Western Michigan University Chemistry Worksheet

1) Calculate the molar volume of reaction in units of cm3 at 298 K and 1 bar for the reaction forming the mineral sphalerite from zinc and sulfur in their reference forms. Use the thermodynamic database of Robie et al. (1979). Provide answer in 3 significant figures.

2)Write a balanced chemical reaction in which red litharge (PbO) reacts with hydrogen sulfide gas to yield galena and water. (1 pt)

Using your reaction, calculate the log fugacity (partial pressure in atmospheres) of hydrogen sulfide gas that would exist in equilibrium with water and the minerals litharge (PbO) and galena (PbS) at 900 K. (1 pt) HINT: Remember to use appropriate units of the gas constant. Assume activities equal concentrations.

What is the molar entropy of reaction for this reaction at 900 K, in J/mol-K? (1 pt)

3 In the 1973 paper, Mueller and Kridelbaugh [Icarus 19(4), pp 531-541] considered how geochemical reactions on the planet Venus might help to explain why the world has an atmosphere of 96.5% CO2 at 93 bars of pressure at the surface.

Mueller and Kridelbaugh (1973) argued that it is possible for a reaction between wollastonite in mafic igneous rocks and atmospheric carbon dioxide to produce calcite and quartz, effectively buffering the CO2 fugacity via what amounts to an ongoing metamorphic reaction with surface basalt.

Write a balanced chemical equation for this potential geochemical reaction on Venus. (1 pt)

Use your equation and the Robie and Hemingway (1995) database to determine what the equilibrium atmospheric pressure would be, in bars, at 700 K if wollastonite-calcite-quartz buffering alone accounted for the air composition. (1 pt)

Discussion: How does your result compare with the real fugacity of carbon dioxide in Venusian air? Is wollastonite-calcite-quartz buffering alone reasonably capable of maintaining atmospheric conditions at Venus in their current state? Why or why not?

4) Chemical weathering of primary igneous or metamorphic minerals by groundwater tends to produce weathering products like clays. An example of this type of reaction would be the aqueous weathering reaction converting muscovite into kaolinite in the presence of water, producing K+ as a dissolved weathering product in solution.

Write a balanced chemical reaction for muscovite reacting with water to produce kaolinite and the dissolved potassium ion (K+), balancing charge with H+ and oxygen with H2O. (1 pt)

Determine the equilibrium constant for this reaction at 298.15 K. HINT: Liquid water is on page 16 of R&H1995. It's kind of hard to find otherwise. (1 pt)

Using your weathering reaction for muscovite altering to kaolinite and releasing K+, determine the aqueous concentration (assume concentration = activity) of dissolved K+ in water at equilibrium (298.15 K) in mmol/L (millimoles/L). (1 pt)

5) Using your results and data from the previous question regarding muscovite weathering to produce kaolinite and dissolved K+, determine what the equilibrium concentration of K+ should be in the presence of muscovite and kaolinite at 298.15 under pH conditions of 2 to 10.

Plot these results on a diagram of pH (x-axis) versus molar (mol/L) concentration of K+ (y-axis), with proper labeling and easy-to-understand format and notation. Attach the plot in your answer or upload it to your OneDrive and provide a link. (2 pts)

Brief discussion: What might account for low natural K+ concentrations (low ppm or micromolar range) in rock containing primary muscovite and secondary kaolinite formed by chemical weathering, regardless of naturally acidic pH in some settings? (1 pt)

6)For the activity coefficient you determined in the previous problem, calculate what the activity of dissolved Nd3+ would be in our hypothetical aqueous solution for which you've already calculated the ionic strength, given an aqueous Nd3+ concentration of 3 ng/g (ppb) in solution. please provide units