What is Electrochemistry?

2. What is Electrochemistry? So in Chem 20 we looked at individual elements and drew electron dot diagrams for them. Non-Metal Metal Last we learned that when metals form compounds they lose electrons and when non-metals form compound they gain electrons. The study of the transfer of electrons during chemical reactions is known as Electrochemistry.

3. CHEM 30 Unit B: ELECTROCHEMICAL change Chapter 13 (P.556 – 609)

4. Electrochemical (Electron Transfer) reactions are the most common type of reactions in both living and non-living systems. Electrochemical reactions were discovered way before their was science to explain it. What do you call chemical reactions before there is science to explain them?

5. Magic! No, I’m kidding… Its sad, but true. But seriously…I’m actually not. The Alchemists main goal was to turn common metals into gold and create a magical potion so they could live forever….

6. Alchemists and Early Science So I know so far I have made the Alchemists out to be sort of…..nut jobs, and they sort of were…. But! Even though the alchemists and other early scientists had somewhat… “unusual” ideas, their contribution to our current understanding of how things work was important. Their empirical knowledge (things learned through mixing things and experimenting) helped our current understanding of chemistry. “Technology Drove Science”

7. So in prehistoric times when people were learning how to turn metal ores (mixtures of different metals) into metals they could make things out of, they actually discovered (by accident) the science of metallurgy. Metallurgy is the science of extracting pure metals from their naturally occurring compounds (ores) and adapting these metals for Primitive's…. Useful Purposes. Tools Weapons

8. Metallurgy Early people learned that if they heated a pile of raw ore (mixture of different metals) in a fire it would reduce (get smaller) into a small amount of pure metal when it cooled. This is where the term Reductioncomes from. Heated In A Fire Pure Metal Metal Ore

9. Reduction Early people didn’t understand that the metal ores were reacting with something (usually gases) which caused them to be reduced. Carbon Monoxide Charcoal (Carbon) Hydrogen Gas

10. Understanding Oxidation and Reduction Oxidation and Reduction were happening during early smelting and metallurgy, the people didn’t know the science, but that didn’t stop them from doing it. It wasn’t until the 1700’s….more than 6500 years after the first copper was produced…that the science was around to explain oxygen’s role in burning and corrosion (rust). 6500 Years

11. Oxidation It was only now that scientists understood that when things interacted with oxygen they did the opposite of being reduced to pure, useable metals. In general scientists noticed that combustion and corrosion were similar processes (both broke down substances, Both reacted with oxygen) Through experimentation scientists found other gases that caused similar reactions. So they referred to all reactions that caused pure metals to turn back into compounds Oxidation.

12. Oxidation Oxidizing Agent: Any substance that causes a metal to oxidize and turn into a metal compound. Oxygen Chlorine Bromine

13. Oxidation and ReductionElectron Transfer Theory This modern theory says that all chemical reactions can be broken in two pieces….2 ½ reactions. When reactants combine to form products, one reactant gains electronsand one reactant loses electrons. Electrons are transferredfrom one reactant to the other in equal amounts.

14. Lets Try One. For example, when zinc metal is dropped in hydrochloric acid…what happens? All those bubbles are hydrogen gas escaping. (Hydrogen’s flammable right…….) So lets break this reaction into two pieces….. First, lets look at the zinc. Second, look at the hydrogen. Zn(s) Zn2+(aq) +2e- Oxidation 2H+(ag) H2(g) Reduction

15. Practice Zn(s) Zn2+(aq) + 2e- Oxidation Metal  Compound Pb2+(aq)  Pb(s) Reduction Compound  Metal Its Just That Easy….

17. Redox Reactions Take Place In (aq) Oxidation/Reduction(Redox) reactions take place in aqueous (liquid) environments. Because reactions happen in water we have to include water in our balanced ½ reaction. AND because water can easily be acidic or basic (just a few ions one way or the other and water isn’t neutral anymore) we balance the equation using the acid or basic ions. In fact most chemical reactions need water to happen…there are very few chemical that can be put together as solids and react. So what is the acidic ion? H3O+(ag) Hydronium OH-(aq) Basic Ion? Hydroxide

18. Balancing Half Reactions In Acids & Bases 1 = 2 1 O HNO2(aq) H+(aq) + NO(g)  + H2O(l) 2 3 Balance hydrogen ions by adding (H+(aq))to the opposite side.

