We examine other important transition-type metals. Zinc is amphoteric, mercury is the only liquid metal at room temperature, and chromium and manganese are central to redox titrations.
Basic Knowledge
Zinc Zn (Group 12, behaves like a typical element): An amphoteric metal. Zn + 2HCl → ZnCl2 + H2, Zn + 2NaOH + 2H2O → Na2[Zn(OH)4] + H2. Used in galvanized iron (Zn-plated steel sheets), dry cells, and brass.
Mercury Hg: The only metal that is liquid at room temperature. Hg(I) is Hg22+ (a dimeric ion), and Hg(II) is Hg2+. Amalgam (a Hg alloy) was traditionally used in dental restoration, but organic mercury (methylmercury) causes Minamata disease. Its use is now strictly restricted.
Chromium Cr: Oxidation states +2/+3/+6. Cr2O3 is a green pigment, and Cr(VI) is a strong oxidizing agent and carcinogen. Chromate K2CrO4 (yellow) and dichromate K2Cr2O7 (orange) convert into each other depending on pH.
Manganese Mn: Oxidation states +2 to +7. Potassium permanganate KMnO4 is a red-purple strong oxidizing agent. Under acidic conditions: MnO4- + 8H+ + 5e- → Mn2+ + 4H2O (colorless). Under neutral to basic conditions, reduction stops at MnO2 (brown).
Amphoteric metals (review) (Al, Zn, Sn, Pb = “Ah, smoothly” mnemonic)
Amalgam (Hg alloy, historically used in dentistry and gold extraction)
Potassium permanganate KMnO4 (red-purple; reagent for redox titration)
Potassium dichromate K2Cr2O7 (orange; oxidizing agent)
Chromium plating (corrosion protection and decoration; transition from hexavalent to trivalent chromium is under way)
Galvanized iron/tinplate (Zn-plated steel sheets/Sn-plated steel sheets)
Further Study (Principles and Applications)
Permanganate titration: KMnO4 (red-purple) becomes Mn2+ (colorless), so no indicator is necessary (self-indicating). It is used to determine Fe2+, oxalic acid, and H2O2. 2KMnO4 + 5H2C2O4 + 3H2SO4 → K2SO4 + 2MnSO4 + 10CO2 + 8H2O.
Dichromate titration is used to determine Fe2+. An indicator (diphenylamine sulfonate) is added.
Regarding toxicity: hexavalent chromium Cr(VI) is carcinogenic and causes skin ulcers, so industrial wastewater standards are strict. It was the subject of the film “Erin Brockovich.” Mercury binds strongly to proteins and methylmercury accumulates in the central nervous system, causing Minamata disease. Cadmium (in the zinc group) is also known for bone-accumulating toxicity and Itai-itai disease.
- Zn is amphoteric and dissolves in both HCl and NaOH
- Hg is liquid at room temperature and forms amalgams
- K2CrO4 is yellow and K2Cr2O7 is orange; they are in pH-dependent equilibrium
- KMnO4 is red-purple and becomes colorless Mn2+ in acid
- Permanganate titration is self-indicating
- Cr(VI) is carcinogenic; Hg and Cd cause pollution-related diseases
- Galvanized iron = Zn plating (sacrificial protection), tinplate = Sn plating
Points to Note (Common Confusions and Frequent Errors)
① Galvanized iron (Zn plating) uses a metal less noble than iron. Even if the coating is scratched and exposed, Zn dissolves first and protects the iron by sacrificial protection. Tinplate (Sn plating) uses a metal more noble than iron, so when scratched, corrosion of the exposed iron progresses instead. ② Zn(OH)2 dissolves in both NaOH and aqueous NH3 (it is amphoteric and forms complexes). ③ The reduction product of MnO4- depends on pH: acidic, Mn2+ (colorless); neutral to weakly basic, MnO2 (brown); strongly basic, MnO42- (green).
Practice
- Write the equations for dissolving Zn in dilute HCl and dilute NaOH, respectively.
- Write the equation for titrating oxalic acid with potassium permanganate.
- Explain the difference in the corrosion-prevention principles of galvanized iron and tinplate.