U4 S1 L3Balancing by redox

Textbook Readings MHR

page 730: The Half-Reaction Method for Balancing Equations page 731: Balancing Half-Reactions page 732: Balancing Half-Reactions for Acidic Solutions page 733: Balancing Half-Reactions for Basic Solutions pages 734-736: Half-Reaction Method for Balancing Redox Reactions pages 736-738: Balancing Redox Reactions for Acidic and Basic Solutions pages 742-744: Stoichiometry and Redox Titrations

Textbook Practice Items MHR

page 732: items 17, 18, 19 and 20 page 734: items 21, 22, 23 and 24 pages 738-739: items 27 and 28 page 744: items 29, 30, 31 and 32 pages 745-746: items 1-9

Upon completion of this lesson, you should be able to:

• write and balance equations for complex oxidation-reduction reactions occurring in acidic or basic solutions 

• solve redox stoichiometry problems for spontaneous redox reactions

• The first thing we have to do when balancing redox equations is to make sure the reaction is indeed a redox reaction.

• Therefore, we must check to make sure that a species is being reduced and one is being oxidized.

• Once the redox species have been identified we can proceed to balance the equation.

Balancing by half reactions: (balancing by electrons!)

Potassium reacts with Chlorine to produce potassium chloride.

Balancing Redox Reactions in Acidic Conditions: ~~The No Fail 6 Step Method~~ p 732

1. Write the half reaction for the species being oxidized and reduced.

2. Balance: – Atoms other than O and H – Balance O using H2O – Balance H using H+ (hence acidic conditions!)

3. Balance each half reaction electrically using electrons (e-). Net charge must be zero.

4. Balance each half reaction so that the number of electrons in each is the same.

5. Simplify. Combine both equations (cross out/add/subtract - just like in math).

6. Check the numbers, kinds of atoms and net charge.

• Write a balanced half reaction for that shows the reduction of permanganate ions to manganese (II) under acidic conditions.

Balance the following chemical equation under acidic conditions

23 3 4AsH Zn H AsO Zn

Balancing Redox Reactions in Basic Conditions: ~~The No Fail 7 Step Method~~ p 733

1. Write the half reaction for the species being oxidized and reduced.

2. Balance: – Atoms other than O and H – Balance O using H2O – Balance H using H+ (hence acidic conditions!)

3. Balance each half reaction electrically using electrons (e-). Net charge must be zero.

4. Change from acidic to basic – look at the H+, add the same number of OH - to both sides. On the side H+ is on form water (H+ + OH - H2O).

5. Balance each half reaction so that the number of electrons in each is the same.

6. Simplify. Combine both equations (cross out/add/subtract - just like in math).

7. Check the numbers, kinds of atoms and net charge.

• Write a balanced half reaction for the oxidation of thiosulphate ions to sulfite ions in basic conditions

• Balance the following under basic conditions:

4 2CN MnO CNO MnO

Balancing redox by Ox # method – p 747 : The short cut!1. Write equation2. Assign Ox #’s3. Identify atoms involved in redox4. Find numerical value of change5. Determine ratio of atoms 6. Balance7. Balance O with H2O and H with H+

2 2 2 2CS O CO SO

2 3 3 2B O Mg MgO Mg B

Balance under acidic conditions

2 3 32 7Cr O Fe Cr Fe

Using the Standard reduction potential Table.

• Write the redox reaction equation for the reaction between zinc metal and aqueous chromium(III) sulphate – List the reactant species present