Have you ever wondered what is inside a battery? You already know electric current results from the movement of electrons. So what are batteries made of so that they can provide flowing electrons? Read on to learn!
- In this article, you will learn about Standard Electrode Potential of a species
- You will learn about reduction potential and oxidation potential of a species.
- Oxidation state or oxidation number of a species.
- What is meant by oxidation or reduction of a species in a chemical reaction.
- Half-reactions or half-cells.
- Redox reactions.
- Galvanic cell.
- How chemical energy can be converted to electrical energy.
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.
The measure of the ability of a species to gain or lose electrons is called Standard Electrode Potential (E°).
The Oxidation State or Oxidation Number is the number of electrons a species has to gain or lose to form bonds with other species.
A species gets oxidized when it loses electrons. The oxidation state (charge on ion) increases.
Conversely, a species gets reduced when it gains electrons. Oxidation state decreases.
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.
Reduction Potential is a measure of the ability of a species to gain electrons and get reduced in the process.
Conversely, Oxidation Potential is a measure of the ability of a species to lose electrons and get oxidized in the process.
Numerically, Oxidation Potential is the negative of Reduction Potential.
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.
Species with highly positive reduction potentials are good oxidizing agents, since they can oxidize other species easily.
Conversely, species with highly negative reduction potentials are good reducing agents, since they can reduce the other species easily.
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.
The reduction potential of H+ is the reference point for all other species, therefore, it is considered 0. Electrode potential for all half reactions is actually measured with reference to H+.
Redox Reactions
Redox reactions are chemical reactions where both an oxidation reaction and a reduction reaction are taking place in tandem.
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+.
we can't have both half reactions giving a receiving electrons because the electrons in the equation have to be balanced too. The electrons have to come from either of these two, they can't come from thin air!
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.
$$Cu^{2+}_{(aq)}+V_{(s)}\rightleftharpoons V^{2+}_{(aq)}+Cu_{(s)}$$
When combining 2 half equations, Balancing the charge on both sides of the equation is of utmost importance!
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.
$$E\degree _{red}=+0.34V$$
$$E\degree _{ox}=-1.20V$$
$$E\degree_{cell}=+0.34-(-1.20)$$
$$E\degree_{cell}=+0.34+1.20$$
$$E\degree_{cell}=+1.54V$$
The two half equations or half cells combine to give a redox reaction. The net electrode potential of these redox reactions can be used as a battery, called an electrochemical cell.
The total E° measured for a cell is called Electromotive Force of the cell, or simply EMF.
Electromotive Force (EMF) is defined as an electrical action produced by a non-electrical source.
Let us take another example - the Zinc-Copper battery. Consider the half-reactions for Copper and Zinc:
$$Cu^{2+}_{(aq)}+2e^{-}\rightleftharpoons Cu_{(s)}\ \ \ \ E\degree=+0.34V$$
$$Zn^{2+}_{(aq)}+2e^{-}\rightleftharpoons Zn{(s)}\ \ \ \ E\degree=-0.76V$$
What do you think will happen if we combine these 2 equations? Which of the two (Zinc and Copper) will undergo oxidation and which will undergo reduction? This is decided by the sign of the electrode potentials. Since E° for Copper half-reaction is positive, it will go in the forward direction (Copper will get reduced).
Thus, giving us the redox reaction:
$$Cu^{2+}_{(aq)}+Zn_{s}\rightleftharpoons Cu_{(s)}+Zn^{2+}_{(aq)}$$
We can also calculate the E° for the combined redox reaction:
$$E\degree_{red}=+0.34V$$
$$E\degree_{ox}=-0.76VV$$
$$E\degree_{cell}=+0.34-(-0.76)$$
$$E\degree_{cell}=+0.34+0.76$$
$$E\degree_{cell}=+1.10V$$
Galvanic Cells
A Galvanic Cell is an electrochemical cell, which can generate electric current from spontaneous redox reactions.
Electrochemical cells can convert either chemical energy to electrical energy, or vice versa. An electrochemical cell that converts chemical energy to electrical energy is called a Galvanic cell. We will only discuss about 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.
An Electrode is a conductor which makes contact with the non-metallic parts in an electric circuit.
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.
- In a Galvanic cell, the electrode at which oxidation takes place is called anode. The electrode at which reduction takes place is called cathode.
