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OXIDATION – REDUCTION REACTIONS

OXIDATION – REDUCTION REACTIONS. Pp 258 – 261 + Chapter 18 in book HW: Read pp 258 – 261 several times. Answer active reading Q on p. 258 Do p. 261 practice problem 8.5. OXIDATION. Originally referred to oxygen combining with other elements to form oxides

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OXIDATION – REDUCTION REACTIONS

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  1. OXIDATION – REDUCTION REACTIONS • Pp 258 – 261 + Chapter 18 in book • HW: Read pp 258 – 261 several times. • Answer active reading Q on p. 258 • Do p. 261 practice problem 8.5

  2. OXIDATION • Originally referred to oxygen combining with other elements to form oxides • Principal sources of energy involve oxidation – combustion of gas, food metabolism in cells, etc. • Oxidation now has a broader meaning – refers to more than just “burning” and forming oxides • Bleaching and rusting are oxidation processes

  3. REDUCTION • Opposite of oxidation • Originally meant the loss of oxygen • Iron ore is reduced in the production of metallic iron by heating it with charcoal– oxygen is removed from the ore

  4. REDOX REACTIONS • Oxidation and reduction occur simultaneously • Can’t have one without the other! • When producing iron from iron ore, the removed oxygen combines with carbon and forms carbon dioxide.

  5. REDOX TODAY • Includes many reactions that do not involve oxygen • Oxidation is redefined as complete or partial loss of electrons OR gain of oxygen • Reduction is complete or partial gain of electrons OR loss of oxygen

  6. REDOX REACTIONS All redox reactions are also classified in other ways: ie, single replacement, combustion, etc. The opposite is not always true Example: some double replacement rxs are not redox – why? Electrons are not transferred

  7. LEO the lion goes GER

  8. Oxidation Complete loss of e- ionic reactions Shift of e- away from atom in covalent bond Gain of oxygen Loss of hydrogen by a covalent compound Increase in oxidation number Reduction Complete gain of e- ionic reactions Shift of e- toward an atom in a covalent bond Loss of oxygen Gain of hydrogen by a covalent compound Decrease in oxidation number Processes leading to oxidation and reduction

  9. Oxidation Numbers • Positive or negative number assigned to an atom and is equal to the ionic charge • Ex. Br1- has an oxidation number of -1, Fe3+ has an oxidation number of +3 • Hydrogen has a +1 charge except in metal hydrides it is -1 (NaH) • Uncombined atoms have an oxidation number of 0 • For neutral compounds sum of oxidation numbers is 0

  10. Changing oxidation numbers • In polyatomic ions, the sum of the oxidation numbers is equal to the charge on the ion. • EX. SO32- Oxygen always (almost) has an oxidation number of -2 so when 3 oxygens bond with 1 sulfur to form a sulfite ion, the oxidation number of sulfur must be +4. • What is sulfur’s oxidation number in SO42-?

  11. More Changing!! • Determine the oxidation number for sulfur in each of the following: • H2S Na2SO4 • S2Cl2 • Determine the oxidation number for the bold element in each of the following: • NaClO4 AlPO4 AsO43-

  12. Oxidation changes in reactions • Increase in oxidation # indicates oxidation • Decrease in oxidation # indicates reduction • Use oxidation numbers to determine what is oxidized and what is reduced +1 +5 -2 0 +2 +5 -2 0 • 2AgNO3 + Cu  Cu(NO3)2 + 2Ag

  13. Using oxidation # • Use oxidation numbers in an equation to determine what is oxidized and reduced 0 +1-1 +1-1 0 • Cl2 + 2HBr  2HCl + Br2 • Chlorine is reduced – oxidation # 0 to -1 • Bromine is oxidized – oxidation # -1 to 0

  14. Identifying Redox Reactions • Any type of reaction may be redox • Assign oxidation numbers to all elements to see if there is a transfer of electrons • If no change, not redox • If there is a change, is redox

  15. Balancing Redox Reactions half reaction method • Write oxidation states • Decide which elements are being oxidized and reduced • Write ½ reactions, bringing full terms containing oxidized and reduced elements • Balance all elements except O & H in normal way. • Balance O’s by adding H2O to appropriate side • Balance H’s by adding H+’s to appropriate side • Balance charge by adding e-’s to appropriate side • Multiply 1/ rxs by an integer to equalize e-’s • Combine, cancel, rewrite & check

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