EN.030.102 notes for entire course - Friday February 1 2013...

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Biology: The Dynamic Science
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Chapter 6 / Exercise 1
Biology: The Dynamic Science
Hertz/Russell
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Biology: The Dynamic Science
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Chapter 6 / Exercise 1
Biology: The Dynamic Science
Hertz/Russell
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Friday, February 1, 2013- Substitute for Professor Karlin, but going over slides 38-60Balancing Redox Reactions: Half-Reaction MethodStep 1: Determine which atoms change oxidation number- Review oxidation states- O = -2- O3- is -6, so N has to be +5- S is -2, so Cu is +2- Sulfer goes from -2 to +6, and losing electrons so OXIDATION- Nitrogen from +5 to +2, gaining electrons so REDUCTIONStep 2: Write out the half-reactions (not balanced, but with key compounds or ions present)- Oxidation in one line, reduction in another- Only the reactions that have changed in electronsStep 3: Balance charges by adding electrons to the right side for oxidation and left side for reduction- balancing and canceling..- ex: +6 + 8e- = 2-Step 4: Balance atoms (mass by adding H2O to balance oxygen and then H+ to balance hydrogen, in that order- This is for acidic media (solutions)- just like stoichiometric balancing- need water and h+ because you donʼt change any redox chemistry- balance the deficiency in O through H2O, then deficiency in H with H+Step 5: Multiply each half-reaction by an integer so the number of electrons in each is the same (so they cancel)- Once again, this is for acidic media (solutions)Step 6: Cancel and add the two half-reactions- The electrons should cancel out, among other things- check the charges and stoichiometry tooStep 7: Solve for acidic media frist. Add OH- to both sides for every H+. H+ + OH- becomes H2O. Cancel water.- this is for BASIC media (solutions)- cancel some of the waters to simplify and rewrite the reaction- add OHCell Potential:- Electrons are “driven” from anode to cathode by an electromotive force (emf)- the driving force that makes the reaction happen
- a negative delta G is spontaneous, but a positive voltage is spontaneous- For Zn/Cu cell, this is indicated by a voltage of 1.10V across the cell at 25ʼC and when [Zn2+] and [Cu2+] = 1.0 M (standard state, so standard cell potential Eʼ)- Standard Cell Potential: A quantitative measure of the tendency of reactants to proceed to products when all are in their standard states, 1 atm (gases) and 1M (solutions)- Zn2+ + Cu -> Zn + Cu2+- Instead of tabulating the potential for every possible electrochemical cell, half-reactions potential s are tabulated and combined- Balanced half-reaction can be added together to get overall, balanced equations- If we know Eʼfor each half-reaction, we can know Eʼfor the net reaction.- Canʼt measure half reaction Eʼdirectly, so measure it relative to a standard hydrogen electrode (SHE, aka NHE)- 2H+ (aq, 1M) + 2e- -> H2 (g, 1atm)- Eʼ= 0.0V- The only thing that matters is the potential DIFFERENCE so they use SHE as the standard, just like in elevation sea level is the zero.So, can hook up Zn|Zn2+ half-cell to a SHE. Delta Eʼmeasured for the cell = +0.76V... if this is negative, just switch the cells- Zn -> Zn2+ + 2e- = oxidation- Oxidation = anode (VOWELS!!) = supplier of electrons- Reduction = cathode (CONSONANTS!!) = acceptor of electronsWhen using oxidation and reduction reactions, you add the cathode and anodeIf you have both reduction reactions, you have to subtract themso, delta E cell = Eʼcathode + Eʼanode+0.76 V = 0.00V + anode

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