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?
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).
In the above cell, the concentrations of
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.
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
Calculating an unknown half-cell potential
We can write the equations for the half-cells in the first image.
#"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