Suggested Homework Set 3

BCH 341 – ASU ONLINE – iCourse – Spring 2018

J.P. Allen, BioPhysical Chemistry, Wiley-Blackwell, 2008. (free PDF version for ASU students)

Chapter 5: problems 6, 7, 12, 13 & 18.

Additional Problems

mix_2gases3-1A: Two gasses are initially in two separate flasks at the same temperature and pressure.  A valve in the tube connecting the two flasks is then opened to alllow complete mixing of the gases.  Derive an equation for the change in energy for mixing the two gases.  Given that this type of process is commonly done under constant temperature and pressure conditions, the Gibbs free energy (G) is the form of energy often most helpful.  Hence, when ‘deriving an equation for the change in energy’, it is most useful to focus on deriving an equation for the change in the Gibbs free energy (ΔG) for mixing the two gases.

3-2A: One hundred milligrams of solid dinitrogen tetroxide (N2O4 (s)) is placed in the bottom of a glass tube and placed under vacuum (more precisely, an active vacuum of 1 mbar), while the sample is kept cold by placing the bottom of the tube in liquid nitrogen.  The glass tube is flame sealed to create a closed container sample of solid dinitrogen tetroxide.  The end result is an evacuated sealed glass tube with one gram of a clear white solid in the bottom of the tube .   The internal volume of the flame sealed glass tube is 10 ml (20 mm inner diameter tube with a length of around 32 mm).  If the tube is removed from the liquid nitrogen bath and allowed to warm up to room temperature (25°C), the solid appears to melt as the sample gets warmer and further vaporizes (boils) as it approaches room temperature.  Given the know melting and boiling points (-11°C and 22°C, respectively) of dinitrogen tetroxide, this is expected.  What is unusual, however, is that the white solid power becomes more and more redish/brownish in color as it liquifies and vaporizes.  Example images of the starting and ending sealed glass tubes are shown below. Using publically available thermodynamic data (for example, from NIST online), predict the final chemical and physical state of the system. So, now explain why the tube turned color during this process and predict the pressure within the sealed tube (is the seal tube safe to handle at room temperature?).


3-3A: A 15 liter volume of gas at room temperature and atmospheric pressure is compressed isothermally to a final volume of 5 liters. What is the change in Gibbs free energy for this process? (or in general what is the change in all the various types of energy associated with this process)

3-4A: As accurately as possible, determine the molar entropy change when liquid water at standard pressure and temperature is taken isobarically to steam (vapor)  at 450F (top end of most home stoves or ovens). (B) How much of difference is there in the calculated molar entropy change (ΔS) when using the explicit temperature dependence heat capacities, versus just taking the values at room temperature and 450F and assuming temperature independent heat capacities?

3-5A: Calculate the equilibrium constant at room temperature for the decarboxylation of pyruvic acid.

3-6A: Over the last few years, cannabis (marijunana) seems to be a growing social and medical topic.  The active pharmaceutical ingredient (API) in cannabis plant and associated extracts is the general class of chemical compounds called cannabinoids.  The most notable cannabinoid is the phytocannabinoid tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis.  The majority of cannabinoids produced from the cannabis plant are found in a carboxylic acid form.  This includes THC, which is actually found as tetrahydrocannabinolic acid (THCA) in living cannabis plants.  The process of going from THCA to THC is decarboxylation.  Calculate the equilibrium constant at room temperature for the decarboxylation of THCA.

3-7A: What are the concentrations of all species in a 150 mM acetic acid (vinegar) solution? (B) What are the concentrations of all species in a 150 mM sodium acetate solution? (i.e., aqueous solutions)

3-8A: How would you make 1 liter of a pH 5.0 buffer solutions from sodium acetate and acetic acid? (and water, obviously)

3-9A: What is the pH of pure water at human body temperature?

3-10A: What are the concentrations of all species in a 150 mM solution of histidine at pH 7.

Brief Answer Set: BCH341_Hwk3_Brief_Answers_Spring2018.pdf

** Students can get a lot of benefit from making general spreadsheets to do the computation for these problems.  This will make the take home exam much easier and take much less time to complete.  Also, with the ability to share documents and spreadsheets, this is an excellent way for students to work together and help each other.

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