Electrochemical Cells (Spontaneous)

Of all species present, the Highest on the Left (std. red’n. pot. chart) will be reduced at the CATHODE. The Lowest on the Right will be oxidized at the ANODE.

e- always flow toward the CATHODE (in the wire) Cations migrate toward the CATHODE Anions migrate toward the ANODE

HOL Species Reduced Oxidizing Agent Electron Acceptor LOR Species Oxidised Reducing Agent Electron donor

Standard Reduction Potential This refers to the amount of work done per electron gained (red’n), under standard conditions (1M solutions, 1atm, 25  C). Since red’n cannot occur without oxid’n, half-reactions cannot be assessed independently. So, each half-reaction is connected to a reference cell (The Hydrogen Half- Cell), and the red’n potential is really a comparison (a potential difference) between half-cells.

Hydrogen Half-cell

The Hydrogen Half-Cell was chosen arbitrarily, and was given a reduction potential of 0.00V. If you are given a series of reactions, which are not on your chart, you can still compare them by setting the lowest half-cell equal to 0.00V. You will be required to make up your own reduction potential chart (activity series).

Ex 1: You have 5 metals A, B, C, D and E, each in 1.00M solutions of their trivalent cations. When some of the half-cells are connected, the following results are found: CathodeAnodeE˚cell BE-0.23V DA+0.36V CE-0.07V BA-0.85V List the reducing agents in order of increasing strength.

When two half-cells are connected in an electrochemical cell (battery), the total cell potential can be calculated. Cr e -  Cr (s) E  = -0.74V Zn e -  Zn (s) E  = -0.76V Highest on the left reacts with lowest on the right. 2x(Cr e -  Cr (s) )E  = -0.74V 3x(Zn (s)  Zn e - )E  = +0.76V 2Cr Zn (s)  2Cr (s) + 3Zn 2+ E  Cell = +0.02V Flip over, flip sign of E˚ Multiply up, don’t multiply E˚

What is the E˚cell? Ex 2:

Ni, Ni 2+, Cl 2, Cl -, K +, NO 3 -, H 2 O

Ex 3: What is the E˚cell when copper and Fe(NO 3 ) 2 are combined?

Spontaneous REDOX reactions occur if the E  cell is positive. Negative E  cell values are non-spontaneous, and must be forced. The SIZE of E  cell does not determine the reaction rate.

Water ½ O 2(g) + 2H + (10 -7 M) + 2e-  H 2 OE  = +0.82V Acidic conditions 2H + + 2e-  H 2(g) E  = 0.00V 2H 2 O + 2e-  H 2 + 2OH - (10 -7 M)E  = -0.41V Basic conditions 2H 2 O + 2e-  H 2(g) + 2OH - E  = -0.83V

Shifting the Equilibria Look at these two reactions: (1M H + ) 2H + + 2e -  H 2(g) E  = 0.00V (10 -7 M H + ) 2H 2 O + 2e -  H 2 + 2OH - (10 -7 M) E  = -0.41V When you lower the [H + ], essentially shifting the equilibrium left, the potential also decreases.

Effect of Surface Area The concentration of a solid (the electrodes) is constant, so changing the amount of solid present does not shift the equilibria; and, has no effect on cell potential. But, changing the surface area DOES increase the reaction rate. Batteries would discharge faster, but with the same voltage.

As Cell Approaches Equilibrium… As cells react, the concentration of ions changes, causing shifts in the equilibria. The effect is that the overall cell potential may decrease to zero at equilibrium (if ion concentrations are changing). Let’s look at example #1 again… After discharging Cl 2(g) + 2e -  2Cl - E  = +1.36V+0.70V Ni (s)  Ni e - E  = +0.26V-0.70V Cl 2(g) + Ni (s)  2Cl - + Ni 2+ E  Cell = +1.62V0.00V

From the species present, pick the highest on the left, and the lowest on the right. Flip the potential of the species on the right. Add up the potentials. Do not multiply E  values when balancing. Sometimes you have to make up your own reduction potential chart based on which species spontaneously react, and their cell potentials.

Ex: Reducing agents yttrium, uranium and vanadium, from the strongest to the weakest, are Y, U and V. Which of the following statements is true? A. V will react with the Y 3+ ion B.V will react with the U 3+ ion C.V will react with both U 3+ and Y 3+ ions D.V will not react with either U 3+ of Y 3+ ions

Ex: Consider the following: Identify the standard potential for the half-cell reaction: A VB VC VD V

Ex: The metals Hg, Cd, Ga, and Pd react as follows: Which of the metals is the strongest reducing agent? A. HgB. CdC. GaD. Pd

Work on Questions… 35, 36abcdf, 37, 38, 40, 41, 43, 46