CHAPTER21CHEM

CHAPTER21CHEM - CH.21 Electrochemistry: Chemical Change and...

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CH.21 Electrochemistry: Chemical Change and Electrical Work 21.1 Redox Reactions and Electrochemical cells Whether an electrochemical process releases or absorbs free energy, it always involves the movement of electrons from one chemical species to another in an oxidation-reduction (redox) reaction. Oxidation-reduction reaction: a process in which there is a net movement of e- from one reactant (reducing agent) to another (oxidation agent). Oxidation: loss of electrons ; Reduction: gain of electrons Oxidizing agent: one that does the oxidizing, taking e- from the substance being oxidized. Reducing agent: one that does the reducing, giving e- to the substance being reduced. After reaction: oxidized substance has a higher (more pos. or less neg.) oxidation number (O.N), and the reduced substance has a lower (less pos. or more neg.) O.N. 1. Oxidation (take away e-) always accompanies reduction (gain e-). 2. The oxidizing agent is reduced (gains e- when it takes e- away from substance being oxidized), and the reducing agent is oxidized (loses e- when it gives e- to substance being reduced). 3. The total number of e- gained by the atoms/ions of the oxidizing agent always equals the total number lost by the atoms/ions of the reducing agent. Half-Reaction Method for Balancing Redox Reactions: Half-reaction method: divides the overall redox reaction into oxidation and reduction half-reactions. Begin w/a “skeleton” ionic reaction, which shows only the species that are oxidized and reduced. If the oxidized form of a species is on the left side of the skeleton reaction, the reduced form of that species must be on right side (and vice versa). H2O, H+, and OH- do not appear in skeleton reaction unless they are being oxidized or reduced: 1. Divide skeleton reaction into 2 half-reactions, each of which contains the oxidized and reduced forms of 1 of the species. 2. Balance the atoms and charges in each half-reaction: -atoms are balanced in order: atoms other than O and H, then O, then H. -charge is balanced by adding electrons: added to the left in the reduction half-reaction b/c the reactant gains them; added to the right in the oxidation half-reaction b/c the reactant loses them. 3. If needed, multiply one or both half-reactions by an integer to make the # of e- gained in reduction equal to # lost in oxidation. 4. Add the balanced half-reactions, canceling substances that appear on both sides, and include states of matter. Check that atoms and charges are balanced. Half reaction is: reduction if e- appear on left (reactant is oxidizing agent), oxidation if e- appear on right (reactant is reducing agent). For redox reactions in : 1. acidic solution: H2O molecules and H+ (or H3O+) are available for balancing. 2.
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CHAPTER21CHEM - CH.21 Electrochemistry: Chemical Change and...

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