# 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?

Jun 11, 2017

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).

In the above cell, the concentrations of ${\text{CuSO}}_{4}$ and ${\text{ZnSO}}_{4}$ 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.

The $\text{Zn}$ 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 $\text{Zn/Zn"^"2+}$ 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

Calculating an unknown half-cell potential

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

$\textcolor{w h i t e}{m m m m m m m m m m m m m m} {E}^{\circ} / V$
"Cu"^"2+" + 2"e"^"-" → "Cu"; color(white)(mmmmmm)?
$\text{Zn" → "Zn"^"2+" +2"e"^"-"; color(white)(mmmmm)"+0.763}$
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 $\text{Zn/Zn"^"2+}$ half cell and the $\text{Cu/Cu"^"2+}$ half-cell potential is +0.337 V.