Reduction-Oxidation Reactions (1) 213 PHC 11th lecture Dr. Mona Alshehri (1) Gary D. Christian, Analytical Chemistry,6 th edition.

By the end of the lecture the student should be able to: Detect the end point in redox reaction. Balance a redox reaction. 2

Detection Of The End Point 3

Self-Indication Highly colored e.g. potassium permanganate (purple) MnO e = Mn 2+ (purple) (colorless) 4

Starch Indicator Used with iodine Strach-I 2 complex (dark blue) 5

Redox Indicators Highly colored dyes Weak reducing or oxidizing agents The color of the reduced and oxidized forms are different The color will change with E change during titration E o of indicator must be near the E of eq. point The redox indicator reaction must be rapid E.g. Diphenylamine 6

Balancing Redox Reactions 7

Half-Reaction Method I - + MnO 4 - → I 2 + Mn 2+ Separate the reaction into two half reactions: I - → I 2 MnO 4 - → Mn 2+ 8

Balance the atoms of each half-reaction: first balance all of the atoms except H and O. For an acidic solution add H 2 O to balance the O atoms and H + to balance the H atoms. In a basic solution use OH - and H 2 O to balance the O and H. Balance the iodine atoms: 2 I - → I 2 The Mn in the permanganate reaction is already balanced, so let's balance the oxygen: MnO 4 - → Mn H 2 O Add H + to balance the 4 waters molecules: MnO H + → Mn H 2 O The two half-reactions are now balanced for atoms. 9

Balance the charges in each half- reaction: The reduction half-reaction should accept the same number of electrons as the oxidation half- reaction loss. This is accomplished by adding electrons to the reactions: 2 I - → I 2 + 2e - 5 e H + + MnO 4 - → Mn H 2 O 10

Multiply the electrons by a number: The two half-reactions should have the same number of electrons so they can cancel each other: 5 (2I - → I 2 +2e - ) 2 (5e - + 8H + + MnO 4 - → Mn H 2 O) 11

Add the two half-reactions: 10 I - → 5 I e - 16 H MnO e - → 2 Mn H 2 O This gives the final equation: 10 I e H MnO 4 - → 5 I Mn e H 2 O 12

Get the overall equation: Cancel out the electrons and H 2 O, H +, and OH - that may appear on both sides of the equation: 10 I H MnO 4 - → 5 I Mn H 2 O 13

Check the numbers: Make certain that the mass and charge are balanced. In this example, the atoms are balanced with a +4 net charge on each side of the reaction. 10 I H MnO 4 - → 5 I Mn H 2 O 14

Follow the link: w6wso&feature=endscreen&NR=1 How to balance a redox reaction in acidic solution. 5BCU&feature=fvwrel How to balance a redox reaction in basic solution. 15

Questions? 16

SUMMARY Indicators used to detect the end point in redox reaction. Balancing redox reactions. Thank You 17