Advanced Geochemistry of Natural Waters: Lesson 6 - Redox Reactions and Balancing, Assignments of Hydrogeology

Download Advanced Geochemistry of Natural Waters: Lesson 6 - Redox Reactions and Balancing and more Assignments Hydrogeology in PDF only on Docsity! 1 GEOL578 Advanced Geochemistry of Natural Waters Start Audio Lecture! LESSON 6 Oxidation-Reduction MODULE 1 Definitions, Assigning Valences and Balancing Redox Reactions 2 GEOL578: Advanced Geochemistry of Natural Waters DEFINITIONS • Oxidation: Loss of electrons. • Reduction: Gain of electrons. • Reductant: Species that loses electrons. • Oxidant: Species that gains electrons. • Valence: the electrical charge an atom would acquire if it formed ions in aqueous solution. 3 GEOL578: Advanced Geochemistry of Natural Waters RULES FOR ASSIGNMENT OF VALENCES 1) The valence of all pure elements is zero. 2) The valence of H is +1, except in hydrides, where it is -1. 3) The valence of O is -2, except in peroxides, where it is -1. 4) The algebraic sum of valences must equal zero for a neutral molecule or the charge on a complex ion. 2 4 GEOL578: Advanced Geochemistry of Natural Waters VARIABLE VALENCE ELEMENTS • Sulfur: SO42-(+6), SO32-(+4), S(0), FeS2(-1), H2S(-2) • Carbon: CO2(+4), C(0), CH4(-4) • Nitrogen: NO3-(+5), NO2-(+3), NO(+2), N2O(+1), N2(0), NH3(-3) • Iron: Fe2O3(+3), FeO(+2), Fe(0) • Manganese: MnO4-(+7), MnO2(+4), Mn2O3(+3), MnO(+2), Mn(0) • Copper: CuO(+2), Cu2O(+1), Cu(0) • Tin: SnO2(+4), Sn2+(+2), Sn(0) • Uranium: UO22+(+6), UO2(+4), U(0) • Arsenic: H3AsO40(+5), H3AsO30(+3), As(0), AsH3(-1) • Chromium: CrO42-(+6), Cr2O3(+3), Cr(0) • Gold: AuCl4-(+3), Au(CN)2-(+1), Au(0) 5 GEOL578: Advanced Geochemistry of Natural Waters BALANCING OVERALL REDOX REACTIONS Example - balance the redox reaction below: Fe + Cl2 ↔ Fe3+ + Cl- Step 1: Assign valences, Fe(0) + Cl2(0) ↔ Fe3+ + 2Cl- Step 2: Determine number of electrons lost or gained by reactants. Fe(0) + Cl2(0) ↔ Fe3+ + 2Cl- ↓ ↑ 3e- 2e- Step 3: Cross multiply. 2Fe + 3Cl20 ↔ 2Fe3+ + 6Cl- 6 GEOL578: Advanced Geochemistry of Natural Waters HALF-CELL REACTIONS The overall reaction: 2Fe + 3Cl20 ↔ 2Fe3+ + 6Cl- may be written as the sum of two half-cell reactions: 2Fe ↔ 2Fe3+ + 6e- (oxidation) 3Cl20 + 6e- ↔ 6Cl- (reduction) All overall redox reactions can be expressed as the sum of two half-cell reactions, one a reduction and one an oxidation.