**Electrochemical Series** is a way of ordering chemical elements in order of how reactive they are. It uses a concept called **Standard** **Electrode Potential** to arrange the elements in the order. It has multiple uses in chemistry.

Index

## What is Electrochemical Series?

The **electrochemical series** is also called the “**activity series**”. It is a method of arranging elements in order of increasing reactivity. The ordering uses a value called “**electrode potential value**”.

### What is Standard Electrode Potential Value?

Electrode potential value is calculated as follows. An electrode consisting of metal (or non-metal) and its ion is connected to the **Standard Hydrogen Electrode** (SHE). SHE consists of H_{2} and H^{+} ions.

Depending on what metal and ions are used, a **characteristic value** of voltage is observed across the electrodes, in standard conditions. This is called the “**standard electrode potential value**” for the given metal/ion combination.

### What Does the Electrochemical Series Tell Us?

The electrochemical series tells us how *electropositive or electronegative* the element/ion combination is, compared to** Standard Hydrogen Electrode**. This combination is also called a **half-cell**.

A more electropositive metal *loses electrons more easily than hydrogen* does in the SHE. On the other hand, a more electronegative element* gains electrons more easily. *

In general, a more electronegative element takes electrons from a more electropositive one. The electrochemical series thus can be said to be a** measure of electronegative nature. **

## Features of Electrochemical Series

Above is a sample of common elements in the Electrochemical Series. Here are some important features of it:

- Hydrogen (the Standard Hydrogen Potential, SHE) has an electrode potential of
**0.00**by definition. All other potentials are defined with respect to it. - The half-cells (element/ion pairs) with very positive Electrode Potential are high in the Electrochemical Series. They are
**strong oxidising agents.** - The half-cells with
**negative electrode potential**are**reducing agents**. The more the reducing power, the more negative is the value. - Metals are in general are electropositive, while non-metals are electronegative.
- The
*metals at the bottom*are the**most reactive**. In comparison, the*non-metals at the top*of the series are the**most active**. Thus, reactivity is the minimum along the middle. - Metals higher in the series can be
**reduced**by metals lower in the series. Likewise, non-metals higher in the series can**oxidise**the metals and nonmetals lower in the series.

## Applications of Electrochemical Series

### 1. Calculation of Cell EMF

Any electrochemical cell consists of two half-cells, at each electrode. Each half cell undergoes a reaction, one is oxidation and the other is reduction. Corresponding to each reaction there is a potential, namely, **oxidizing potential** and** reducing potential.**

Cell EMF (\(E^circ_{\text{cell}}\)) is the **sum of oxidizing and reducing potentials of the cell**. It measures the spontaneity of the overall reaction in the cell. It also is a measure of the work that can be done by the cell.

The electrochemical series helps us measure the cell EMF by taking **Standard Electrode Potential** values of the half cells, and then adding them suitably.

\(E^\circ_{\text{cell}} = E^\circ_{\text{red}} – E^\circ_{\text{ox}}\)

Here, \(E^\circ_{\text{red}}\) is the **Standard Reduction Potential** *for the reduction half-cell*, and \(E^\circ_{\text{ox}}\) is the **Standard Reduction Potential** *for the oxidation half-cell.*

### 2. Measuring Spontaneity of a Reaction

The *feasibility or spontaneity* of a redox reaction is directly related to the cell EMF of the corresponding reaction.

- If the cell
**EMF is positive**, the reaction is*spontaneous.* - If the cell
**EMF is negative**, the reaction is*nonspontaneous.*

Thus, we can tell if a redox reaction can proceed spontaneously by examining the reactants and products. We write the equations for both reduction and oxidation half-reaction. Then we suitably add their Standard Electrode Potentials based on the electrochemical series.

The resulting cell EMF tells us if the reaction is spontaneous or not.

### 3. Estimating Gibbs Free Energy

**Gibbs free energy** (\(\Delta G^\circ_{\text{cell}}\)) is another measure of the **spontaneity** of a reaction. It is related to the cell EMF (\(E^\circ_{\text{cell}}\)) as follows.

\(\Delta G^\circ_{\text{cell}} = -n F E^\circ_{\text{cell}}\)

Where \(n\) is number of electrons involved in the reaction,

\(F\) is** Faraday’s constant,** which is equal to \(96,485 \text{ Coulomb}{\text{ mole}}^{-1}\)

Again, based on sign of cell EMF, we have the following:

- If cell EMF is
**negative**, Gibbs Free energy is**positive**and the reaction is**not spontaneous.** - If the cell EMF is
**positive**, Gibbs Free energy is**negative**, and reaction is**spontaneous**.

