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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!
Consider this equation below.
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.
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.
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.
The standard reduction potential (E°red) for some more species is given in the Electrode Potential Table.
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 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.
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 -
Electrons and the net charge on both sides are balanced.
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. -
For the Copper-Vanadium cell, Copper undergoes the reduction reaction, and Vanadium undergoes the oxidation reaction.
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 -
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 -
We can also calculate the E° for the combined redox reaction.
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 -
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 -
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.
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.
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 -
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 -
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.
Electrode potential of a species is the emf generated by a Galvanic cell made from a reference electrode (Hydrogen electrode) and an electrode of the species in question.
Single electrode potential for individual species is calculated by measuring the emf generated by a galvanic cell made from an electrode of that species, and a reference electrode (Hydrogen electrode)
Standard electrode potential is the electrode potential of a half cell when the concentration of all species involved in the electrolyte is unity.
It is also defined as the ability of a species to gain or lose electrons.
The net electrode potential of a cell is called emf (electromotive force) (E°cell).
E°cell = E°red - E°ox
Reduction Potential - Ability of a species to get reduced/oxidize other species.
Oxidation Potential - Ability of a species to get oxidized/reduce other species
Oxidation potential is the negative of reduction potential.
A Galvanic cell is an electrochemical cell which can convert chemical energy to electrical energy.
What is the electrode potential of Cu2+?
Cu2+ is a ______ agent.
What is a redox reaction?
Redox reaction is a reaction in which oxidation and reduction reactions are taking place in tandem.
What is the oxidation state of O2- ion?
Oxidation potential is the ability of a species to...
What is a half reaction?
Half reaction shows the reduction reaction of a single species.
What species is considered as reference point for the calculation of reduction potentials of all other species?
H+. It is called the Standard Hydrogen Electrode. E° = 0.
What is the strongest oxidizing agent?
E° = 2.87 Volts
What is the strongest reducing agent?
E° = -3.04 Volts
Which of the following is true?
Stronger oxidizing agent = Weaker reducing agent
How do you define Electromotive Force?
Electromotive Force (EMF) is an electrical action produced by a non-electrical source.
What is an electrode?
Electrode is a conductor which makes contact with non-metallic parts of an electrical circuit.
What is the direction of flow of electrons in a Galvanic cell?
In a Galvanic cell, electrons flow from the anode to the cathode.
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