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Oxidation and Reduction Or, “Do you know where your electrons are?”
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Definitions Oxidation is the process of losing electrons (oxidation state becomes more positive) Na Na + + 1e - Reduction is the process of gaining electrons (oxidation state becomes more negative) Cl + 1e - Cl -
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Definitions L osing E lectrons O xidation goes G aining E lectrons R eduction
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Definitions O xidation I s L osing R eduction I s G aining
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Oxidation state Charge on an ion Na +, Ca +2, O -2 The number of electrons unequally shared in a covalent bond. H 2 O : H is +1, O is -2
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Oxidation state assignment rules Any element has oxidation number of zero Oxygen has an oxidation number of -2, except in peroxides where it is -1 Hydrogen is +1 except in hydrides, where it is -1 – in HCl the H is +1, but in NaH it is -1 Nitrogen is -3 except with oxygen
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Oxidation state assignment rules Halogens are -1 except with oxygen or each other All other oxidation numbers are assigned so that the sum of all the oxidation numbers equals the charge on the particle. In examples not covered here the atom with greater electronegativity gets the negative charge.
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Oxidation state assignment rules NH 3 H= +1, N= -3 NI 3 N= -3, I = +1
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Oxidation state assignment rules NF 3 N= +3, F= -1 H3O+H3O+ H= +1, O= -2
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Oxidation state assignment rules NO 3 - O= -2, N= +5 Cr 2 O 7 -2 O= -2, Cr= +6
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Redox reaction Any reaction that results in a change of oxidation state for any reactant. N 2 + 3H 2 2NH 3 0 3Cu + 8HNO 3 3Cu(NO 3 ) 2 + 2NO + 4H 2 O 0 0-3,+1 +5+2
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Redox Reaction 2Fe + 3CuSO 4 3Cu + Fe 2 (SO 4 ) 3 0 +2 0 +3 Oxidizing agent – the reactant that is reduced C + O 2 CO 2 Oxygen is reduced (0 to -2), so it is the oxidizing agent
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Oxidizing and reducing agents RR educing agent – the reactant that is oxidized 33 H 2 + 2Cr +3 6H + + 2Cr HH ydrogen is oxidized (0 to +1), so it is the reducing agent EE xample: Identify the oxidizing and reducing agents in the following reaction: 22 HCl + Zn ZnCl 2 + H 2 ZZ n – reducing agent HH + – oxidizing agent
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Redox and electronegativity C + O 2 CO 2 Carbon is oxidized because it has lost some electron density to oxygen, which has greater electronegativity. Oxygen is reduced because it gained some electron density from carbon
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Balancing redox equations Charge Balance Redox is a transfer of electrons, so the number of electrons lost by the reducing agent = number of electrons gained by oxidizing agent Total charge of reactants must = total charge of products Cr +6 + Fe +2 Cr +3 + Fe +3 Even though the atoms are balanced, the charge is not.
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Balancing redox equations Oxidation number method: Identify all changes in oxidation number Cr +6 + Fe +2 Cr +3 + Fe +3 -3 +1
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Balancing redox equations Use coefficients to make the changes cancel Cr +6 + Fe +2 Cr +3 + Fe +3 -3+1x3 = +3 33
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Balancing redox equations Check charge balance Cr +6 + 3Fe +2 Cr +3 + 3Fe +3 +12 +12 +5 +3 +2 +5 HNO 3 + H 3 AsO 3 NO + H 3 AsO 4 + H 2 O -3 +2 Use least common multiple – 6 2HNO 3 + 3H 3 AsO 3 2NO + 3H 3 AsO 4 + H 2 O
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Balancing Redox Equations Half reactions method Every redox reaction consists of two half reactions Fe + Cu +2 Fe +3 + Cu oxidation Fe Fe +3 + 3e - reduction Cu +2 + 2e - Cu Oxidation and reduction reactions always happen in pairs
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Balancing Redox Equations Sum of appropriate numbers of half reactions yields a balanced equation – use coefficients to make # electrons lost = # electrons gained 2(Fe Fe +3 + 3e - ) + 2(Cu +2 + 2e - Cu) = 2Fe + 3Cu +2 2Fe +3 + 3Cu
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Balancing Redox Equations AA toms and electrons have to balance II f the electrons balance, the charge will also balance (but be sure to check it!) CC u + HNO 3 Cu(NO 3 ) 2 + NO 2 + H 2 O OO xidation: Cu Cu +2 + 2e - RR eduction: NO 3 - + 1e - NO 2
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Reduction half reaction must be balanced – in acid solution use 2H + and H 2 O for each missing oxygen 2H + + NO 3 - + 1e - NO 2 + H 2 O Number of electrons in oxidation and reduction must be equal Add half reactions to get balanced equation
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Balancing Redox Equations 2(2H + + NO 3 - + 1e - NO 2 + H 2 O) Cu Cu +2 + 2e - 4H + +2NO 3 - +2e - +Cu Cu +2 +2e - +2NO 2 +2H 2 O Electrons cancel; addition of nitrates to each side (spectators) gives overall equation 4HNO 3 +Cu Cu(NO 3 ) 2 +2NO 2 +2H 2 O
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Balancing Redox Example #2 Zn + VO 3 - Zn +2 + VO +2 (in acid solution) Half reactions: Oxidation: Zn Zn +2 + 2e - VO 3 - V is +5, VO +2 V is +4 Reduction: VO 3 - + 1e - VO +2
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Balancing Redox Example #2 balance with H + and H 2 O 2(4H + + VO 3 - + 1e - VO +2 + 2H 2 O) Balanced equation is sum of half reactions 8H + +2VO 3 - +Zn 2VO +2 +4H 2 O+Zn +2
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Balancing in Base Solution Use 2OH - and H 2 O for each missing oxygen Cr(OH) 3 + ClO 3 - CrO 4 2- + Cl - Oxidation Cr(OH) 3 CrO 4 -2 + 3e - + 3OH - Hydroxides are added to balance hydrogens. Balance oxygen (four missing on left) with 2OH - /H 2 O.
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Balancing in Base Solution 88 OH - + Cr(OH) 3 CrO 4 -2 + 3e - + 3OH - + 4H 2 O CC ancel hydroxides on both sides. 55 OH - + Cr(OH) 3 CrO 4 -2 + 3e - + 4H 2 O RR eduction: CC lO 3 - + 6e - Cl - BB alance oxygen (three missing on right) with 2OH - /H 2 O.
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Balancing Redox in Base Solution 3H 2 O + ClO 3 - + 6e - Cl - + 6OH - Add equations and eliminate spectators 2[5OH - + Cr(OH) 3 CrO 4 -2 + 3e - + 4H 2 O] 3H 2 O + ClO 3 - + 6e - Cl - + 6OH - 10OH - + 2Cr(OH) 3 + 3H 2 O + ClO 3 - 2CrO 4 -2 + 8H 2 O + Cl - + 6OH - 4 4OH - + 2Cr(OH) 3 + ClO 3 - 2CrO 4 -2 + 5H 2 O + Cl - 5
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