In a reaction, the side that “oxidizes the other substance” is the oxidizing agent, and the side that “reduces the other substance” is the reducing agent. For metals, their relative strengths can be understood from the ionization tendency.
Basic Knowledge
Representative oxidizing agents (take electrons):
・KMnO4 (powerful in an acidic sulfate solution)
・K2Cr2O7
・Concentrated HNO3, dilute HNO3, hot concentrated H2SO4
・Cl2, O3, H2O2 (normally)
Representative reducing agents (give electrons):
・H2S, SO2 (normally)
・KI, FeSO4
・(COOH)2 (oxalic acid)
・H2O2 (acts as a reducing agent toward strong oxidizing agents such as KMnO4)
Ionization tendency of metals (memorization aid: K Ca Na Mg Al Zn Fe Ni Sn Pb (H) Cu Hg Ag Pt Au): The farther left, the more easily the metal is ionized (= oxidized), and the stronger it is as a reducing agent.
Oxidizing agent（A substance that receives electrons and is itself reduced）
Reducing agent（A substance that gives electrons and is itself oxidized）
Half-reaction equation（An equation showing the electron transfer of an oxidizing agent or reducing agent separately）
Ionization tendency（The order of the tendency of metals to become positive ions）
Ionization series（K>Ca>Na>Mg>Al>Zn>Fe>Ni>Sn>Pb>(H)>Cu>Hg>Ag>Pt>Au）
Dual nature of H2O2（Normally an oxidizing agent, but acts as a reducing agent toward strong oxidizing agents）
Further Study (Background・Significance)
Examples of half-reaction equations for oxidizing and reducing agents:
・KMnO4 (in an acidic sulfate solution): MnO4- + 8H+ + 5e- → Mn2+ + 4H2O (deep purple → nearly colorless)
・H2O2 (as an oxidizing agent): H2O2 + 2H+ + 2e- → 2H2O
・(COOH)2 (as a reducing agent): (COOH)2 → 2CO2 + 2H+ + 2e-
Applications of ionization tendency:
・Metals that react with water: K, Ca, Na (water at room temperature), Mg (hot water), Al, Zn, Fe (high-temperature steam)
・Metals that react with acids: those with an ionization tendency greater than H
・Cu, Hg, and Ag react only with strongly oxidizing acids (HNO3, hot concentrated H2SO4)
・Pt and Au react only with aqua regia (concentrated HNO3:concentrated HCl = 1:3)
- Representative oxidizing agents: KMnO4, K2Cr2O7, HNO3, hot concentrated H2SO4
- Representative reducing agents: H2S, SO2, KI, FeSO4, (COOH)2
- H2O2 has a dual role (normally an oxidizing agent, but a reducing agent toward strong oxidizers)
- Ionization series: K Ca Na Mg Al Zn Fe Ni Sn Pb (H) Cu Hg Ag Pt Au
- Greater ionization tendency = stronger reducing agent = greater reactivity
- Metals above H dissolve in dilute acids while producing hydrogen
- Pt and Au react only with aqua regia
Points to Note (Easily Confused)
① H2O2 and SO2 have dual roles (depending on the conditions, they can act as either oxidizing or reducing agents). ② The products of concentrated and dilute nitric acid differ (concentrated: NO2, dilute: NO). ③ Cu and Ag do not dissolve in dilute hydrochloric or dilute sulfuric acid (the oxidizing power is insufficient). They dissolve in HNO3 or hot concentrated H2SO4. ④ Al, Fe, and Ni become passive in concentrated nitric acid, causing the reaction to stop.
Practice
- Write the half-reaction equation when KMnO4 acts as an oxidizing agent in an acidic sulfate solution.
- Which has the greater ionization tendency, Cu or Zn?
- What is the special acid that dissolves gold and platinum called?