Electrochemistry

Oxidation-Reduction 2Na (s) + Cl2 (g)  2NaCl (s) What happened to each of the reactants in this situation? Neutral atoms became ions by the transferring of electrons, this is a Redox Reaction (Oxidation-Reduction)

Oxidation-Reduction The concept of Oxidation states provides a way to keep track of electrons during Redox reactions Oxidation states: in a covalent compound it is defined as the imaginary charges the atoms would have if the shared electrons were divided equally between identical atoms bonded to each other, or for different atoms, electrons are assigned to the atom in each bond that has the greater attraction for electrons Example: H2O

Oxidation-Reduction The oxidation state of… Summary Examples Is it in its elemental form? Element:0 Na (s) , O2 (g) A monatomic ion is the same as its charge Monatomic ion: charge of ion Na+ , Cl- Fluorine is -1 in its compounds Fluorine: -1 HF, PF3 Oxygen is usually -2 in its compounds (exception of peroxides H2O2, and ozoneO3 Both are -1) Oxygen: -2 H2O, CO2 Hydrogen is +1 in its covalent compounds Hydrogen: +1 H2O, HCl, NH3

Oxidation and Reduction GER Gain of Electrons is Reduction LEO Loss of Electrons is Oxidation

Oxidation-Reduction CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g) -4 +1 +4 +1 -2 -2 Oxidation is an increase in oxidation state (a loss of electrons) (Oil) C is oxidized and CH4 is considered a reducing agent Reduction is a decrease in oxidation state (a gain of electrons) (Rig) O is reduced and O2 is considered an oxidizing agent

Balancing Redox Reactions It is often difficult to balance Redox reactions by inspection alone , therefore a special technique called Half Reaction Method is used. A half reaction can be written for both the oxidized and reduced substances Ce4+ (aq) + Sn2+ (aq)  Ce3+ (aq) + Sn4+ (aq) Reduction Half Reaction: Ce4+ (aq)  Ce3+ (aq) Oxidation Half Reaction: Sn2+ (aq)  Sn4+ (aq)

Balancing Redox Reactions In Acidic Solutions: 1. Write separate equations for the oxidation and reduction half reactions 2. For each half reaction: a. balance all elements except H and O b. balance O using H2O c. balance H using H+ d. balance charge using e- 3. If necessary, multiply one or both balanced half reactions to equalize the number of e- transferred 4. Add half reactions, cancel identical species 5. Check that elements and charges are balanced

Balancing Redox Reactions In Basic Solutions: 1. Use the half reaction method for acidic solutions to obtain the final balanced reaction as if H+ ions were present 2. To both sides add OH- ions equal to the number of H+ ions 3. Form H2O on each side, cancel the equal number of H2O molecules that form 4. Check that the elements and charges are balanced

Voltaic cells 2Ag1+ + Cu(s) 2Ag(s) + Cu 2+ Electrons are transferred from one half-cell to the other using an external metal conductor. Ag Cu Ag+ Cu2+