A closed system at equilibrium

QUESTION 2 [6 marks]
The following defines a closed system at equilibrium
Recipe: Species: Reactions:
[NaHCO3]T = 2 × 10-3 M; [NaHS]T = 5 × 10-4 M H2O, H+, OH-, Na+, H2CO3, HCO3-, CO32-, H2S, HS-, S2- H2CO3 = HCO3- + H+ HCO3- = CO32- + H+
H2S=HS- +H+ HS- =S2- +H+
K = 10-6.3 K = 10-10.3
K = 10-13.9
K = 10-14.0
H2O = H+ + OH-
a) Write all the mass law and mole balance equations
b) Sketch the logC-pH diagram (on a graph paper or using Excel) for the system (2)
c) Find the pH at equilibrium. Briefly justify your assumption(s) (2)
d) Calculate the alkalinity of the system (1)
QUESTION 3 [3 marks]
Monochloramine (NH2Cl) is commonly used as an alternative disinfectant to free chlorine for disinfecting drinking water. This is because monochloramine forms less disinfection by-products (DBPs). It also lasts much longer in the distribution system than free chlorine. Monochloramine is formed when dissolved ammonia (NH3(aq)) reacts with hypochlorous acid (HOCl)
NH3(aq) + HOCl → NH2Cl + H2O
with the second order-rate constant, k, and follows the second-order rate law d[NH2Cl] = ?[NH3(aq)][HOCl]
a) Write an expression for the rate of formation of monochloramine in terms of total NH3(aq)
and total HOCl, that is,
[HOCl]T = [HOCl] + [OCl-] and assuming pseudo-equilibrium for acid-base reactions:
NH4+ = NH3(aq) + H+
HOCl = OCl- + H+
KNH4+ = 10-9.3
KHOCl = 10-7.5
d[NH2Cl] = ?obs[NH3(aq)]T[HOCl]T dt
[NH3(aq)]T = [NH3(aq)] + [NH4+]
b) How does kobs change with changing pH when (1) pH < 7.5, (2) 7.5 < pH < 9.3, (3) pH > 9.3? In what pH range would the fastest rate of monochloramine be expected? (It is interesting to note that while the true rate constant, k, has remained unchanged, the observed rate constant, kobs, depends strongly on pH). (1.5)
c) (OPTIONAL) It is desirable to maximize the rate of formation of monochloramine. Find the pH value at which the maximum kobs is expected. (bonus 1 mark)

Sample Solution