25.The following cell is initially at standard-state conditions. Which of the fo
ID: 562256 • Letter: 2
Question
25.The following cell is initially at standard-state conditions. Which of the following statements is true after the cell is allowed to discharge (do work) for a period of time? Vol tmetere Cu sa Cu Zn2+(aq) + 2e Zn(s); Eo =-0.76 V Cu2+(aq) + 2e-=-Cu(s); Eo = 0.34 V A) Initially Ecel,--1.10 V, and it will become more negative with time. B) Ecell does not change with time. C) Initially Ecell 1.10 V, and it will become more positive with time. D) Initially Ecell = + 1.10 V, and it will become more negative with time. E) Initially Ecel,-+ 1 . 10 V, and it will becomes more positive with time.Explanation / Answer
E°cell = Ered - Eox = 0.34 --0.76 = 1.1 V
intiially --> 1.1 V is true, so ignore A and C
clearly, as reactants start to form new products, there is change in concentrations
When the cell is NOT under standard conditions, i.e. 1M of each reactants at T = 25°C and P = 1 atm; then we must use Nernst Equation.
The equation relates E°cell, number of electrons transferred, charge of 1 mol of electron to Faraday and finally, the Quotient retio between products/reactants
The Nernst Equation:
Ecell = E0cell - (RT/nF) x lnQ
In which:
Ecell = non-standard value
E° or E0cell or E°cell or EMF = Standard EMF: standard cell potential
R is the gas constant (8.3145 J/mol-K)
T is the absolute temperature = 298 K
n is the number of moles of electrons transferred by the cell's reaction
F is Faraday's constant = 96485.337 C/mol or typically 96500 C/mol
Q is the reaction quotient, where
Q = [C]^c * [D]^d / [A]^a*[B]^b
pure solids and pure liquids are not included. Also note that if we use partial pressure (for gases)
Q = P-A^a / (P-B)^b
substitute in Nernst Equation:
Ecell = E° - (RT/nF) x lnQ
therefore, if Q increases/decreases, E changes
ignore B
as time passes by, Ecell < E°cell
therefore
it becomes more negative with time
choose "D"
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