19. Balancing Half Reactions In Acids & Bases O NO(g) + H2O(l)  e- + H+(aq) + HNO2(aq) But what if it happened in a Basic solution!?? AKA….add e- to the same side as you added the H+(aq).

20. Balancing Half Reactions In Acids & Bases a basic H+(aq) + e- + HNO2(aq)  NO(g) + H2O(l) *** First 4 steps are exactly the same*** Balance hydrogen ions by adding (H+(aq))to the opposite side. Add e- to the same side as you added the H+(aq).

21. Balancing Half Reactions In Acids & Bases a basic / OH-(aq) + H+(aq) + e- + HNO2(aq)  NO(g) + H2O(l) Add OH-(aq) to both sides equal to the number of H+(aq) you added back in step 3.

22. Write This Down

23. Practice (Acidic)

24. Practice (Basic)

25. So we have seen so far when we looked at theElectron Transfer Theory that Redox reaction involve the transfer of electrons. But we have also seen that when elements form compounds or ions they do it so they can be more stable…….So they don’t want to transfer electrons and undergo redox reactions. So then why do they do it? Any ideas?

26. Tug of War For Electrons The two players in a redox reaction compete with each other seeing who is strong enough to take the other players electrons. If one of the players is strong enough to win the competition and take the others electrons a redox reaction occurs….if not, nothing happens. Example:

27. Example: Cu Zn Oxidized So which one is oxidized and which one is reduced? ****Reduced: Gains electrons and its charge decreases (Less -) as a result. Oxidized: Loses electrons and its charge increases (more +) as a result.****

28. Oxidizing and Reducing Agents (RA & OA) Reducing Agent (RA): The reactant that causes the other reactant to be reduced. (It actually gets oxidized) Oxidizing Agent (OA): The reactant that causes the other reactant to be oxidized. (It gets reduced in the process)

29. RA & OA Cu Zn Zinc gets oxidized from Zn(s) to Zn2+(aq)….it causes Cu2+(aq) to be reduced to Cu(s)……therefore it’s the RA. Copper gets reduced from Cu2+(aq) to Cu(s)….it causes Zn(s) to be oxidized to Zn2+(aq)…therefore it’s the OA. RA OA

30. Recap of Oxidation/Reduction So far we have learned that when a reaction occurs, one reactant gets their valence electrons taken away and gets oxidized. AND The other reactant takes the electrons and thus becomes more negatively charge and gets reduced. The reactant that gets oxidized and thus causes the other reactant to become reduced is referred to as the Reducing Agent (RA). AND The other reactant that gets reduced and thus causes the other reactant to become oxidized is referred to as the Oxidizing Agent (OA).

31. So…. You put two reactants….one solid and one not then one will reduce and one will oxidized? Copper(II) Nitrate….lots of copper ions. ERRRRR. Wrong again…..Reactions are NOT always spontaneous. So there’s the question. How do we know if the reaction will happen or not?

32. To Be or Not To Be……. Spontaneous So to find out what combinations of solid reactants will react spontaneous (right away as soon as their mixed) and which will not… We could just randomly mix different combinations of metals and solutions and see which ones happen spontaneously. It would take forever….and why? Really, who has the time for that? And besides, somebody already did it for us…..so lets look at the Redox table in your text book and learn how to use it. That way we can predict without having to do all that work. And Remember! Its not guessing…I prefer to call it scientific guestimation…..sounds more scientific.

33. Redox Table and Ranking RA’s and OA’sRanked According to Strength P. 569 So in this experiment scientists mixed a bunch of metals and metal ions together to see which combinations would be spontaneous and which would not. They just tried every possible combination and listed them according to which reactant work and caused a reaction the most times when combined with another reactant. (The same way as you rank a team in a round robin tournament in sports. 3

34. Redox Table and Ranking RA’s and OA’sRanked According to Strength P. 569 3 Strongest Oxidizing Agent Weakest Reducing Agent Weakest Oxidizing Agent Strongest Reducing Agent

