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Oxidation Numbers

Oxidation Numbers. & Balancing equation. Oxidation Number. Oxidation number is defined as The charge an atom has Or appears to have When electrons are distributed according to certain rules. Oxidation Number Rules. The oxidation number of an Element is 0 group One elements is +1

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Oxidation Numbers

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  1. Oxidation Numbers & Balancing equation

  2. Oxidation Number • Oxidation number is defined as • The charge an atom has • Or appears to have • When electrons are distributed • according to certain rules

  3. Oxidation Number Rules • The oxidation number of • an Element is 0 • group One elements is +1 • group Two elements is +2 in compounds

  4. The oxidation number of an ion is equal to the charge on the ion • halogens is -1 (in binary compounds) (except ……????)

  5. The oxidation number of H in a compound is +1 • except in metal hydrides when it is -1

  6. The oxidation number of O in a compoundis -2 • except (x2) in peroxides when it is -1 (H2O2) in OF2 when it is +2 (why?)

  7. Oxidation numbers • add up to zero in a compound • add up to the charge of a complex ion

  8. What is the oxidation number of each element in :- (write down before you go on) H20 MnO4¯ I2 KBrO3 Na2S2O3 H2O2 NaOCl

  9. The oxidation number of each element is :- H20 MnO4¯ I2 KBrO3 Na2S2O3 H2O2 NaOCl +1 -2 +7 -2 0 +1 +5 -2 +1 +2 -2 +1 -1 +1 -2 +1

  10. Learning Check • Can I give the oxidation number RULE for • Oxygen • Hydrogen • free element • Neutral atom (sum) • Ion (simple and complex) • Group 1 element • Group 2 element • HALOGEN STILL NOT The End - click to go on

  11. Balancing Equations with oxidation numbers STEPS • Assign oxidation numbers • Identify what is oxidised and reduced • Write halfequation SIDE by SIDE for each (showing number of electrons on the move for one atom of each) • Rewrite for the number of atoms given e.g. Cr2 • Balance the electrons • REWRITE the original equation using these “prefixes” • Balance remainder by inspection • CHECK – do the charges on each side cancel out??

  12. Example Assign & Identify Cr2O72- + Fe2+ + H+Cr3+ + Fe3+ + H20 +6 -2+2+1+3+3+1 -2 Oxidised x1 Reduced x3

  13. 2 Identifyoxidised or reduced & number of electrons lost or gained per atom & as given Reduced Oxidised Cr + 3e-Cr Fe – e- Fe ATOM Cr2+ 6e-2Cr Fe – e- Fe GIVEN • Balance Electrons Cr2+ 6e-2Cr Fe – e-Fe • Rewrite and sub back Cr2O72- + 6Fe2+ + H+ 2Cr3+ + 6Fe3+ + H20 • Balance remainder by inspection Cr2O72- + 6Fe2++ H+ 2Cr3+ + 6Fe3++ H20 6 6 6 14 7

  14. Check • Charges on each side should balance • Cr2O72- + 6Fe2++ 14 H+2Cr3+ + 6Fe3++ 7 H20 • LEFT RIGHT 2- 6+ 12+ 18+ 14+ ___ 24+24+

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