SCH4U Electrochemistry: Cell Reactions

The Standard Half-Cell and its Reactions

If the potential of a standard half-cell is assumed to be zero (0), then the experimentally measured difference in potential between the standard and a second half-cell is the relative potential of that second half-cell.

The standard half-cell assumed to be zero (0), is the H₂ half-cell. It is a half cell with H₂ gas bubbled in at SATP over an inert platinum electrode.

The other type of cell is the Daniell cell. It involves 2 separate beakers, each with a 1/2 of the cell. The cells are connected by a physical U-tube filled with a salt solution. Below is a standard cell with Ag and H₂.

In any standard cell the electrolytic concentrations are always 1 M. Below is a H₂ | Zn standard cell.

Ex #1: Zn | Zn⁺² || Ag | Ag⁺¹ Galvanic Cell

Go to the redox table and find both sets of equations:

E^o = +0.80 V Ag⁺¹_(aq) + e^-1 ---> Ag_(s) reduction
E^o = -0.76 V Zn⁺²_(aq) + 2e^-1 ---> Zn_(s) oxidation
Zn has the weaker reduction potential therefore it will be oxidized. Reverse the direction of the arrow and the polarity of E^oof the Zn reaction.

E^o = +0.80 V                  Ag⁺¹_(aq) + e^-1 ---> Ag_(s)     reduction
E^o = +0.76 V                  Zn_(s) ---> Zn⁺²_(aq) + 2e^-1     oxidation
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E_cell = +1.56 V        2 Ag⁺¹_(aq) + Zn_(s) ---- > 2Ag_(s) + Zn⁺²_(aq)

Ex.#2 Ag | Ag⁺¹ || Cu⁺² | Cu
E^o = +0.80 V Ag⁺¹_(aq) + e^-1 ----> Ag_(s) reduction
E^o = +0.34 V Cu⁺² + 2 e^-1 ----->Cu_(s) oxidation

Therefore
E^o = +0.80 V     2 Ag⁺¹_(aq) + 2 e^-1 ----> 2 Ag_(s) reduction
E^o = -0.34 V      Cu_(s) ----> Cu⁺² + 2 e^-1          oxidation
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E_cell = +0.54 V   Cu_(s) + 2 Ag⁺¹_(aq) ---->   Cu⁺²_(aq) + 2 Ag_(s)

Ex #3: What is the standard E^o potential for the reaction between Ag | Ag⁺¹ || Al⁺³ | Al ?

E^o = +0.80 V Ag⁺¹_(aq) + e^-1 ---> Ag_(s) reduction
E^o = -1.66 V Al⁺³_(aq) + 3 e^-1 -----> Al_(s) oxidation

Therefore
E^o = +0.80 V      3 Ag⁺¹_(aq) + 3 e^-1 ------> 3 Ag_(s) reduction
E^o = +1.66 V     Al_(s) ------>   Al⁺³_(aq) + 3 e^-1          oxidation
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E_cell = + 3.46 V     Al_(s) + 3 Ag⁺¹_(aq) ----> Al⁺³_(aq)   +   3 Ag_(s)

Go to the Standard Half-Cell Potentials Worksheet