Exam 4A, Spring 2003
Exam 4A, Spring 20031. Complete and balance the following reactions.
a. manganese (II) hydroxide plus phosphoric acid
b. Ni(OH)2(s) + H2SO4(aq)
c. Cd2+(aq) + NH3(aq)
d. HNO3(aq) + SO2(g) → NO(g) + H2SO4(aq)
e. Co(OH)2(s) + ClO4–(aq) → Co(OH)3(s) + ClO3–(aq)
a. 3 Mn(OH)2(s) + 2 H3PO4(aq) → Mn3(PO4)2(s) + 6 H2O(l)
b. Ni(OH)2(s) + H2SO4(aq) → Ni2+(aq) + SO42–(aq) + 2 H2O(l)
c. Cd2+(aq) + 4 NH3(aq) → ← [Cd(NH3)4]2+(aq)
d. HNO3(aq) + SO2(g) → NO(g) + H2SO4(aq)
Oxidation: SO2(g) + 2 H2O(l) → H2SO4(aq) + 2 H+(aq) + 2 e–
Reduction: HNO3(aq) + 3 H+(aq) + 3 e– → NO(g) + 2 H2O(l)
Net: 2 HNO3(aq) + 3 SO2(g) + 2 H2O(l) → 2 NO(g) + 3 H2SO4(aq)
e. Co(OH)2(s) + ClO4–(aq) → Co(OH)3(s) + ClO3–(aq)
Oxidation: Co(OH)2(s) + H2O(l) → Co(OH)3(s) + H+(aq) + e–
Reduction: ClO4–(aq) + 2 H+(aq) + 2 e– → ClO3–(aq) + H2O(l)
Net in acid: 2 Co(OH)2(s) + ClO4–(aq) + H2O(l) → 2 Co(OH)3(s) + ClO3–(aq)
Neutralization: None
Net in base: 2 Co(OH)2(s) + ClO4–(aq) + H2O(l) → 2 Co(OH)3(s) + ClO3–(aq)
2. Look at the periodic trend for the enthalpies of formation and absolute entropies of the group 2 chlorides and hydroxides. Notice that the values for strontium hydroxide are missing. Make an estimate of the enthalpy of formation and the absolute entropy of strontium hydroxide, accurate to 10%.
For the enthalpies of formation, there is a large jump from magnesium chloride (–641 kJ/mole) to calcium chloride (–796 kJ/mole) but the changes are much smaller from calcium chloride to strontium chloride to barium chloride. The same trend appears to hold for the hydroxides, although all of the changes are much smaller. Thus, a reasonable estimate for the enthalpy of formation for strontium hydroxide is about half way between the values for calcium hydroxide and barium hydroxide, ~ –966 kJ/mole.
For the absolute entropies, the same trend holds, so a reasonable estimate for the absolute entropy of strontium hydroxide is ~ 95 J/mole·K.
3. Consider the reaction between solid barium hydroxide and aqueous hydrochloric acid at 25 °C.
a. Write the balanced reaction.
b. Find ΔH° for the reaction in units of kJ.
c. Find ΔS° for the reaction in units of J/K.
d. Without doing any calculations, would you expect the reaction to be spontaneous or nonspontaneous? Why?
a. Ba(OH)2(s) + 2 HCl(aq) → Ba2+(aq) + 2 Cl–(aq) + 2 H2O(l)
b. ΔH° = [(–537.6) + 2(–167.2) + 2(–285.8)] – [(–946.0) + 2(–167.2)] = –163.2 kJ/mol
c. ΔS° = [(9.6) + 2(56.5) + 2(69.91)] – [(107) + 2(56.48)] = 42 J/mol·K
d. Spontaneous because ΔH° is negative and ΔS° is positive.
4. Consider the zinc air cell: Zn(s) | Zn2+(aq), pH=2 || O2(g), pH=2 | Pt(s)
a. What is the balanced oxidation half-reaction?
b. What is the balanced reduction half-reaction?
c. What is the balanced net reaction?
d. What is the standard potential for the cell?
e. Is the reaction spontaneous or nonspontaneous? Why?
f. Does the cell potential increase of decrease as the pH increases? Explain.
a. Zn(s) → Zn2+(aq) + 2 e–
b. O2(g) + 4 H+(aq) + 4 e– → 2 H2O(l)
c. 2 Zn(s) + O2(g) + 4 H+(aq) → 2 Zn2+(aq) + 2 H2O(l)
d. E° = +0.763 + 1.229 = +1.992 V
e. Spontaneous, E° > 0.
f. H+ decreases with increasing pH. which will drive the reaction towards reactants, the nonspontaneous direction. Thus, the potential will become more negative, a decrease.