Question

Consider a galvanic cell that consists of a Ni^2+/Ni half-cell and a standar hydrogen electrode. The concentration of Ni^2+ is 0.1M, the pressure of H2 is 1atm and the concentration of H+ is 0.05M.

A. write a balanced reaction for the cell.

B. what is the oxidizing and reducing agent?

C. Calculate the Eo cell

D. Calculate the Ecell

E. Is the reaction spontaneous under these conditions?

Answer 1

Ni2+ + 2 e− ⇌ Ni(s) −0.25

2 H+ + 2 e− ⇌ H2(g) 0.00

The reaction will favour H2(g) formation and Ni2+ formation as well (forward direction)

a)

the balanced equation

2H+(aq) + Ni(s) = Ni2+(aq) + H2(g)

b)

Reducing agent = The species that favors reduction, i.e. it will oxidize in order to reduce another species. In this case, this is the solid Nickel

Oxidizing agent = The species that favors oxidation, i.e. it will reduce in order to oxidise another species. In this case, this is the H+ in solution

c)

Remember that each species will have a specific reduction potential. Remember that this is, as the name implies, a potential to reduce. We use it to compare it (numerical) with other species.

Note that the basis if 2H+ + 2e- -> H2(g) reduction. Therefore E° = 0 V

Then

E°cell = Ecathode - Eanode = 0 - (-0.25) = 0.25 V

D)

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

Q = P-H2 * Ni2+ / (H+)^2

Ecell = 0.25 - 8.314*298/(2*96500) * ln((1*0.1)/(0.05^2)) = 0.202645

e)

Since Ecell > 0, this must be spontaneous