Oxidation Numbers For example: ClO 4 - So, X Cl + 4 X O = -1 X Cl + 4 (-2) = -1 m X A + n X B = +/- q A m B n q +/- X Cl = -1 - 4 (-2) 9 Oxidation-Reduction Reactions Fe ( s ) + Cu 2+ ( aq ) → Fe 2+ ( aq ) + Cu ( s ) Coating of iron nail with copper: 0 0 +2 You can see that iron loses 2 electrons to form Fe 2+ and the copper ion gains 2 electrons to form elemental Cu. The transfer of electrons indicates that this is an oxidation-reduction (redox) reaction . When an atom gains electrons (or its oxidation number decreases) it is said to be reduced . When an atom losses electrons (or its oxidation number increases) it is said to be oxidized . Fe → Fe 2+ + 2 e- Cu 2+ + 2 e - → Cu +2 10 Oxidation-Reduction Reactions An oxidation-reduction reaction is described as having two parts (two half-reactions ). Describing Redox Reactions Oxidation is the half-reaction that involves the loss of electrons ( oxidation number increases ). Reduction is the half-reaction that involves the gain of electrons ( oxidation number decreases ). Oxidation-Reduction Reactions 11 Describing Redox Reactions Fe (s) + Cu 2+ (aq) → Fe 2+ (aq) + Cu (s) 0 0 +2 +2 oxidation reduction + 2e - Cu 2+ ( aq ) → Cu ( s ) and the reduction half-reaction is The oxidation half-reaction is Fe ( s ) → Fe 2+ ( aq ) + 2e - When adding and balancing red-ox reactions, the electrons must cancel on each side of the equation. 12 Balancing Oxidation-Reduction Reactions • Balance all atoms except O and H. • Balance O atoms by adding H 2 O’s • Balance H atoms by adding H + ions • Balance electric charge by adding electrons (e - ) to the more positive side. Assign oxidation numbers to each atom. 1 . Split the skeleton equation into two half-reactions (one for oxidation and one for reduction). 2 . Complete and balance both half-reactions 3 .
3 13 Balancing Oxidation-Reduction Reactions Combine half-reactions to acquire balanced equation. 4 . •Multiply each half-reaction by some factor that will cancel all electrons when the half-reactions are added. Simplify overall reaction. 5 . •Cancel species that occur on both sides of the reaction. •Reduce the coefficients to their smallest whole numbers by multiplying all coefficients by some common factor. 14 Balancing Oxidation-Reduction Reactions Additional steps for reactions in basic solutions Note the number of H + ions in the equation and add this number of OH - ions to both sides of the equation. 6 . Simplify the equation by noting that H + reacts with OH - to give H 2 O and reduce equation to simplest terms. 7 . & 5. Balancing Oxidation-Reduction Reactions 15 Oxidation-Reduction Reaction in Acidic Solution + 2 e - + 2 H 2 O + 2 e - Balance the following reaction in acidic solution. Br 2 ( l ) + SO 2 ( g ) Br - ( aq ) + SO 4 2- ( aq ) 0 -1 -2 -2 1 . +6 +4 (reduction half-reaction) Br 2 Br - (oxidation half-reaction) SO 2 SO 4 2- 2 . 4 . + + 4 H + Br - 3 . SO 2 SO 4 2- Br 2 2 SO 2( g ) + 2 H 2 O ( l ) + Br 2( l ) SO 4 2- ( aq ) + 4 H + ( aq ) + 2 Br - ( aq ) Balancing Oxidation-Reduction Reactions 16 Oxidation-Reduction Reaction in Basic Solution + H 2 O + 2 e - + e - Balance the following reaction in basic solution.
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