### 4. Predicting End-Products of a Redox Reaction

If we are given only the reactants of a reaction, we can calculate the **end products** of the reaction as follows.

We use the electrochemical series to write the Standard Electrode Potential values of each of the reactants. Then, we note which has the** highest** and **lowest** reduction potential.

Once we have these values, we can predict the end-products as follows:

- The ions with
*highest reduction potential*are**reduced at the cathode**. - The ions with
*least reduction potential*are**oxidized at the anode.**

The oxidised and reduced ions give us the end-products of the reaction.

## Examples

**Question 1.** Which of the following is the best oxidizing agent and which is the best reducing agent?

\(\mathrm{Fe}^{2+}, \mathrm{Fe}^{3+}, \mathrm{Pb}^{4+} and \mathrm{Fe}^{2+}\)

**Solution.** From the values of Standard Electrode Potential from Electrochemical Series, we have that \(\mathrm{Pb}^{4+}\) has the **most positive** Standard Electrode Potential, thus it is b**est oxidizing agent. **

\(\mathrm{Fe}^{2+}\) has the **least** Standard Electrode Potential thus \(\mathrm{Fe} |\mathrm{Fe}^{2+}\) is the **best reducing** agent.

**Question 2.** Calculate the cell EMF of the following reaction:

\($\mathrm{Fe}^{2+} + \mathrm{Cu} \rightarrow \mathrm{Cu}^{2+} + \mathrm{Fe}\)

Is it a spontaneous reaction?

**Solution.** Clearly, the reaction consists of the **following half-reactions.**

\(\mathrm{Fe}^{2+} + 2e^- \rightarrow \mathrm{Fe}\)

\(\mathrm{Cu} \rightarrow \mathrm{Cu}^{2+} + 2e^-\)

From the Electrochemical Series, we have the following:

**Standard Reduction potential **for \(\mathrm{Fe}^{2+} | \mathrm{Fe}\) reduction half-cell \(E^\circ_{\text{red}} = -0.44 \text{ V}\).

**Standard Reduction Potential** for \(\mathrm{Cu} | \mathrm{Cu}^{2+}\) oxidation half-cell \(E^\circ_{\text{ox}} = 0.34 \text{ V}\).

Thus, cell EMF is given by,

\(\begin{align}

E^\circ_{\text{cell}} &= E^\circ_{\text{red}} – E^\circ_{\text{ox}} \\

&= -0.44 – 0.34 \text{ V} \\

&= -0.78 \text{ V}

\end{align}

\)

**As cell EMF is negative**, it is not a **spontaneous reaction**.

## FAQs

**What is electrochemical series?**Electrochemical series is a way of ordering chemical elements in decreasing order of Standard Electrode Potential. In simple terms, it arranges them in order of their reactivity.

**How to remember electrochemical series?**Here is a **mnemonic** for remembering electrochemical series:**Like King Canute Named Magnificent Altered Zebras For Conquering Nile, Snake Hunter Completed Investigating Silver Mercedes Breakdown Claims Over a Flight. ****Li**ke – Li – Lithium**K**ing – K – Potassium**Ca**nute – Ca – Calcium**Na**med – Na – Potassium**Magn**ificent – Mg – Magnesium**Al**tered – Al – Aluminium**Z**ebras – Zn – Zinc**F**or – Fe – Iron**Co**nquering – Co – Cobalt**Ni**le – Ni – Nickel**Sn**ake – Sn – Tin**H**unter – H – Hydrogen**C**ompleted – Cu – Copper**I**nvestigating – I – Iodine**Silver** – Silver**Merc**edes – Mercury**Br**eakdown – Br – Bromine**Cl**aims – Cl – Chlorine**O**ver – Oxygen

A**F**light – F – Fluorine

This **Electrochemical Series trick** gives the elements in increasing order of Standard Electrode Potential.

**What decides order of electrochemical series?**The order of electrochemical series is based on the Standard Electrode Potential of the half-cells, specifically, the Standard Reduction Potential.

**What is the strongest and weakest reducing agents in electrochemical series?**The *strongest reducing agent* in electrochemical series is **Lithium**, which has Standard Electrode Potential -3.05 V.

The *weakest reducing agent* in electrochemical series is** Fluorine**, which has Standard electrode Potential +2.87 V.