CHEM 161: Aqueous Reactions and Redox Problems
1. In alkaline dry-cell batteries, the half-reactions involve Zn and manganese compounds. Complete
and balance the following reaction in basic solution:
Zn(s) + MnO2(s) → ZnO(s) + Mn2O3(s)
steps i-iv.
2 OH
0
-
+2 -2
+ H2O + Zn → ZnO + 2 e- + 2 H+ + 2 OH2 H2O
-
+
2 OH + 2 H + 2 e
2 H2O
-
+4 -2
+3
+ 2 MnO2 → Mn2O3 + H2O + 2 OH-
2 OH- + Zn + H2O + 2 MnO2 →
step v:
overall reaction:
-2
Zn(s) + 2 MnO2(s)
→
ZnO + H2O + Mn2O3 + 2 OHZnO(s) + Mn2O3(s)
2. Although copper is less active than hydrogen, copper reacts with nitric acid because nitrate ion can
act as a strong oxidizing agent in acidic solution. Complete and balance the following reaction (in
acidic solution):
Cu(s) + HNO3(aq) → Cu2+(aq) + NO2(g)
0
+2
Cu → Cu2+ + 2 e-
steps i-iii.
+
-
2( H + 1e +
+1 +5 -2
+4 -2
HNO3 → NO2 + H2O )
x2 to balance e-
step v:
Cu + 2 H+ + 2 HNO3 → Cu2+ + 2 NO2 + 2 H2O
overall:
Cu(l) + 2 H+(aq) + 2 HNO3(aq) → Cu2+(aq) +2 NO2(g) + 2 H2O(l)
3. Calculate the concentration of the ions that remain in solution after 50.0 mL of a 1.25M sodium
phosphate solution is mixed with 75.0 mL of a 2.00M calcium nitrate solution.
2 Na3PO4(aq) + 3 Ca(NO3)2(aq) → Ca3(PO4)2(s) + 6 NaNO3(aq) 0.0500L
0.0750L
1.25molNa PO
L
3molCa NO
2molNa PO
2.00molCa NO
L
0.09375molCa NO
→
0.150 mol Ca NO
Na3PO4 aq isL.R.
→ AllPO4‐3reactstoform
theCa3 PO4 2 ppt
0.150 molCa2 atstart
–0.09375 molCa2 react
0.05625molCa2 remain
Ca
.
→ PO4‐3
.
.
.
NO3–isaspectatorion→AllNO3–ionsstillinsolution:
0.0500L
1.25molNa PO
L
.
0M
.
0.0625molNa PO .
Na isaspectatorion→AllNa ionsstillinsolution:
.
.
4. A student pipetted 25.00 mL of phosphoric acid to a 125 mL Erlenmeyer flask and titrated the
solution with potassium hydroxide.
a. If 26.09 mL of potassium hydroxide solution was required for complete neutralization to a
phenolphthalein endpoint, calculate the molarity of the potassium hydroxide solution. The
concentration of phosphoric acid was 0.3816M.
H3PO4(aq) + 3 KOH(aq) → K3PO4(aq) + 6 H2O(l) 0.02500L
0.3816 mol H PO
L
KOH
3 mol KOH
1molH PO
0.02862molKOH
0.02609 L
0.02862molKOH
.
b. If the student added 50.0 mL of deionized water to the flask described in part a after the acid was
transferred but before the titration began, calculate the molarity of the ions remaining in solution
in the flask at the endpoint.
Attheendpoint correspondingtotheequivalencepoint ,#molofH3PO4 #molKOH,
soalltheOH–ionsreactwiththeH’sintheH3PO4andtheonlyionsremaininginsolution
aretheK andPO43‐ions.Vtotal 25.00 26.06 50.00mL 101.09mL
0.02500L
K
0.02862molK
0.10109L
.
0.3816molH PO
L
PO
0.009540molH PO
0.009540mol PO
0.10109 L
.
5. Mixing household ammonia with chlorine bleach can be dangerous because ammonia can react with
hypochlorite ion, ClO, the active ingredient in bleach, to form hydrazine, N2H4, a toxic gas.
Complete and balance the following reaction in basic solution:
ClO(aq) + NH3(g) → N2H4(g) + Cl(aq)
steps i-iv.
-
2 OH + 2 H
2 H2O
+
+ 2e
-
2 OH
-
+1
-1
-
+ ClO → Cl- + H2O + 2 OH-3 +1
-2 +1
+ 2 NH3 → N2H4 + 2 e- + 2 H+ + 2 OH2 H2O
H2O + ClO- + 2 OH- + 2 NH3 → Cl- + 2 OH- + N2H4 + 2 H2O
step v:
ClO-(aq) + 2 NH3(g) →
overall reaction:
Cl-(aq) + N2H4(g) + H2O(l)
6. Complete and balance the following reaction (in basic solution) that occurs in nickel-cadmium
(NiCad) batteries:
Cd(s) + NiO2(s) → Cd(OH)2(s) + Ni(OH)2(s)
-
steps i-iv.
2 OH +
-
+
0
2 H2O + Cd → Cd(OH)2 + 2 e- + 2 H+ + 2 OH2 H2O
-
2 OH + 2 H + 2 e +
2 H2O
step v:
+2 -2 +1
+4 -2
+2 -2 +1
NiO2 → Ni(OH)2 + 2 OH-
2 OH- + Cd + 2 H2O + NiO2 → Cd(OH)2 +
Cd(s) + 2 H2O(l) + NiO2(s)
overall reaction:
→
Ni(OH)2 + 2 OH-
Cd(OH)2(s) + Ni(OH)2(s)
7. Limestone is a mixture of calcium carbonate and other substances. A 1.356 g sample of limestone
rock is pulverized then analyzed with a standard solution of hydrobromic acid. The sample required
25.87 mL of a 0.750M hydrobromic acid solution for complete neutralization to a phenolphthalein
endpoint. Calculate the mass percent concentration of calcium carbonate in the limestone.
CaCO3(s) + 2 HBr(aq) → CO2(g) + H2O(l) + CaBr2(aq) 0.02587L
0.750molHBr
L
%CaCO
1molCaCO
2molHBr
0.970998gCaCO
1.356glimestone
100.09gCaCO
molCaCO
100%
0.970998gCaCO
71.6%CaCO