Study Assignment 9 Electrochemistry Chemistry R122 1. a) What is the electromoti
ID: 547276 • Letter: S
Question
Study Assignment 9 Electrochemistry Chemistry R122 1. a) What is the electromotive force at 25 for a cell made up of the following Zn! Zn2+(0.10M) 11 Cu2+ (1.0 x 10"M) 1 Cu b) What is G for the above cell? c) Calculate the equilibrium constant for this reaction at 25°C. 2. The following cell is set up: 2. The following cell is setup Pereus Barrier pH-10.0 0.1M KMnO4 0.01M PDINOal2 Pt Pb a) What are the reactions taking place at the electrodes? b) What will the cell voltage be? c) What is the maximum amount of work you can get from this cell (per mole of electrons)? A galvanic cell consisting of a Cu2"- Cu half cell versus a H - H2 half cell was used to determine the pH of an unknown solution. The unknown solution was placed in the hydrogen half cell and the pressure of the hydrogen gas controlled at 1 atm. The concentration of Cu" in the Cu"-Cu half cell was 1.0 M and the emf of the cell at 25°C was determined to be +0.48V 3. a) Draw the cell and label the anode and the cathode. b) Label which electrode is positive and which is negative. c) Write the balanced equation for the reaction d) Calculate the pH of the unknown solution.Explanation / Answer
Q1
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
Zn2+ + 2 e Zn(s) 0.7618; Cu2+ + 2 e Cu(s) +0.337
E° = Ered - Eox = 0.337 - -0.7618 = 1.0988 V
n = 2 e-,
Q = (10^-4)/(0.1) = 0.001
Ecell = E0cell - (RT/nF) x lnQ
Ecell = 1.0988 - (8.314*298/(2*96500) * ln (0.001)
Ecell = 1.18747 V
b)
dG = -nF*Ecell
dG = -296500*1.18747
dG = -352084.855J/mol
dG = -352.085 kJ/mol
c)
dG = -RT*ln(K)
K = exp(-dG/(RT))
K = exp(352084.855/(8.314*298))
K = 5.212*10^61