Oxidation-Reduction Reactions (electron transfer reactions) (redox reactions)

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Presentation transcript:

Oxidation-Reduction Reactions (electron transfer reactions) (redox reactions)

Oxidation-Reduction Reactions (electron transfer reactions) 2Mg (s) + O 2 (g) 2MgO (s) 2Mg 2Mg e - O 2 + 4e - 2O 2- Oxidation half-reaction (lose e - ) Reduction half-reaction (gain e - ) 2Mg + O 2 + 4e - 2Mg O e - 2Mg + O 2 2MgO (redox reactions)

Oxidation-Reduction Reactions (electron transfer reactions) 2Mg (s) + O 2 (g) 2MgO (s) 2Mg 2Mg e - O 2 + 4e - 2O 2- Oxidation half-reaction (lose e - ) Reduction half-reaction (gain e - ) L E OG E R

Oxidation-Reduction Reactions (electron transfer reactions) 2Mg (s) + O 2 (g) 2MgO (s) 2Mg 2Mg e - O 2 + 4e - 2O 2- Oxidation half-reaction (lose e - ) Reduction half-reaction (gain e - ) Reducing agent causes oxidation Oxidizing agent causes reduction ? ? ? ?

Oxidation number The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred. 1.Free elements (uncombined state) have an oxidation number of zero. Na, Be, K, Pb, H 2, O 2, P 4 = 0 2.In monatomic ions, the oxidation number is equal to the charge on the ion. Li + = +1; Fe 3+ = +3; O 2- = -2 3.The oxidation number of oxygen is usually –2. In H 2 O 2 and O 2 2- it is –1. Don’t forget the free element rule for O 2.

4.The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds (hydride). In these cases, its oxidation number is –1. 6. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion. 5.Fluorine is always –1. (except elemental F2), Other halogens have negative numbers when occurring as halides (eg Cl - ). In oxoacids and oxoanions, they have positive numbers. 7. Oxidation numbers do not have to be integers (but they usually are in this class). One common example is the oxidation number of O = -1/2 in superoxide, O 2 -.

Figure 4.10 The oxidation numbers of elements in their compounds

Oxidation Number 2Mg (s) + O 2 (g) 2MgO (s) Rules: Elements = zero Sum of oxidation numbers = charge on molecule Sum of oxidation numbers reactants = sum of oxidation numbers products

NaIO 3 Na = +1 O = -2 3x(-2) ? = 0 I = +5 IF 7 F = -1 7x(-1) + ? = 0 I = +7 K 2 Cr 2 O 7 O = -2K = +1 7x(-2) + 2x(+1) + 2x(?) = 0 Cr = +6 Harder examples: ? ? ?