Electrochemistry Lecture Notes PDF

Topic 6 Electrochemistry ELECTROCHEMISTRY: LECTURE 6.1 REDOX REACTIONS Specific Learning Objectives I. Define oxidation and reduction II. Identify the oxidizing and reducing agent in a redox reaction III. Identify the changes in oxidation numbers in redox reactions  Redox rea...

Topic 6 Electrochemistry ELECTROCHEMISTRY: LECTURE 6.1 REDOX REACTIONS Specific Learning Objectives I. Define oxidation and reduction II. Identify the oxidizing and reducing agent in a redox reaction III. Identify the changes in oxidation numbers in redox reactions  Redox reactions What are redox reactions? https://youtu.be/B-vZc2Ty96Q  Electrochemical processesprocesses: oxidation- Electrochemical reduction reactions in which: the energy released by a spontaneous reaction is converted to electricity (galvanic cells) or electrical energy is used to cause a non- spontaneous reactions (electrolysis)  Redox processes Oxidation-reduction (redox) processes: reactions where oxidation and reduction occur simultaneously, Reduction Cu2+ (aq) + Zn(s) -> Cu(s) + Zn2+(aq) Oxidation  Redox processes Oxidation - defined by one of the following: – loss of electrons – loss of hydrogen – gain of oxygen Egs. Na Na+ + e- (loss of electron) CH3CH2OH  CH3CHO (loss of hydrogen) 2Mg + O2  MgO (gain of oxygen)  Redox processes Reduction - defined by one of the following: – gain of electrons – gain of hydrogen – loss of oxygen Egs. Cl + e- Cl- (gain of electron) 2Li + H2  2LiH (gain of hydrogen) Fe2O3 + 3CO -> 2Fe + 3CO2 (loss of oxygen)  Redox processes Oxidizing and reducing agent 2Na + Cl2 2Na+ + 2Cl- (full reaction) Reducing agent Oxidizing agent Causes reduction Causes oxidation Is oxidized Is reduced Loses electrons Gains electrons  Redox processes dentify the oxidizing and reducing agent Zn (s) + I2 (s) ZnI2 (s) 2e- Zn2+ and 2 I- n donates electrons; is oxidized; the reducing agent accepts electrons; is reduced; the oxidising agent  Pause & check Given the following redox reactions, complete the tables that follow. 1. Cu2+(aq) + Fe(s)  Cu(s) + Fe2+(aq) Oxidized/Reduced? Oxidizing Agent/Reducing Agent? Cu2+ Reduced Oxidizing agent Fe Oxidized Reducing agent 2. Cl2(aq) + 2I-(aq)  2Cl-(aq) + I2(aq) Oxidized/Reduced? Oxidizing Agent/Reducing Agent? Cl2 Reduced Oxidizing agent I- Oxidized Reducing agent  Oxidation numbers Oxidation numbers (ON) Oxidation number is a charge on an atom which indicates its state of oxidation (the charge on the atom will take if it is an ion in the compound, also called the oxidation state) Oxidation number is simply the charge of the monatomic ion For a neutral element, oxidation number is zero Element/Ion A Oxidation No. monatomi Na 0 c ion is an Na+ +1 ion Cl2 0 consisting Cl- -1 of a single atom.  Oxidation numbers Rules for oxidation numbers (ON) Species ON Egs Free elements 0 Na Alkali/Gp 1 metals in compounds +1 Na+ Alkaline earth/Gp 2 metals in +2 Mg2+ compounds Halogens/Group 7 in ionic -1 Cl- compounds Hydrogen in covalent compounds +1 HCl (non-metal) EXCEPT for hydrogen in ionic metal -1 NaH hydrides Oxygen in compounds -2 MgO  Oxidation numbers Rules for oxidation numbers (ON) Elements and Egs ON compounds Oxidation state of a Na+ +1 MONATOMIC ion is the Mg2+ +2 same as its charge O2- -2 SUM of oxidation states NO2 OS of N + 2x(OS of of all atoms = net O) = 0 charge on the species OS of N + 2x(-2) = 0 NO3 OS of N = +4 OS of N + 3x(-2) = -  Redox reactions edox reactions and oxidation numbers REDUCTION (decreases in ON) REDOX reactions OXIDATION (increases in ON) Eg. 2Mg(s) + O2(g) 2MgO(s) MAGNESIUM is oxidized (ON increases ) OXYGEN is reduced (ON decreases) Mg (s) Mg 2+ (s) + 2 e- O2 (g) + 4 e- 2O 2- (s) (0) ( +2 ) (0) ( -2 )  Pause & check Calculate the oxidation numbers of the following: Mn Answers: Mn 2+ 0 MnO2 +2 +4 MnO4  +7 NH3 3 HNO2 +3  Pause & check 2. Consider the following redox reaction: Cr2O72-(aq) + 6Fe2+(aq) + 14H+(aq)  2Cr3+(aq) + 6Fe3+(aq) + 7H2O(l) a. Identify the changes in the oxidation numbers b. Name the oxidizing and reducing agents Reduction + O R + + + 6 Cr AO A 1 + 3 1 2 7 (aq) + 6Fe2+(aq) 14H+(aq)  2Cr3+(aq) + 6Fe3+ 2- (aq) 2 + 7H2O(l) + + 2 2 3 Oxidatio n  In summary Oxidation is the addition of oxygen to an element or compound, removal of hydrogen, loss of electron/s Reduction is the removal of oxygen, addition of hydrogen, gain of electron/s Oxidation-reduction reactions are called redox reactions when they happen together