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Recap

Recap. Precipitation Reactions: ions combine to form insoluble products Neutralization Reactions: H + ions and OH - ions combine to form H 2 O Next: Oxidation-Reduction (Redox) Reactions: Atoms or ions donate or accept electrons. Redox Reaction. Corrosion of a metal battery terminal

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Recap

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  1. Recap Precipitation Reactions: ions combine to form insoluble products Neutralization Reactions: H+ ions and OH- ions combine to form H2O Next: Oxidation-Reduction (Redox) Reactions: Atoms or ions donate or accept electrons

  2. Redox Reaction • Corrosion of a metal battery terminal iron rusting metal Oxygen, acid

  3. Metal Corrosion • metal loses electron forms a cation = metal is oxidized AND another reactant gains electrons = is reduced

  4. Oxidation-Reduction Reactions • An oxidationoccurs when an atom or ion loses electrons. • A reduction occurs when an atom or ion gains electrons. • One cannot occur without the other.

  5. Redox Reactions • Oxidation-Reduction Reactions (Redox Reactions) • reactions that involve the transfer of electrons between two reactants • an element in one reactant is oxidized while an element in another reactant is reduced Mg (s) + 2 H+ (aq)  Mg2+ (aq) + H2 (g) oxidized reduced

  6. Redox Reactions • Oxidation: • the loss of electrons • chemical species becomes more positively charged • the gain of oxygen • the loss of hydrogen

  7. Redox Reactions • Reduction: • the gain of electrons • the chemical species becomes more negatively charged • the gain of hydrogen • the loss of oxygen

  8. Redox Reactions • LEO: • Lose Electrons Oxidation • GER: • Gain Electrons Reduction GER LEO LEO says GER

  9. Redox Reactions • Oil : • OxidationInvolves Lossof e- • Rig : • ReductionInvolves Gainof e- Oil Rig

  10. Redox Reactions • Electrons are not explicitly shown in chemical equations. • Oxidation Numbers are used to keep track of electrons gained and lost during redox reactions. • Oxidation number • a hypothetical number assigned to an individual atom present in a compound using a set of rules. • May be positive, negative, or zero

  11. Rules for Oxidation Numbers • Oxidation numbers are always reported for individual atoms or ions not groups of atoms or ions!!!!!!!!!!! • For an atom in its elemental form, the oxidation number is always zero. • H2: oxidation # = 0 for each H atom • Cu: oxidation number = 0 • Cl2: oxidation # = 0 for each Cl atom

  12. Rules for Oxidation Numbers • For any monoatomic ion: oxidation # = charge on ion • K+ oxidation # = +1 • Cl- oxidation # = -1 • S2- oxidation # = -2

  13. Rules for Oxidation Numbers • Group 1A Metal Cations: Always +1 • Group 2A Metal Cations: Always +2 • Hydrogen (H) +1 when bonded to nonmetals -1 when bonded to metals

  14. Rules for Oxidation Numbers • Oxygen (O) -1 in peroxides (O22-) -2 in all other compounds • Fluorine (F) always -1

  15. Rules for Oxidation Numbers • The sum of the oxidation numbers of all atoms in any chemical species (ion or neutral compound) is equal to the charge on that chemical species H2O: MgCl2: MnO4-: • 1 + 1 + (-2) = 0 2 + (-1) + (-1) = 0 • 7 + (-2) + (-2) + (-2) + (-2) = -1

  16. Oxidation Numbers • For many compounds, you will be able to directly apply the rules to determine the oxidation number of all atoms except for one. • Use the last two rules to determine the oxidation number of that last element.

  17. Practice determining oxidation numbers Example: Determine the oxidation state of all elements in SO3. Is it elemental? No Are any monoatomic ions present? No Which elements have rules? O = -2 Set up an equation to find the remaining oxidation number. S + 3(-2) = 0  S = +6

  18. Oxidation Numbers Example: Determine the oxidation number of Mn and O in MnO4-. Is it elemental? No Are any monoatomic ions present? No Which elements have rules? O = -2 Set up an equation to find the remaining oxidation number. Mn + 4(-2) = -1 so Mn = +7

  19. Oxidation Numbers Example: Determine the oxidation state of all elements in NaNO3 Is it elemental? No Are any monoatomic ions present? Na+ Which elements have rules? Na = +1, O = -2 Set up an equation to find the remaining oxidation number. 1 + N + 3(-2) = 0  N = +5

  20. Practice • Determine the oxidation number of P in HPO42-. • Determine the oxidation state of all elements in Cr2O72-. • Determine the oxidation state of Sn in SnBr4.

