What is standard potential? Is standard potential for a particular substance is constant ( standard potential for zinc = #-0.76# v ) ? How to calculate the same?

1 Answer
Jun 11, 2017

Answer:

See below.

Explanation:

There are two types of standard potential: standard cell potential and standard half-cell potential.

Standard cell potential

Standard cell potential is the potential (voltage) of an electrochemical cell under standard conditions (concentrations of 1 mol/L and pressures of 1 atm at 25 °C).

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In the above cell, the concentrations of #"CuSO"_4# and #"ZnSO"_4# are each 1 mol/L, and the voltage reading on the voltmeter is the standard cell potential.

Standard Half-cell potentials

The problem is, we don’t know what portion of the voltage comes from the zinc half-cell and how much comes from the copper half-cell.

To get around this problem, scientists have agreed to measure all voltages against a standard hydrogen electrode (SHE), for which the standard half-cell potential is defined as 0 V.

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The #"Zn"# turns out to be negative, so the zinc/SHE cell has a standard cell potential of -0.76 V, and the standard potential of the #"Zn/Zn"^"2+"# half cell is -0.76 V.

We can measure the half-cell potentials of many reactions against the SHE and put them in a list of standard half-cell potentials.

If we list them all as reduction half-reactions, we have a table of standard reduction potentials. Here is a short list

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Calculating an unknown half-cell potential

We can write the equations for the half-cells in the first image.

#color(white)(mmmmmmmmmmmmmm)E^@//V#
#"Cu"^"2+" + 2"e"^"-" → "Cu"; color(white)(mmmmmm)?#
#"Zn" → "Zn"^"2+" +2"e"^"-"; color(white)(mmmmm)"+0.763"#
#stackrel(————————————)("Cu"^"2+" + "Zn" → "Cu" + "Zn"^"2+");color(white)(m)"+1.100"#

If we find the cell potential to be 1.100 V, we know that 0.763 V comes from the #"Zn/Zn"^"2+"# half cell and the #"Cu/Cu"^"2+"# half-cell potential is +0.337 V.