3 BALANCING REDOX REACTIONS – USING OXIDATION NUMBERS A)

3 BALANCING REDOX REACTIONS – USING OXIDATION NUMBERS A)
BALANCING CHEMICAL EQUATIONS CLASS COPY PLEASE DO NOT WRITE
BALANCING CHEMICAL EQUATIONS WORKSHEET 1 H2 + O2 →

BALANCING EQUATIONS NA + NANO3 → NA2O + N2
BALANCING EQUATIONS WORKSHEET 1 ANSWERS BALANCE THE FOLLOWING
BALANCING OXIDATIONREDUCTION EQUATIONS IN THE LABORATORY IT IS IMPORTANT

3

3. Balancing Redox Reactions – Using Oxidation Numbers


a) What are “Oxidation Numbers”?


Sample: K2Cr2O7 K is +1 Cr is +6 O is -2


 They are not actual charges!

 They are a bookkeeping system used to keep track of an atoms electrons!



i) Rules for Assigning Oxidation Numbers


  1. Oxidation number of an atom as an isolated ion is the charge of the ion.


Eg: Cu2+ has oxidation number of +2


  1. Oxidation number of an atom in elemental form is zero.


Eg. Cu (s) has oxidation number of 0


Eg. O2 (g) has oxidation number of 0


Eg. S8 (s) has oxidation number of 0


  1. Oxidation of common atoms in a compound is its combining capacity


Common atoms include:


Group 1 (+1); Group 2 (+2); Halogens (-1); Oxygen (-2); Aluminum (+3)







  1. The sum of the positive and negative charges in a compound equals zero


Eg. K2Cr2O7











  1. The sum of the positive and negative charges in a polyatomic ion equals the charge of the ion.


Eg. MnO4-








ii) Example: What are the oxidation numbers for all the atoms in Al(NO3)3?




iii) Example: What are the oxidation numbers for each atom in ClO4-? (Tricky!)




iv) Example: What are the oxidation numbers for each atom in KH2PO4?




v) Example: Use changes in oxidation number to identify which species is oxidized and

which is reduced:


Cu (s) + 2AgNO3 (aq) Cu(NO3)2 (aq) + 2Ag (s)



vi) Example: Identify the oxidizing agent and the reducing agent:


3H2S (g) + 2HNO3 (aq) 3S (s) + 2NO (g) + 4H2O (l)




Do Questions: #3-6 page 194-195; #20-23 page 204

b) Balance Redox Using Oxidation Numbers (optional)


i) Procedure:


Separate redox into its two half reactions

Assign oxidation numbers to all atoms that change oxidation number

(not oxygen or hydrogen)

Find change in oxidation number (ON) for each half reaction

Multiply half reactions by a factor to make the overall ON zero when added.

 Add the two half reactions together

 Balance the O’s by adding H2O and balance the H’s by adding H+


ii) Balance: Cr2O72- + Fe+2 Cr+3 + Fe+3 (acidic)


1st Separate half reactions and 2nd Assign oxidation numbers and 3rd Find ON





4th Make overall ON zero and 5th Add together





6th Balance O’s and H’s













Do Questions: #25 page 210



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Tags: balancing redox, numbers, balancing, reactions, using, redox, oxidation