- Electrons flow from Anode to Cathode. Current flows from Cathode to Anode.
Copper sulphate solution provides Cu2+ ions for the Copper half cell. As more and more Zn atoms release electrons and get dissolved in the electrolyte, you will see the Zn electrode get thinner with time, just as the Cu electrode gets thicker.
The salt bridge helps balance the net charge in both electrolytes of the 2 half cells. As more cations are released into the ZnSO4 solution, K+ ions from the salt bridge permeate into the solution to balance the net charge. Similarly, Cl- ions permeate into the CuSO4 solution.
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.
Electrode Potential is the potential difference between the electrode and the electrolyte in a half cell.
There is no way of measuring the electric potential of an electrolyte, therefore, electrode potential for a species is measured according to this definition:
Electrode Potential of a species is the electromotive force (emf) / E°cell of a galvanic cell built from a half cell of that species and a reference half cell (H+).
Electrode potential is called Standard Electrode Potential when the concentration of all species involved in a half cell is unity.
Disproportionation reactions are chemical reactions in which an element undergoes oxidation as well as reduction, like so:
$$2Cu^{+}_{(aq)}\rightarrow 2Cu_{(s)}+Cu^{2+}_{(aq)}$$
The oxidation and reduction halves of this equation can be written as:
$$Cu^{+}+e^{-}\rightarrow Cu\ (red)$$
$$Cu^{+}\rightarrow Cu^{+}+e^{-}\ (Ox)$$
This redox reactions can be seen as an electrochemical cell. Therefore, the two half equations of this electrochemical cell would be:
$$Cu^{+}+e^{-}\rightarrow Cu\ (red)\ E\degree=0.52V$$
$$ Cu^{2+}+e^{-}\rightarrow Cu^{+}\ E\degree=0.16V$$
Note that half reactions are written as reduction reactions according to IUPAC convention.
If you calculate the electrode potential of the whole cell:
$$E\degree=E_{red}-E_{ox}$$
$$E\degree=0.52-0.16=0.36V$$
The electrode potential of the disproportionation reaction of Copper is 0.36V.
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:
$$E\degree_{cathode}=0V$$
$$E\degree_{cathode}=0.76V$$
$$E{cell}=0-0.76$$
$$E_{cell}=E_{Zn}=-0.76V$$
It is important to note that Platinum only serves as an electrical contact and a catalyst. It does not contribute to the EMF of the cell.
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.
Standard electrode potential / standard reduction potential is the ability of a species to reduce a standard hydrogen electrode at conditions of 298 K, 100 kPa and 1.00 mol dm−3 ion concentration.
A cell can be made using two species (two half reactions). The net cell potential depends on the electrode potential of the two species.
Standard cell potential, Eo, is the difference between the potentials of the reduction half and the oxidation half of a cell. It is measured at standard conditions of 298 K, 100 kPa and 1.00 mol dm−3 ion concentration.
$$E\degree=E\degree_{red}-E\degree_{ox}$$
Electrode Potential - Key takeaways
- Standard Electrode Potential is a measure of the ability of a species to gain or lose electrons.
- Species which gains electrons is said to be reduced. Species which loses electrons is said to be oxidized.
- Reduction potential is the ability of a species to get reduced. It is the ability of a species to oxidize other species.
- Oxidation potential is the negative of reduction potential. It is the ability of a species to get oxidized. It is its ability to reduce other species.
- Species with highly positive reduction potential are good oxidizing agents. Species with highly negative reduction potentials are good reducing agents.
- Redox reactions are reactions where a reduction reaction and an oxidation reaction is taking place in tandem.
- Each half of a redox reaction (oxidation/reduction) is called a half reaction or half cell.
- Galvanic cells are electrochemical cells which can convert chemical energy to electrical energy.
- Galvanic cells are based on redox reactions. They combine the electrode potentials of the two electrodes of the two half reactions to form an electrical circuit
- The potential difference between the electrode and the electrolyte is called Electrode Potential.
- In a Galvanic cell, the electrode at which oxidation takes place is called anode. The electrode at which reduction takes place is called cathode.
- Electrons flow from anode to cathode. Current flows from cathode to anode.
- Electrode potential for all species is calculated against the Standard Hydrogen Electrode (SHE). It is the reference electrode and its potential is considered to be 0.
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