Electrode Potential

Standard Electrode Potential

Consider this equation below.

$$Cu^{2+}_{(aq)}+2e^{-}\rightleftharpoons Cu_{(s)}$$

This equation shows equilibrium between Cu²⁺ ions and ground state Cu. These are the two oxidation states of Copper - +2 and 0. The Copper ion has to gain 2 electrons to get to its ground state with 0 net charge.

Electrode potential is measured in Volts. Standard electrode potential for Cu²⁺ is +0.34 volts.

Let's consider the Chlorine atom. You know that Chlorine has 7 electrons in its outermost electron shell, and only needs 1 more to have a completely filled stable electron shell. This means Chlorine has a high tendency to gain 1 electron and to exist with an oxidation state of -1. Since Standard Electrode Potential is a measure of the ability of a species to gain or lose an electron, Chlorine has a high Standard Electrode Potential.

$$Cl_{2}+2e^{-}\rightleftharpoons 2Cl^{-}$$

E° for Chlorine is E° = +1.36V.

A species that gains electrons is said to undergo reduction and the electrode potential measured for these reactions is called the reduction potential of that species. Therefore, the reduction potential for Chlorine is +1.36V. The reduction potential of Cu²⁺ is +0.34V.

Let us see for Vanadium.

$$V^{2+}+2e^{-}\rightleftharpoons V$$

V²⁺ has a reduction potential of -1.2V. What does the negative sign imply? It implies that Vanadium is more likely to lose electrons than gain them i.e., more likely to get oxidized than get reduced. Therefore, Vanadium has the oxidation potential of +1.20V.

The 3 reactions which are written above (Cu²⁺, Cl₂, V²⁺) are called half equations, or half cells. They only show the reduction side of a chemical reaction. But, in a chemical reaction, oxidation and reduction is happening in tandem. Therefore, these reactions are called redox reactions (reduction + oxidation) (Cool name, no?).

Generally, half equations are written as reduction equations, and standard electrode potential is written as standard reduction potential. This is done to avoid confusion and maintain uniformity. Since oxidation potential is just the negative of reduction potential, only one of them needs to be calculated.

Electrode Potential Table

Standard electrode potentials, E°, can be listed as an electrochemical series. This is also called the electrode potential Table. This table consists of half reactions of species undergoing reduction and stating the reduction potential of that half reaction. It is the IUPAC convention to always write the half reactions as reduction reactions i.e., The species are always shown to be gaining electrons. This is why the standard electrode potential of any species is the same as its standard reduction potential. This is done to establish a standard when comparing electrode potentials of any two species.

Fig. 1: Electrode Potential Table |Inspirit

Standard electrode potential, E°, refers to conditions of 298 K, 100 kPa and 1.00 mol dm⁻³ solution of ions.

Redox Reactions

Let us consider the half equations for Copper and Vanadium.

$$Cu^{2+}_{(aq)}+2e^{-}\rightleftharpoons Cu_{(s)}$$

$$V^{2+}+2e^{-}\rightleftharpoons V$$

What If we combined these 2 half equations or half cells? One of the two (either Copper or Vanadium) will have to be the one that gives electrons and the other will have to be the one that accepts electrons. You have already seen that Copper has a tendency to accept electrons (get reduced), and Vanadium has a tendency to lose electrons (get oxidized). So naturally, the half-reaction for Copper will go in the forward direction, and the half-reaction for Vanadium will go in the reverse direction. In other words, Cu²⁺ will get reduced to Cu, and V will get oxidized to V²⁺.

Giving the following reaction:

$$Cu^{2+}_{(aq)}+V_{(s)}+2e^{-}\rightleftharpoons V^{2+}_{(aq)}+Cu_{(s)}+2e^{-}$$

Electrons and the net charge on both sides are balanced.

$$Cu^{2+}_{(aq)}+V_{(s)}\rightleftharpoons V^{2+}_{(aq)}+Cu_{(s)}$$

This whole equation will also have a net electrode potential value. The net electrode potential value for the reaction is the difference between the electrode potential of the reduction reaction and electrode potential of the oxidation reaction. -

$$E\degree_{cell}=E\degree_{red}-E\degree_{ox}$$

For the Copper-Vanadium cell, Copper undergoes the reduction reaction, and Vanadium undergoes the oxidation reaction.

