Manganese ions (Mn2+) present in water can be removed by oxidation with chlorine dioxide (ClO2) asshown in the balanced redox reaction below.Mn2+ (aq) + 2ClO2 (aq) + 2H2O (l) ⇌ MnO2 (s) + 2ClO2– (aq) + 4H+ (aq)The reaction is an overall second order reaction with the following rate law:Rate = k[Mn2+][ClO2], where k = 1 x 104 units at pH 7 and 25°C[Mn2+] and [ClO2] are in the units of concentration mol/L.(a) What is the units of the rate constant, k? (1 point)(b) Describe how (i) the reaction rate and (ii) rate constant, k, would change when the concentration ofchlorine dioxide is increased to twice the initial amount (with all other factors remaining the same).(2 points)Page 3 of 3RestrictedOzone is another chemical that can be used to react with manganese in water:Mn2+ (aq) + O3 (g) + 2H2O (l) ⇌ MnO2 (s) + O2 (g) + 2H+ (aq)(c) Based on the reaction equilibrium, should the reaction be carried out at low or high pH for theeffective removal of Mn2+ in water? Briefly explain why. (2 points)(d) Refer the table of Standard Reduction Potential given. Based on the Eo values, name two chlorinecontaining compounds that can be used to react with Mn2+ present in water. (2 points)Reduction Half-Reaction Eo (V)MnO4− (aq) + 8H+ (aq) + 5e− ⇌ Mn2+ (aq) + 4H2O (l) +1.49MnO2 (s) + 4H+ (aq) + 2e−⇌ Mn2+ (aq) + 2H2O (l) +1.21HOCl (aq) + H+ (aq) + e− ⇌ Cl2 (g) + H2O (l) +1.63ClO3− (aq) + 6H+ (aq) + 5e− ⇌ ½Cl2 (g) + 3H2O (l) +1.47ClO4− (aq) + 2H+ (aq) + 2e− ⇌ ClO3− (aq) + H2O (l) +1.19Hg2Cl2 (s) + 2e− ⇌ 2Hg (l) +2 Cl− (aq) +0.27O3 (g) + H2O (l) + 2e− ⇌ O2 (g) +2OH− (aq) +1.24(e) In practice, one of the comparts in Part (d) does not removed manganese (Mn) from watereffectively. Which compound is it and why is it ineffective in the removal of manganese fromwater? (2 points)(f) Mn2+ is reacted to form MnO2 in the reaction shown above. Name one method in which MnO2 canbe removed from water. (1 point)
Question
Manganese ions (Mn2+) present in water can be removed by oxidation with chlorine dioxide (ClO2) asshown in the balanced redox reaction below.Mn2+ (aq) + 2ClO2 (aq) + 2H2O (l) ⇌ MnO2 (s) + 2ClO2– (aq) + 4H+ (aq)The reaction is an overall second order reaction with the following rate law:Rate = k[Mn2+][ClO2], where k = 1 x 104 units at pH 7 and 25°C[Mn2+] and [ClO2] are in the units of concentration mol/L.(a) What is the units of the rate constant, k? (1 point)(b) Describe how (i) the reaction rate and (ii) rate constant, k, would change when the concentration ofchlorine dioxide is increased to twice the initial amount (with all other factors remaining the same).(2 points)Page 3 of 3RestrictedOzone is another chemical that can be used to react with manganese in water:Mn2+ (aq) + O3 (g) + 2H2O (l) ⇌ MnO2 (s) + O2 (g) + 2H+ (aq)(c) Based on the reaction equilibrium, should the reaction be carried out at low or high pH for theeffective removal of Mn2+ in water? Briefly explain why. (2 points)(d) Refer the table of Standard Reduction Potential given. Based on the Eo values, name two chlorinecontaining compounds that can be used to react with Mn2+ present in water. (2 points)Reduction Half-Reaction Eo (V)MnO4− (aq) + 8H+ (aq) + 5e− ⇌ Mn2+ (aq) + 4H2O (l) +1.49MnO2 (s) + 4H+ (aq) + 2e−⇌ Mn2+ (aq) + 2H2O (l) +1.21HOCl (aq) + H+ (aq) + e− ⇌ Cl2 (g) + H2O (l) +1.63ClO3− (aq) + 6H+ (aq) + 5e− ⇌ ½Cl2 (g) + 3H2O (l) +1.47ClO4− (aq) + 2H+ (aq) + 2e− ⇌ ClO3− (aq) + H2O (l) +1.19Hg2Cl2 (s) + 2e− ⇌ 2Hg (l) +2 Cl− (aq) +0.27O3 (g) + H2O (l) + 2e− ⇌ O2 (g) +2OH− (aq) +1.24(e) In practice, one of the comparts in Part (d) does not removed manganese (Mn) from watereffectively. Which compound is it and why is it ineffective in the removal of manganese fromwater? (2 points)(f) Mn2+ is reacted to form MnO2 in the reaction shown above. Name one method in which MnO2 canbe removed from water. (1 point)
Solution
(a) The units of the rate constant, k, can be determined by rearranging the rate law equation to solve for k. The rate law is given as Rate = k[Mn2+][ClO2]. The units of rate are mol/(Ls), and the units of concentration are mol/L. Therefore, the units of k are (mol/(Ls))/(mol/L)^2 = L/(mol*s).
(b) (i) If the concentration of chlorine dioxide is doubled, the reaction rate will also double. This is because the reaction is second order, meaning the rate is directly proportional to the concentration of the reactants. (ii) The rate constant, k, would not change when the concentration of chlorine dioxide is increased. The rate constant is a measure of the inherent reactivity of the reactants and is not affected by changes in concentration.
(c) The reaction should be carried out at low pH for the effective removal of Mn2+ in water. This is because the reaction produces H+ ions, which would shift the equilibrium to the left according to Le Chatelier's principle if the pH is high (meaning [H+] is high). At low pH, the equilibrium will shift to the right, favoring the formation of MnO2 and thus the removal of Mn2+.
(d) Based on the Eo values, the two chlorine-containing compounds that can react with Mn2+ present in water are HOCl and ClO3−. These compounds have higher reduction potentials than MnO2, meaning they are more likely to be reduced (and thus to oxidize Mn2+ to MnO2).
(e) ClO3− is the compound that does not remove manganese effectively from water. This is because its reduction potential is lower than that of MnO2, meaning it is less likely to be reduced and thus less likely to oxidize Mn2+ to MnO2.
(f) MnO2 can be removed from water through filtration. The solid MnO2 particles can be trapped in a filter and thus separated from the water.
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