Sticking with the topic of sulfides, consider the kinetics of the following elem

Question

Sticking with the topic of sulfides, consider the kinetics of the following elementary reaction: H_2S HS + H^+ with rate constants k_f and k_b for the forward and back reactions, respectively. a. Write the overall rate equation for this reaction. b. If k_1 = 10^3 s^-1, and K_a1 = 10^-7 M, what are the value and units of k_a? What are the "orders" of k_f and k_b? And what is the "order" of the overall rate equation? c. If the pH = 7, what are the concentrations of H_2S and HS if the [S]_T = 10^-4 M? d. What are the forward and reverse exchange rates at this pH and S_T concentration at equilibrium? e. If a large amount of "marsh gas" (methane; CH_4) were to bubble out of the sediments, and thereby strip much of the gaseous H_2S out of the water, how long would the sulfide system take to return to equilibrium? f. If the pH of this water were to drop to 5, what would happen to the rates of the forward and back reactions? And what would happen to the relaxation time of the sulfide system?

Explanation / Answer

pH = -log[H+]

7 = -log[H+]

[H+] = 10-7 M

So according to reaction: [H2S] = [HS-] = [H+] = 10-7 M

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