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Redox Reactions.

Redox Reactions. Reduction. Oxidation. Oxidation and Reduction. Oxidation : Gain of oxygen Loss of electrons. Reduction : Loss of oxygen Gain of electrons. Increase in oxidation number. Decrease in oxidation number. 4 Experiments:. Burning magnesium

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Redox Reactions.

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  1. Redox Reactions. Reduction Oxidation

  2. Oxidation and Reduction • Oxidation: • Gain of oxygen • Loss of electrons • Reduction: • Loss of oxygen • Gain of electrons Increase in oxidation number Decrease in oxidation number

  3. 4 Experiments: • Burning magnesium • Copper in silver nitrate solution • Chlorine solution and potassium iodide solution • Exploding hydrogen • Word equation • Balanced symbol equation

  4. Oxidised – gains oxygen 2Mg(s) + O2(g)  2MgO(s) Must be a redox! Mg  Mg2+ Oxidised – loss of e- +2e- Put the e- in. O  O2-CHARGE OF A - = GAINED ELECTRONSCHARGE OF A += LOST ELECTRONS Reduced – gain of e- +2e-

  5. Cu(s) + 2AgNO3(aq)  Cu(NO3 )2(aq) + 2Ag(s) Complete the half-equations Oxidised? Reduced? Cu  Cu2+ Oxidised – loss of e- +2e- Ag+ Ag Reduced – gain of e- +e-

  6. reduction H2(g) + ½ O2(g)H2O(g) Reducing agent oxidation Oxidising agent Covalent! No H+ or OH- Need a new definition.

  7. agents • Anoxidising agent isa substance that brings about oxidation(itself reduced) • example- hydrogen peroxide for bleaching hair • A reducing agent is a substance that brings about reduction.(itself oxidised)aa sulphur dioxide used to bleach straw

  8. In terms of oxidation number • Oxidation: • Gain of oxygen • Loss of electrons • Reduction: • Loss of oxygen • Gain of electrons Increase in oxidation number Decrease in oxidation number

  9. Oxidation Numbers- the seven rules • The oxidation number of an atom in an uncombined element is zero. E.g. Mg in Mg, O in O2. • The oxidation number of an ion of an element is the same as its charge. • O.N Br-= -1 • O.N Mg in Mg 2+ = +2 • ALKALI METALS= +1 • ALKALINE EARTH METALS=+2 • HALOGENS= -1

  10. Oxidation Numbers • The oxidation numbers of atoms in a compound add up to zero. Oxidation state of C in CO2? x – 4 = 0 x = +4 Put the +!

  11. Oxidation Numbers • The oxidation numbers of atoms in a compound add up to zero. Oxidation state of Mg in MgCl2? +2

  12. Oxidation Numbers • The oxidation numbers of atoms in a compound add up to zero. Oxidation state of N in NH3? -3

  13. Oxidation Numbers • The oxidation numbers of atoms in an ion add up to the charge on the ion. Oxidation state of S in SO42-? x – 8 = -2 x = +6

  14. Oxidation Numbers • The oxidation numbers of atoms in an ion add up to the charge on the ion. Oxidation state of S in S2-? -2

  15. Oxidation Numbers • The oxidation numbers of atoms in an ion add up to the charge on the ion. Oxidation state of N in NH4+? -3

  16. Oxygen has a charge Oxygen has a charge of –2 EXCEPT in peroxides where the charge is –1 In the compound OF2 Where it has a value of +2. This is because F has a greater electronegative number than oxygen Hyrogen has a charge of +1 except in Metal hydrides where it has an O.N of –1 Metal Hydrides are ionic compounds NaH (+1)(-1) OXYGEN HYDROGEN

  17. Halogens-assign charge of –1unless bonded to more electronegative elementCl2O(+1)2(-2)Cl= +1when writing formulas the most electronegative is placed second. • Halogen

  18. H2(g) + ½ O2(g)H2O(g) Covalent! Need a new definition. No H+ or OH-

  19. Oxidation: • Gain of oxygen • Loss of electrons • Reduction: • Loss of oxygen • Gain of electrons Increase in oxidation number Decrease in oxidation number

  20. H2(g) + ½ O2(g)H2O(g) H Covalent! +1 0 O 0 -2 No H+ or OH- Need a new definition.

  21. H2(g) + ½ O2(g)H2O(g) H +1 0 O 0 -2 Oxidised? Reduced? O – decrease in oxidation number H – increase in oxidation number

  22. Balancing Redox Reactions • Using Oxidation Numbers balance the following equation. • Solution: • 1 assign oxidation number • 2. Note element that changes oxidation number. • 3.show the number of electrons lost and gained.

  23. 4.Work out ratio of oxidising agent to reducing agent. • 5.Balance remaining items by inspection method.

  24. Worked Example MnO 4ֿ+Fe2+ +H+Mn2+ +Fe3+ +H2O (+7)4(-2) (+2)(+1) (+2) (+3) 2 (+1)(-2) MnO 4ֿ+Fe2+ +H+Mn2+ +Fe3+ +H2O (+7)4(-2) (+2)(+1) (+2) (+3) 2 (+1)(-2) ֿֿֿֿֿֿֿֿֿֿֿֿֿ GAINS 5 ELECTRONS LOSES 1 ELECTRON

  25. Balance remaining items • 1MnO 4ֿ:5Fe2+ • 1MnO 4ֿ+5Fe 2+ +H+ 1Mn 2+ +5Fe3+ +H2O • 1MnO 4ֿ+5Fe2+ +8H+ 1Mn2+ +5Fe3+ +4H2O

  26. Well done!

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