QUESTION 1 [2 marks]
The standard molar free energies for hypochlorous acid (HOCl), proton (H
+
), and hypochlorite
(OCl-
) are:
?HOCl
0 = −79.9 kJ/mol ?H+ (aq)
0 = 0 kJ/mol ?OCl−
0 = −36.8 kJ/mol
a) Calculate ΔG0
values for the dissociation of hypochlorous acid to hypochlorite (1)
HOCl = H
- + OClb) Calculate the equilibrium constant for HOCl dissociation (Ka) at 25 oC. (1)
QUESTION 2 [6 marks]
The following defines a closed system at equilibrium
Recipe: [NaHCO3]T = 2 × 10-3 M; [NaHS]T = 5 × 10-4 M
Species: H2O, H+
, OH-
, Na+
, H2CO3
*
, HCO3
, CO3
2-
, H2S, HS-
, S2-
Reactions: H2CO3
- = HCO3
- + H+ K = 10-6.3
HCO3 - = CO3
2- + H+ K = 10-10.3
H2S = HS- + H+ K = 10-7
HS- = S2- + H+ K = 10-13.9
H2O = H+ + OH- K = 10-14.0
a) Write all the mass law and mole balance equations (1)
2
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 (CH2Cl) 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]
dt = ?[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, (1.5)
d[NH2Cl]
dt = ?obs[NH3(aq)]T[HOCl]T
where
[NH3(aq)]T = [NH3(aq)] + [NH4
+
]
[HOCl]T = [HOCl] + [OCl-
]
and assuming pseudo-equilibrium for acid-base reactions:
NH4 - = NH3(aq) + H+ KNH4+ = 10-9.3
HOCl = OCl- + H+ KHOCl = 10-7.5
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)
3
QUESTION 4 [4 marks]
Stream A, which is contaminated by acid mine drainage, mixes with Stream B, an unpolluted
stream, in a 1:1 volume ratio as shown below. Before mixing, both streams are at equilibrium with
the atmospheres.
Sample Solution