35. Redox Table and Ranking RA’s and OA’sRanked According to StrengthTextbook Pg. 828 or Data Booklet Pg. 7 “Table of Selected Standard Electrode Potentials*” The table has all the possible redox reactions listed by relative strength. Along the right hand side all the Electrical Potentials E° (V) are listed for each reaction as well….. DON’T WORRY ABOUT THE E° (V), its used for something else….they just list them their because its convenient. You will also notice that on the bottom “*For 1.0mol/L solutions at 298.15K (25.00 °C) and a pressure of 101.325 kPa” Nice that they put it there…..BUT I HAVE NEVER USED IT FOR ANYTHING. It actually give ½ STP and ½ SATP….How useless.

36. RA If OA is above RA on the table SPONTANEOUS SPONTANEOUS! Decreasing Strength Decreasing Strength NOT SPONTANEOUS! OA

37. Predicting Redox Reactions The first step to predicting a redox reaction is to list all the species (Atoms/Ions/H+/OH-) that are present (reactants only). ***REMEMBER to add H2O(l) as a species that’s present because it’s a solution…ALWAYS! ***Keep an eye out to see if the example is acidic or basic….if it is make sure to also include H+ or OH-.

38. Predicting Redox ReactionsExample HNO3(aq) H2O(l) Au(s) Did we get everything? H+(aq) NO3-(aq) Au(s) H+(aq) NO3-(aq) H2O(l)

39. Predicting Redox ReactionsExample OA RA RA Next, you use the Redox table to assign all of the entities as either an RA or an OA. ***Look for the entities in the table EXACTLY as they are in your question…SAME STATE. Au(s) H+(aq) NO3-(aq) H2O(l)

40. Predicting Redox ReactionsExample Sixth, you rewrite the two equations using the balancing amounts and add the two equations together. Fifth, balance the number of electrons so both ½ reactions have the same amount. You balance by multiplying the whole ½ reaction by a number in front. SOA OA Au(s) H+(aq) NO3-(aq) H2O(l) SRA RA RA Fourth, write out the ½ reactions for the SOA and SRA exactly as they appear in the Redox table. *****Remember to write the SRA Right To Left (Backwards)*** 2 X Third, from your choice (SA/RA), choose the strongest SA and OA. 2H2O(l)  O2(g) + 4H+(aq) +4e-

41. Predicting Redox ReactionsExample Last, we cancel out anything that shows up on both sides. + 4e-  + 8H+(aq) 2N2O4(g) 4 NO3-(aq) + 4H2O(l) + 2H2O(l)  O2(g) + 4H+(aq) +4e- 2 / / / / 4 NO3-(aq) +8H+(aq) +2H2O(l) + 4e-2N2O4(g)+ 4H2O(l)+O2(g) + 4H+(aq) +4e- 2 X 2H2O(l)  O2(g) + 4H+(aq) +4e- 2H2O(l)  O2(g) + 4H+(aq) +4e- 4 NO3-(aq) +8H+(aq)2N2O4(g)+ 2H2O(l)+O2(g) + 4H+(aq)

42. Predicting Redox ReactionsPractice

43. Predicting Redox ReactionsPractice SOA SRA

44. Predicting Redox ReactionsPractice 1 X 5 X

45. Predicting Redox ReactionsPractice

46. Can A Reactant React/Redox With Itself? If you have looked closely at your Redox table and practiced assigning RA and OA to species in a redox question... Anyway, there ARE species that can act as either an OA or an RA, and in fact, react with themselves. This is called Disproportionation. ***If you run into a question where you find the SRA AND SOA are the same species, its ok...your not wrong!*** Just treat it like any other Redox and follow the same 5 steps. Which you all have…. Right?

47. (p. 583) So lets set the stage for all this … So when elements join together to form compounds electrons are transferred. e- This transfer of electrons is what gives elements their + or – charge.

48. Oxidation States These + and – charges we have talked about so far are known as FORMAL CHARGES. Oxidation States are similar to this with assigning + and – charges, but that’s about where the similarity ends. The + and – charges we assign for oxidation states ARE NOT the charge of the element from the periodic table!

49. Rules For Assigning Oxidation Numbers

50. Oxidation Numbers *****Elements by themselves not combined with anything have an oxidation number of 0!!!!