Redox reactions involve the transfer of electrons between substances, causing some substances to be oxidized (lose electrons) and others to be reduced (gain electrons). In the examples given, sodium metal is oxidized when it reacts with bromine gas, losing electrons to become sodium ions. Bromine gas is reduced, gaining electrons to become bromide ions. Zinc metal also undergoes oxidation when reacting with hydrochloric acid, losing electrons to form zinc ions, while hydrogen ions are reduced, gaining electrons to form hydrogen gas. Oxidation numbers are used to indicate the charge of atoms in their elemental or ionic states and can help identify whether a substance is being oxidized or reduced in a redox reaction.

1. By Mr Jeremy Lee Redox Reaction

2. REDOX stands for REDuction & OXidation Both reduction and oxidation occur simultaneously. What is redox reaction? Substance that reduces another substance is called reducing agent A Substance that oxidizes another substance is called oxidizing agent

3. Which substance is being oxidized and reduced?2Na + Br2 2NaBr Sodium Bromine Bromine gas become bromide ions Chemical equation Br2 + 2e- 2Br - The 2 electrons that sodium metal loses has been taken by bromine gas to become bromide ions Sodium metal become sodium ions Chemical equation 2Na (s) 2Na + + 2e- OR Na (s) Na + + e- (Simplified) Sodium metal has loses 2 electrons to become sodium ions

4. More example : Metal and acid reactionZn + 2H+ Zn2+ + H2 Zinc metal Hydrogen Ion (from acid) Hydrogen ions becomes Hydrogen gas 2H+ + 2e- H2 Hydrogen ions will receive the 2 electrons to become hydrogen gas. Zinc metal has become Zinc ions ZnZn2+ + 2e- It becomes a Zinc ions after it happens

5. From the 2 examples, what are the conditions to consider a substance to become oxidized or reduced?

6. Oxidation number What are oxidation numbers? - Oxidation state or oxidation number is a + or - value given to an element to represent the charge of an atom or ion after Redox reaction. - Even for atoms inside a covalent compound, they contains oxidation number.

7. Overall charge of this compound is zero KMnO4 Examples Now you see it, now you don’t Potassium permanganate Potassium has a charge of +1 and permanganate ion has a charge of -1 Potassium permanganate (purple) can be reduced to manganese ion Mn2+(Colourless!) Decrease in oxidation state from +7 to become +2 REDUCTION! Oxygen carries a -2 charge each -2 X 4 = -8 +1 ?

8. Potassium dichromate Overall charge is Zero K2Cr2O7 Chromate ion has a charge of -2 Cr2O72- Each chromium has a charge of +6 Potassium dichromate (orange) can be reduced to chromate ion (green), Cr3+ Cr2O72−(aq) + 14H+ + 6e− -> 2Cr3+(aq) + 7H2O Decrease in oxidation state Reduction!!!! Now you see it, Now you don’t Potassium Dichromate -2 X 7 = -14 +1 X 2 = +2 ?

9. Iodide Ions Sodium Iodide Aqueous Sodium iodide (colourless), NaI, can be turned into brown solution. How? Solution contains iodide ions ( I- ) Iodide ion can be turned into Iodine ( I2 ) 2I-(aq) I2 (aq) + 2e- Iodine is brown and has a oxidation state of zero Increase in oxidation state from -1 to O. oxidation has taken place