06-G2-Electrochemistry

06-G2-Electrochemistry - Section 6 Electrochemistry Recall...

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Electrochemistry Section 6 Recall… Oxidation ≡ Loss of electrons (oxidation state more +) Reduction ≡ Gain of electrons (oxidation state more −) e.g ., Zn (s) in CuSO 4 (aq) Zn (s) + Cu 2+ (aq) + SO 4 2− (aq) Zn 2+ (aq) + Cu (s) + SO 4 2− (aq) Net reaction… Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s) REDOX
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Oxidation Numbers Oxidation number for an ion : the charge on the ion. Oxidation number for an atom : the hypothetical charge that atom would have if it was an ion. Oxidation numbers are assigned by a series of rules: 1. If the atom is in its elemental form, the oxidation number is zero. E.g., Cl 2 , H 2 , P 4 . 2. For a monoatomic ion, the charge on the ion is the oxidation state. Oxidation-Reduction Oxidation-Reduction Reactions Reactions
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Oxidation Numbers 3. Nonmetal usually have negative oxidation numbers: a) Oxidation number of O is usually –2. The peroxide ion, O 2 2- , has oxygen with an oxidation number of –1. b) Oxidation number of H is +1 when bonded to nonmetals and –1 when bonded to metals. c) The oxidation number of F is –1. 3. The sum of the oxidation numbers for the atom is the charge on the molecule (zero for a neutral molecule). Oxidation-Reduction Oxidation-Reduction Reactions Reactions
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Assign Oxidation Numbers Assign oxidation numbers to the underlined elements: NaCl C O 2 S O 4 -2 Cl F 3
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Assign Oxidation Numbers Assign oxidation numbers to the underlined elements: NaCl -1 C O 2 +4 S O 4 -2 +6 Cl F 3 +3
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Law of conservation of mass : the amount of each element present at the beginning of the reaction must be present at the end. Conservation of charge : electrons are not lost in a chemical reaction. Half Reactions Half-reactions are a convenient way of separating oxidation and reduction reactions. Balancing Oxidation- Balancing Oxidation- Reduction Reactions Reduction Reactions
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Half Reactions The half-reactions for Sn 2+ ( aq ) + 2Fe 3+ ( aq ) Sn 4+ ( aq ) + 2Fe 3+ ( aq ) are Sn 2+ ( aq ) Sn 4+ ( aq ) +2e - 2Fe 3+ ( aq ) + 2e - 2Fe 2+ ( aq ) Oxidation: electrons are products. Reduction: electrons are reagents. Balancing Oxidation- Balancing Oxidation- Reduction Reactions Reduction Reactions
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Balancing Equations by the Method of Half Reactions • Consider the titration of an acidic solution of Na 2 C 2 O 4 (sodium oxalate, colorless) with KMnO 4 (deep purple). • MnO 4 - is reduced to Mn 2+ (pale pink) while the C 2 O 4 2- is oxidized to CO 2 . The equivalence point is given by the presence of a pale pink color. • If more KMnO 4 is added, the solution turns purple due to the excess KMnO 4 . Balancing Oxidation- Balancing Oxidation- Reduction Reactions Reduction Reactions
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Balancing Equations by the Method of Half Reactions 1. Write down the two half reactions. 2. Balance each half reaction:
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06-G2-Electrochemistry - Section 6 Electrochemistry Recall...

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