$$E\degree _{red}=+0.34V$$

$$E\degree _{ox}=-1.20V$$

$$E\degree_{cell}=+0.34-(-1.20)$$

$$E\degree_{cell}=+0.34+1.20$$

$$E\degree_{cell}=+1.54V$$

Galvanic Cells

You have seen how 2 half reactions can combine to give a redox reaction. You have also seen that these redox reactions have a net voltage. Thus, redox reactions can be used to form an electrochemical cell called galvanic cell, which can produce electric current. They combine the electrode potentials of the two half reactions form an electrical circuit with a net EMF.

To make a galvanic cell, you will need:

• 2 containers to contain electrolyte of the 2 half reactions.
• 2 electrodes.
• A salt bridge. Salt bridge is needed to facilitate the movement of ions from one electrolyte to the other.
• A wire to connect the two electrodes.
• A voltmeter to measure the voltage across he cell.

For the Zn-Cu galvanic cell, you need:

• A Zinc electrode and a copper electrode.
• Zinc Sulphate (ZnSO₄) and Copper Sulphate (CuSO₄) solutions for the electrolytes.
• Potassium Chloride (KCl) solution for the salt bridge.

Zn-Cu Galvanic Cell | Periodni Chemistry Glossary

You already know that in the Zn-Cu cell, Copper undergoes reduction and Zinc undergoes oxidation i.e., Copper will gain electrons and Zinc will lose electrons. Zn atoms from the solid electrode will release electrons and get dissolved as Zn²⁺ ions in the ZnSO₄ electrolyte solution. The electrons released by Zn atoms will travel through the contact wire over to the Cu electrode. Cu²⁺ ions from the CuSO₄ electrolyte will gain these electrons and convert into solid Cu with the electrode. This whole process is facilitated due to the electric potential difference between the two half cells, which can be measured in the voltmeter attached to the wire connecting the 2 electrodes.

Recall the definition for electrode potential you read at the beginning of this article. Now that you have understood the concept, let us define electrode potential, and revise the definition of standard electrode potential.In a half cell, due to the separation of charges between the electrode and the electrolyte, the electrode may be positively or negatively charged with respect to the electrolyte. This charge difference is what causes the potential difference between the electrode and the electrolyte. This potential difference is the electrode potential. The electrode potential is called standard electrode potential when the concentration of all species involved in a half cell is unity.

There is no way of measuring the electric potential of an electrolyte, therefore, electrode potential for a species is measured according to this definition:

Measuring Standard Electrode Potential

In the section "Electrode Potential Table", we briefly mentioned that electrode potential for H⁺ is considered 0. It is the reference electrode for all other species and standard electrode potential for all species is measured against H⁺ , called the Standard Hydrogen Electrode (SHE). In this section, we will see how standard electrode potential is measured against the Standard Hydrogen Electrode.

First of all, how can an electrode be made out of Hydrogen? It's a tube in which Hydrogen gas is passed. A piece of Platinum serves as the electrical contact and also as a catalyst in the half reaction of Hydrogen. The tube is dipped in an electrolyte containing H⁺ ions. The other half of the galvanic cell consists of the electrode of which the electrode potential has to be measured.

The setup to calculate the standard electrode potential of Zinc is described in the figure -

Measure Electrode Potential of Zinc against Standard Hydrogen Electrode | Saylor.org

In this cell, Zinc forms the oxidation half cell and the SHE forms the reduction half cell. That means Zinc undergoes oxidation and Hydrogen undergoes reduction. You already know that the electrode where oxidation takes place is called the anode, and the electrode where reduction takes place is called the cathode. Since Zinc forms the anode in this cell, and electrode potential of cathode 0 (Since it is a SHE), the EMF measured for this cell is negative. Mathematically, it can be calculated as:

$$E\degree_{cathode}=0V$$

$$E\degree_{cathode}=0.76V$$

$$E{cell}=0-0.76$$

$$E_{cell}=E_{Zn}=-0.76V$$

At the beginning of this article, we defined the term "standard electrode potential". Now that we have understood everything around the term, let us redefine it.

A cell can be made using two species (two half reactions). The net cell potential depends on the electrode potential of the two species.