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Assigning Oxidation Numbers Rules & Half-Rxn 8 Rules for Oxidation Numbers 1. of a free, uncombined element = 0. Na He O2 N2 S8 Cl2 2. of a monatomic ion = charge on ion. +2 -1 +3 Ca = +2. Cl = -1. Al P = +3. Remember: Ions occur in ionic compounds: CaCl2, Al(NO3)3, etc. 8 Rules for Oxidation Numbers 3. Fluorine is always -1. CF4 4. Hydrogen is nearly always +1, except when it’s bonded to a metal. Then it’s -1. H2O, HNO3, H2SO4 LiH CaH2 NaH 8 Rules for Oxidation Numbers 5. Oxygen is nearly always -2 except when it’s… -Bonded to fluorine, where O is +2 OF2 -In the peroxide ion, where O is -1. eg. H2O2 and Na2O2 O22- 8 Rules for Oxidation Numbers 6. The sum of oxidation numbers in a neutral compound is 0. H2O CO2 NO SO3 7. The sum of oxidation numbers in a polyatomic ion = charge of the ion. Sum in SO42- = -2. Sum in NO3- = -1. 8 Rules for Oxidation Numbers 8. In covalent compounds, the oxidation number of the more electronegative atom is the negative charge it would have if it was an ion. *NH3: N = -3, H = +1. SiCl4: Si = +4, Cl = -1. +4 +3 +2 +1 0 -1 -2 -3 -4 2) And if you’re lucky you strike oil & it shoots up 1) You dig down with an oil rig Assign Oxidation Nos • • • • • • • • • KCl K = +1, Cl = -1 CaBr2 Ca = +2, Br = -1 CO C = +2, O = -2 C = +4, O = -2 CO2 Al(NO3)3 Al = +3, O = -2, N = +5 Na3PO4 Na = +1, O = -2, P = +5 H2S H = +1, S = -2 NH4+1 N = -3, H = +1 SO3-2 S = +4, O = -2 Electrons are Negative! • Why do we use the word “reduced” when electrons are gained? Look at how the oxidation number changes. For example, if Cl gains an electron it becomes Cl-1. The oxidation number decreased from 0 to -1. The oxidation number was reduced. Agents • When an element is OXIDIZED it caused something else to be reduced so it is called the REDUCING AGENT. • Likewise, when an element is REDUCED it caused something else to be oxidized so it is called the OXIDIZING AGENT. Non-metals metals Vocabulary Interlude • Oxidizing Agent: Is itself reduced. Accepts electrons from something else – aids oxidation for another species. • Reducing Agent: Is itself oxidized. • Loses electrons to something else – aids reduction for another species. Reduction Half-Reactions Electrons are gained so • I2 + 2e- 2Ithey are like a reactant! • O2 + 4e- 2O-2 • Half-reactions must demonstrate conservation of mass & conservation of charge. • # of atoms of each element on LHS equals “ “ “ “ “ “ “ RHS. • Total charge on LHS = Total charge on RHS Oxidation Half-reactions • K K1+ + 1e• Fe2+ Fe3+ + 1e• Cu Cu2+ + 2e- Electrons are lost so they appear on the product side! • Total Charge on LHS = Total Charge on RHS • # atoms LHS = # atoms RHS