The Nernst Equation and pH Sulfuric acid is a very strong acid that can act as a

Question

The Nernst Equation and pH Sulfuric acid is a very strong acid that can act as an oxidizing agent at high concentrations (very low pH, or even negative pH values). Under standard conditions, sulfuric acid has a low reduction potential, so42-(aq) + 4H + (aq) + 2e- SO2 (g) + 2H2O(l), + 0.20 V which means it cannot oxidize any of the halides F2, Cl2, Br2, or I2. If the H ion concentration is increased, however, the driving force for the sulfuric acid reduction is also increased according to Le Châteliers principle. Sulfuric acid cannot oxidize the fluoride or chloride anions, but it can oxidize bromide and iodide anions when there are enough H ions present. The standard reduction potentials of the halogens are as follows: F2 + 2e- Cl2+2e- Br2+2e- 12 + 2e- ZF-, 2C -, 2Br-, 21, +2.87 V +1.36 V +1.07 V +0.54 V The Nemst equation allows us to determine what nonstandard conditions allow the reaction to occur (have a positive E value) The Nernst equation relates a nonstandard potential E, to the standard potential, E,and the reaction quo ent, Q by o 2.303RT 0.0592V where R = 8.314 J/ mol . K T is the Kelvin temperature, n s the number of moles of electrons transferred in the reaction, and F-96, 485 C/mol e Part A At 68.0 °C, what is the maximum value of the reaction quotient, Q, needed to produce a non-negative E value for the reaction O4 (a)+4H (a)+ 2Br (aq) Br2(aq) +SO2()+2HO() n other words, what is Qwhen E 0 at this temperature? Express your answer numerically to two significant figures. Hints Q- Submit My Answers Give Up

Explanation / Answer

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

get E°cell

SO4 2 + 4H+ + 2e SO2(g) + 2H2O +0.17;

Br2(aq) + 2e 2Br +1.0873

E°cell = 0.17-1.0873 = -0.9173

then

Ecell = E° - (RT/nF) x lnQ

0 = -0.9173 - (8.314*(68+273) / (2*96500) * ln(Q)

-0.9173 /(8.314*(68+273) ) *  (2*96500) = ln(Q

Q = exp(-62.445)

Q = 7.5941*10^-28

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