For a component present in liquid in equilibrium with vapor, its vapor pressure is equal to its partial pressure in the vapor. True or False. Explain why True or why False.
False; if its not pure then yP=xp*
In a room containing humid air, will the relative humidity increase or decrease if we increase the pressure while maintaining temperature? Explain your answer briefly.
Relative humidity = relative saturation for water = ywater P / p
water(T) If temperature is constant, vapor pressure p
water is constant. Increasing pressure will increase the partial pressure of water in air (ywater P), which will increase the relative humidity. It's possible that increasing pressure too much will result in saturation (100% relative humidity) and water will condense; but even in this case, the relative humidity will not drop but stay at 100%.
[T/F] A process can be both continuous and unsteady state.
The atomic species balance in a steady state system with reactions can be simplified to in = out.
An exothermic reaction occurs inside an adiabatic reactor. Compared to the reactants, the products will have a
[higher | lower | same | can be either] temperature and a [higher | lower | same | can be either] total enthalpy.
higher temperature; lower enthalpy
Give a brief description of standard heat of formation.
The heat of reaction to form a compound from its elemental building blocks at 25*C ad 1 atm
The heat of combustion of CO2 (g) is 0; the heat of combustion of H2O (g) is not 0. Briefly explain why.
By convention, CO2(g) is one of the products of combustion reaction, therefore there is no heat of reaction to combust it.
H2O (l) is also a product by convention, but H2O (g) is not.
Specific volume, defined as volume per gram of a substance, is an extensive property.
A batch process is always a transient process
For the chemical reaction 3A + B -> 2C, the extent of reaction is independent of the species
In the production of ammonia, N2 + 3H2 -> 2NH3, 2mol/hr N2 and 5 mol/hr of H2 is fed to a reactor. In this case, N2 is the limiting reagent.
By definition, gauge pressure is zero when the absolute pressure is greater than the atmospheric pressure.
The chemical reactions A + 2B -> 5C and 2A + 2B -> are independent reactions.
Can argon at 25C and 25 bars be considered ideal gas?
No, pressure is too high
Under what circumstance is the following statement true? When a mixture of two species A & B in vapor phase gets chilled, the first dew drop formed is almost pure A.
B is much more violate than A and B has low solubility in A
If the vapor pressure of a substance (put into a system at 35C, 1 atm) is 0.5 atm, is the substance a liquid, gas, a mixture, or would it depend on something else?
a liquid because the vapor pressure it less than the system pressure
10 m3 (STP) of argon and 10 m3 (STP) of oxygen have the same ______
mole or number of molecules
Critical point of a substance is defined as
T and P were liquid and gas cannot be distinguished
A 60:40 (by mole) mixture of SiH4 and H2 are fed to a chemical vapor deposition reactor at a rate of 100
gmol/s. In the reactor, 75% of the SiH4 decomposes, depositing Si atoms onto a glass substrate. The remaining SiH4 and H2 continuously exit the reactor at a total molar rate of 140 gmol/s. Classify this process as
batch, semi-batch, or continuous and steady or unsteady state. Explain your reasoning.
semi-batch stay inside; unsteady 75% decomposes
Liquid acetone is kept in a sealed flask at temperature T. The vapor pressure of acetone at T is 0.25 bar.
The gas above the liquid contains air and acetone vapor at equilibrium and at a pressure of 1.25 bar. If
temperature is raised, the vapor mol fraction of acetone _________. Assume air pressure does not change significantly with T.
C. remains the same
D. none of the above
Water is in equilibrium at 50°C with air in a piston-cylinder system. The total pressure is 1 bar. The
pressure is raised to 2 bar by pushing down on the piston at constant temperature. At equilibrium, the
partial pressure of the water ____________.
B. remained the same
B. The partial pressure is equal to the saturation pressure, which does not
change at constant temperature.
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