Transcript Slide 1
Chapter 20 Oxidation-Reduction Reactions Anything in black letters = write it in your notes (‘knowts’) 20.1 – The Meaning of Oxidation and Reduction Original definitions, still useful… Oxidation Combining with oxygen Reduction Loss of oxygen Oxidation cannot occur without reduction. Circle the substance that is being oxidized CH4(g) + 4Fe(s) 2O2(g) → + 3O2(g) CO2(g) + 2H2O(g) → 2Fe2O3(s) 2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2(g) Oxidation Reduction Combining with oxygen Loss of oxygen Modern definitions, more useful… Oxidation Loss of electrons Reduction Gain of electrons Helpful acronym… ‘LEO the lion says GER’ ger Loss of Electrons is Oxidation Gain of Electrons is Reduction A substance can be oxidized without reacting with oxygen. Mg(s) + S(s) heat MgS(s) Mg is oxidized because it lost 2 e- Oxidation Reduction Loss of electrons Gain of electrons Oxidizing Agent – causes another substance to be oxidized. Reducing Agent – causes another substance to be reduced. Oxidizing Agent 2C + O2 2CO Reducing Agent Carbon is oxidized, oxygen is reduced. The oxidizing agent will always be reduced Circle the oxidizing agent 2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s) 20.2 – Oxidation Numbers This section is really a review… Oxidation Number – charge on an ion. Elements & compounds have an oxidation # of zero. 0 0 +2 -1 Mg + Cl2 MgCl2 Mg0 Mg+2 + 2e- Loss of e- = oxidation Cl20 + 2e- 2Cl-1 Gain of e- = reduction These should make sense…don’t write them all down! Rules for Assigning Oxidation Numbers 1. The oxidation number of a monatomic ion is equal in magnitude and sign to its ionic charge. For example, the oxidation number of the bromide ion (Br1–) is –1; that of the Fe3+ ion is +3. 2. The oxidation number of hydrogen in a compound is +1, except in metal hydrides, such as NaH, where it is –1. 3. The oxidation number of oxygen in a compound is –2, except in peroxides, such as H2O2, where it is –1, and in compounds with the more electronegative fluorine, where it is positive. 4. The oxidation number of an atom in uncombined (elemental) form is 0. For example, the oxidation number of the potassium atoms in potassium metal (K) or of the nitrogen atoms in nitrogen gas (N2) is 0. 5. For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal 0. 6. For a polyatomic ion, the sum of the oxidation numbers must equal the ionic charge of the ion. What is the oxidation number of each kind of atom in the following ions and compounds? a. SO2 b. Na2SO4 c. CO32– d. Cu2S ASSIGNMENT: Chapter 20 Worksheet #1 20.3 – Describing Redox Rxns Not covered…