  21. Redox Reactions • There are many different kinds of redox reactions. • Combustion CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g) • Oxidation of Metals by Acids or Metal Salts Cu (s) + H2SO4 (aq)  CuSO4 (aq) + H2 (g)

  22. Redox ReactionsOxidation of Metals by Acids or Metal Salts • The reaction between a metal and an acid or between a metal and a metal salt is called a displacementreaction. • a reaction in which an element reacts with a compound and displaces an element from that compound A + BXAX +B

  23. Redox ReactionsOxidation of Metals by Acids or Metal Salts Examples: Mg(s) + 2HCl (aq) MgCl2 (aq) + H2(g) Zn(s) + 2 HBr (aq) ZnBr2(aq) + H2(g) Mn (s) + Pb(NO3)2 (aq) Mn(NO3)2 (aq) + Pb (s)

  24. Redox ReactionsOxidation of Metals by Acids or Metal Salts • How do you know if a redox reaction has occurred? • You must examine the oxidation number of each of the elements present in the reactants and products. • If the oxidation number changes, then a redox reaction has occurred.

  25. Redox ReactionsOxidation of Metals by Acids or Metal Salts • When oxidation occurs: • Electrons are lost • Oxidation number increases • When reduction occurs: • Electrons are gained • Oxidation number is reduced (decreases) e- e-

  26. -1 -1 +1 +2 0 0 Oxidation # of Mg increased oxidation Oxidation # of H+ ion reduced reduction Redox ReactionsOxidation of Metals by Acids or Metal Salts Mg(s) + 2HCl (aq)  MgCl2 (aq) + H2 (g)

  27. Redox ReactionsOxidation of Metals by Acids or Metal Salts Example: Identify the element that has been oxidized and the one that has been reduced. Zn (s) + 2 HCl (aq) ZnCl2(aq) + H2(g)

  28. -1 -1 0 +2 0 +1 oxidized reduced Redox ReactionsOxidation of Metals by Acids or Metal Salts • Find the oxidation numbers: Zn (s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g) Zn is the reducing agent. HCl is the oxidizing agent.

  29. Redox ReactionsOxidation of Metals by Acids or Metal Salts • Oxidizing agent: • The reactant that causes another reactant to be oxidized • The reactant that contains the element that was reduced • Reducing agent: • The reactant that causes another reactant to be reduced • The reactant that contains the element that was oxidized.

  30. Redox ReactionsOxidation of Metals by Acids or Metal Salts Example: Identify the oxidizing andreducingagents in the following reaction. CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)

  31. Redox ReactionsOxidation of Metals by Acids or Metal Salts • You can also write a net ionic equation to describe a redox reaction: • Write all soluble strong electrolytes as ions • Eliminate the spectator ions Zn (s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g) Zn (s) + 2 H+ (aq) + 2 Cl- (aq)  Zn2+ (aq) + 2 Cl- (aq) + H2 (g) Zn (s) + 2H+ (aq) Zn2+ (aq) + H2(g)

  32. Redox ReactionsOxidation of Metals by Acids or Metal Salts Example: Write the complete ionic and net ionic equations for the reaction. Which element is oxidized? What is the oxidizing agent? 2 Al (s) + 3 Ni(NO3)2(aq) 2 Al(NO3)3 (aq) + 3 Ni (s) 2 Al (s) + 3 Ni2+(aq) + 6 NO3- (aq) 2 Al3+ (aq) + 6 NO3-(aq) + 3 Ni (s) 2 Al (s) + 3 Ni 2+(aq)  2 Al3+ (aq) + 3 Ni (s)

  33. +5 +5 -2 oxidized Redox ReactionsOxidation of Metals by Acids or Metal Salts 2 Al (s) + 3 Ni(NO3)2 (aq) 2 Al(NO3)3 (aq) + 3 Ni (s) -2 0 +3 0 +2

  34. Redox ReactionsOxidation of Metals by Acids or Metal Salts • Based on the previous equation, we wouldn’t want to store a solution of Ni(NO3)3 in an aluminum container. • The aluminum container would react and dissolve!!! • Metals differ in the ease with which they are oxidized. • Al(s) is oxidized by Ni(NO3)3 (aq) • Ag(s) is NOT oxidized by Ni(NO3)3 (aq)

  35. Redox ReactionsOxidation of Metals by Acids or Metal Salts • Activity series: • A list of metals arranged in order of decreasing ease of oxidation • Used to predict whether a metal will react with an acid or with a metal salt • See Table 4.5 • Any metal on the list can be oxidized by, i.e. will lose electrons to, the ions of a metal below it.

  36. Activity Series

  37. Activity series:Any metal on the list can be oxidized by, i.e. will lose electrons to, the ions of a metal below it.

  38. Cu can lose electrons to Ag+

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