Redox worksheet # 1 key



Redox worksheet # 1 key

Part I

Rules for assigning oxidation numbers

• Uncombined elements = 0 Ex: Al(s), Cl2 (g), O3(g)

• The total oxidation number of a neutral Compounds = 0 (CO2, H2O)

• The oxidation number of a monatomic ion is equal to its charge. Thus the oxidation number of Cl in the Cl- ion is -1, that for Mg in the Mg2+ ion is +2.

• The oxidation number of alkali metals in compounds is +1, and that of alkaline earth metals in compounds is +2. The oxidation number of F is -1 in all its compounds.

• The sum of the oxidation numbers of all of the atoms in a polyatomic ion is equal to the ionic charge of the complex ion. (H3O+, OH-)

• The oxidation number of oxygen is always -2, except in peroxides (H2O2) where it is -1, and when it combined with fluorine (OF2) it is +2.

• The oxidation number of hydrogen is always +1 unless it is combined with a metal is -1.

Practice

What are the oxidation numbers of each of the elements in the following substances?

1. Na = 0

2. NaCl Na = +1 Cl = -1

3. Cl2 Cl = 0

4. H2SO4 H = +1 S = +6 O = -2

5. Cr2O7-2 Cr = +6 O = -2

6. H2O H = +1 O = -2

7. HNO3 H = +1 N = +5 O = -2

8. PCl5 P = +5 Cl = -1

9. NO2 N = + 4 O = -2

10. LiH Li = +1 H = -1

11. MnO4-1 Mn = +7 O = -2

12. C2H6 C = -3 (not in the periodic Table, but it does exist) H = +1

13. Xe = 0

14. NH3 N = -3 H = +1

15. OF2 O = +2 F = -1

16. H2O2 H = +1 O= -1

18. H3O+ H = _____+1______ O = _____-2_____

19. NaOH H = __+1_ O = ______-2____ Na = ______+1______

20. MgBr2 Mg = ____+2_____ Br = _____-1_______

21. FeS Fe = _____+2______ S = ________-2______

22. SO2 S = ______+4______ O = _______-2______

23. SO3 S = ______+6______ O = ______-2_______

Part II

Identify the species that are oxidized and the species that are reduced

1. Au3+ (aq) + I-(aq) (( Au(s) + I2(s)

Oxidized I- Reduced Au3+

2. 2 O2 + N2 ( 2 NO2

Oxidized N2 Reduced O2

3. 2 Li + F2 ( 2 LiF

Oxidized Li Reduced F2

4. If a substance is “reduced” does it gain or lose electrons?

The substance gains electrons

5. If a substance is “oxidized” does it gain or lose electrons?

The substance loses electrons

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