1. Chapter 20

2.  Salt on icy roads can make driving safer, but the salt that clings to the metallic parts of cars can cause them to corrode or rust relatively quickly. This corrosion is one example of a chemical reaction called oxidation- reduction. You will learn about oxidation-reduction reactions. 20.1

3.  Oxidation originally meant combining with oxygen. 20.1

4.  A Bunsen burner oxidizes the methane in natural gas to carbon dioxide and water. 20.1

5.  Carbon is oxidized when charcoal burns. 20.1

6.  Iron is oxidized when it rusts. 20.1

7.  Today the meaning is different: ◦ Oxidation means the loss (or partial loss) of electrons. ◦ Reduction means the gain (or partial gain) of electrons.  They are interdependent: one can’t occur without the other.

8. LEO the lion says GER…

9. ◦ Redox reactions that form ions involve a total loss and gain of electrons. 20.1

11.  The substance that loses electrons is called the reducing agent. The substance that accepts electrons is called the oxidizing agent. 20.1

14. ◦ Redox reactions with covalent compounds involve the partial loss and gain of electrons. 20.1

16.  When a metal corrodes it combines with oxygen. ◦ Iron, a common construction metal often used in the form of the alloy steel, corrodes by being oxidized to ions of iron by oxygen. 20.1

17. ◦ Aluminum resists corrosion because it forms a protective coating of aluminum oxide. 20.1

18. ◦ Painting a surface protects it from corrosion. Chromium metal also serves as a protective coating. 20.1

19. ◦ Zinc blocks attached to the steel hull of this ship oxidize instead of the iron, preventing corrosion. 20.1

20.  When aluminum or magnesium powder in fireworks are heated to high temperatures in the explosion, they burn with an intense white light. As elements burn, their oxidation numbers change. You will learn how oxidation and reduction are defined in terms of a change in oxidation number. 20.2

21. ◦ An oxidation number is a positive or negative number assigned to an atom to indicate its charge or partial charge when bonded to another atom.  In a compound, the more electronegative atom will get a negative number, and the less electronegative atom will get a positive number. 20.2

22.  The oxidation number of any element in the free or uncombined state is zero. 20.2

24.  Oxidation numbers are often written above the chemical symbols in a formula. 20.2

25. K 2 CrO 4 Cr oxidation number = +6 Cr 2 O 3 Cr oxidation number = +3 20.2

26. “Multiply down; divide up” to find an unknown oxidation state.

28. ◦ An increase in the oxidation number of an atom or ion indicates oxidation. ◦ A decrease in the oxidation number of an atom or ion indicates reduction. 20.2

30.  When an iron nail is placed in a copper(II) sulfate solution, the iron reduces Cu 2+ ions in solution and is simultaneously oxidized to Fe 2+ The iron becomes coated with metallic copper. 20.2

33.  When concentrated hydrochloric acid (HCl) and nitric acid (HNO 3 ) are mixed with gold, oxidation and reduction reactions produce gaseous nitrogen monoxide (NO) and soluble, stable AuCl 4 – ions. You will learn how to write and balance chemical equations for redox reactions such as this one. 20.3

34. ◦ If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction. Therefore, the reaction as a whole must be a redox reaction.  Single replacement reactions are always redox, and double replacement reactions are never redox. 20.3

35. K + H 2 O  KOH + H 2 2 Zn + HCl  ZnCl 2 + H 2 20.3

36.  A redox reaction takes place between nitrogen and oxygen when lightning bolts heat the air. 20.3

37.  A color change can signal a redox reaction. 20.3

40.  Shows either the oxidation or reduction portion of a redox reaction and it shows the exchange of electrons.

41.  An equation that shows an atom or ion losing one or more electrons while its oxidation number increases is an oxidation half- reaction. ◦ The law of conservation of mass must be followed. ◦ Also, the law of conservation of charge applies. ◦ Ex:

42.  An equation that shows an atom or ion gaining one or more electrons while its oxidation number decreases is a reduction half-reaction. ◦ Must follow the law of conservation of mass and charge. ◦ Ex:

43. 1. Assign oxidation numbers. 2. Leave out spectator ions (no change in ox #). 3. Write half-reactions to show conservation of mass. 4. Add electrons to show conservation of charge. 5. Balance using coefficients if necessary. *Make sure the # electrons lost = # electrons gained.

44. 1. Mg + Fe(NO 3 ) 2  Fe + Mg(NO 3 ) 2 2. Zn + 2 HCl  H 2 + ZnCl 2 3. 2 Al + 3 ZnCl 2  3 Zn + 2 AlCl 3

45.  Show only participating atoms and ions.  Spectator ions and electrons are left out. ◦ Still have to be balanced! ◦ Mass and charge must be conserved!

46. 1. Balance mass. Use coefficients if necessary. 2. Write half-reactions. 3. Balance charge. Multiply half-reaction(s) by a coefficient to balance electrons. 4. Incorporate coefficients from the half- reactions into the equation.

47. 1. H + + Al  H 2 + Al 3+ 2. Fe 3+ + Ni  Fe 2+ + Ni 2+ 3. Cr 3+ + Mg  Cr + Mg 2+