REDOX Reactions

Redox means… Oxidation – Reduction Reactions Redox Rxns involve the… Complete or partial transfer of electrons Change in Oxidation Numbers (note that this change is a “pseudo” change – not real – it just looks like it’s changing)

OXIDATION The oxidation of an element is the: LOSS OF ELECTRONS - Associated w/ an increase in oxidation number For example: change in oxidation numbers from (-1) to (0) is the oxidation of that element

REDUCTION The reduction of an element is the: GAINOF ELECTRONS - Associated w/ a decrease in oxidation numbers (i.e. it gets more negative) For example: the oxidation number of an element decreases from -1 to -3 (i.e. it has gained two electrons)

Remember… LEO the lion says GER Lose electrons = oxidation (LEO) Gain electrons = reduction (GER)

In a redox reaction… You CANNOT HAVE AN OXIDATION Rxn without a REDUCTION Rxn….they are partners. One does not exist without the other!!!

Rules for Assigning Oxidation Numbers 1. Free elements are assigned an oxidation number of 0 (as are diatomic molecules: Br 2 I 2 N 2 Cl 2 H 2 O 2 F 2 ) 2. The oxidation state (number) for any simple one-atom (monatomic) ion is equal to its charge (i.e. Na + is +1) 3. Usually an atom’s oxid state is the charge it would have if it were an ion (i.e. Group 1 = +1 ; Group 17 = -1, etc.) 4. H is always +1 unless bound to a metal (then its -1) 5. O is always -2 (unless it’s a peroxide, and then its -1) 6. The sum of all of the oxidation numbers within a compound must equal the overall charge of that compound

Lets Assign Some Ox Numbers! NaCl H 2 O H 2 SO 4

Assigning Oxidation Numbers NaCrO 2 NaOH H 2 O 2 Na 2 CrO 4 OH -1 Ag HNO 3 AgNO 3 NOCa(OH) 2 Cl 2 CaCl 2 Ca(ClO 3 ) 2 KNO 3

Redox Reactions Again…one CANNOT happen without the other. They are pairs!! LEO  Lose e- = oxidation GER  Gain e- = reduction

Lose Electrons = OXIDATION An oxidation reaction looks like this: Fe +2  Fe +3 + e - In this reaction: 1. Oxidation numbers increases (more positive) from +2  +3

Gain Electrons = REDUCTION A reduction reaction looks like this: Fe +3 + e -  Fe +2 In this reaction: 1. The oxidation number Decreases - is changing from +3  +2 (it is getting more negative)

Agents – things that cause redox rxns Oxidizing Agent Substance that is being reduced Hence, causing the oxidation Reducing Agent Substance that is being oxidized Causing the reduction So…they HAVE to happen together!!

But remember…they happen together in a chemical reaction… S(s) + HNO 3 (aq)  SO 2 (g) + NO(g) + H 2 O (l) In the above reaction, both oxidation and reduction are occurring. Can you tell where the oxidation is? What about the reduction? But, the above rxn is unbalanced – Redox rxns are oftentimes super complex, so we have a special way to balance them… How?? Use HALF REACTIONS!!

What is a half reaction you say?? The idea behind using half reactions is simple:  They are reactions that show oxidation and reduction separately. Then, they can be combined (or added together) to give the overall, BALANCED, redox reaction But…prepare yourself, there are a lot of rules & it can be confusing….you need to be very methodical about it.

Buckle in…here we go…HALF RXNS

BAHHH…What does this mean?? Let’s an example and try to make sense of this… S(s) + HNO 3 (aq)  SO 2 (g) + NO(g) + H 2 O (l) Ok, let’s break it down to help us see what’s being oxidized and what’s being reduced: S(s) + H +1 (aq) + NO 3 -1 (aq)  SO 2 (g) + NO(g) + H 2 O(l) Step 1: Assign oxidation numbers to figure out what was reduced & oxidized

Oxidation Half Reaction What was oxidized: S (went from 0  +4) So, let’s look at the species involving sulfur S (s)  SO 2 (g)

Reduction Half Reaction What was reduced? N (+5  2) Half reaction: NO 3 -1 (aq)  NO (g) Let’s balance the reaction using our rules…

NO 3 -1 (aq)  NO (g)

Now that I have my half reactions, let’s continue…

Also…you must add the spectator ions back in….and then balance to ensure everything is equal!! Did we have any spectator ions here?? No.

KMnO 4 (aq) + HCl (aq)  MnCl 2 (aq) + Cl 2 (g) + H 2 O (l) + KCl (aq) With a partner & a white board – find lab tables, desks or the floor to try to figure this out Remember my addition to Step 5: Add the spectator ions back in & balance them… GOOD LUCK

Br 2 (l) + SO 2 (g)  Br - (aq) + SO 4 2- (aq)

Small Scale Half-Rxns Lab (from book)