4. [20 points] Heart pacemakers are often powered by lithium-silver chromate \"b

Question

4. [20 points] Heart pacemakers are often powered by lithium-silver chromate "button" batteries. The overall cell reaction is 2 Li(s) + Ag2CrO4(s) Li2CrO4(s) + 2 Ag(s) (a) Lithium metal is the reactant at one of the electrodes of the battery. Is it the anode or the cathode? (b) Choose the two half-reactions from Appendix E that most closely approximate the reactions that occur in the battery. What standard emf would be generated by a voltaic cell based on these half-reactions? (c) The battery generates an emf of +3.5 V. How close is this value to the one calculated in part (b)? (d) Calculate the emf that would be generated at body temperature, 37 °C. How does this compare to the emf you calculated in part (b)?

Explanation / Answer

a)

Lithium metal is going from Li(s) to Li2CrO4(s), at products side Li is present in +1 oxidation state since CrO4 is having a -2 charge

whereas Li is present in 0 oxidation state in left hand side

Hence the lithium metal must be present at anode for oxidation purpose

b)

Two Half cell reactions

Oxidation Half

Li(s) ------ Li(+)(aq) +e-

Reduction Half

Ag+(aq) + e- -------- Ag(s)

Net Reaction

Li(s) + Ag+(aq) ------ Li+(aq) + Ag(s)

Net EMF = Oxidation half + reduction half = 3.85V

c)

Percentage Error = |3.85-3.5|/3.85 * 100 = 9.09%

d)

Since the reaction is having Ecell positive, hence the reaction will be having negative G which means increasing temperature will make Delta G more negative and increase the value of Ecell

Since observed value will increase, hence it will reach more closer to 3.85

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