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Redox 2: Oxidation Numbers Redox reactions are all about electrons being transferred from one substance to another, so it would be useful if we had a system for keeping track of what gains and what loses electrons, and how many electrons are involved. We do - our record-keeping system is called Oxidation Numbers Oxidation Numbers A positive or negative number assigned to an atom in a molecule or ion that reflects a partial gain or loss of electrons What connections do you make with other chemistry concepts you have studied? Knowing the oxidation number of each individual element in a molecule will be a key step in our understanding of redox reactions. Common oxidation states can be found in Appendix A pg 856 Merrill Chemistry. Fortunately there are a series of rules that we can use to determine oxidation numbers: Example(s)/Explanation Rules: 1. Pure elements have an oxidation number of 0 2. If the compound is an ionic compound, the oxidation number for each element is the ion’s charge 3. The oxidation number of hydrogen in a compound is +1 4. The oxidation number of oxygen in most compounds is –2 (peroxides are the exception; in peroxides oxygen has an oxidation number of –1) 5. The sum of the oxidation numbers in a compound is zero. 6. The sum of the oxidation numbers in a polyatomic ion is equal to the ion charge. It is important to note that oxidation number always refers to each individual atom in the compound, not to the total for that element. For example in H2O - the total positive "charge" for both hydrogen atoms will be +2 (which balances with the -2 from oxygen), but EACH HYDROGEN has an oxidation number of +1. Practice Assigning Oxidation Numbers Oxidation Numbers for each Element a. SnCl4 Sn Cl b. Ca3P2 Ca P c. SnO Sn O d. Ag2S Ag S e. HI H I f. N2H4 N H g. Al2O3 Al O h. S8 S i. HNO2 H j. O2 O k. H3O+ H O l. ClO3- Cl O m. S2O32- S O n. KMnO4 K Mn O o. (NH4)2SO4 N H S N O 2. Determine the oxidation number of carbon in each of the following compounds: a. methane, CH4 b. formaldehyde, CH2O c. carbon monoxide, CO d. carbon dioxide, CO2 O