Redox reactions and electrode processes are both related to the transfer of electrons between elements and the conversion of chemical energy into electrical power.
Redox Reactions: The Basics
Redox reactions, short for reduction-oxidation reactions, are the transfer of electrons between chemical species. These reactions are characterised by two simultaneous processes:
Oxidation: The loss of electrons by a substance.
Reduction: The gain of electrons by another substance.
It is easily to remember this processes in this way, think of the acronym “OIL RIG”: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
For example, we consider the reaction between zinc and
copper sulfate:
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Zinc loses electrons to form (oxidation), and copper ions gain those electrons to form copper metal (reduction).
Oxidising and Reducing Agents
Oxidising Agent: The substance that gains electrons and gets reduced. It helps another substance to oxidise.
Reducing Agent: The substance that loses electrons and gets oxidised. It helps another substance to reduce.
In the example above, is the oxidising agent, and zinc is the reducing agent.
Balancing Redox Reactions
Balancing redox reactions ensures that the number of atoms and charges are equal on both sides of the reaction. There are two common methods:
1.Oxidation Number Method: This is assigning oxidation states to each element and ensuring the total increase and decrease in oxidation states are balanced.
2. Ion-Electron Method (Half-Reaction Method): This splits the reaction into two half-reactions one for oxidation and one for reduction. Each half-reaction is balanced separately, and then they are combined.
For example, in acidic medium, the redox reaction is balanced as follows:
Oxidation Half-Reaction:
Reduction Half-Reaction:
Combining these, we get the balanced reaction:
Electrode Processes and Electrochemistry
Electrode processes occur at the surface of electrodes in electrochemical cells. These processes are redox reactions and to know how the electrical energy is generated or used in chemical reactions.
Electrochemical Cells
Electrochemical cells are devices that convert chemical energy into electrical energy (or vice versa). They are broadly classified into:
1. Galvanic (Voltaic) Cells: These cells generate electrical energy from spontaneous redox reactions. For example, the Daniell cell consists of a zinc electrode in solution and a copper electrode in solution.
At the anode (negative electrode), oxidation occurs:
At the cathode (positive electrode), reduction occurs
2. Electrolytic Cells: These cells use electrical energy to drive non-spontaneous chemical reactions. For example, the electrolysis of water splits it into hydrogen and oxygen gases.
Standard Electrode Potential
Each electrode has a specific potential, called its electrode potential, which depends on the nature of the electrode and the ions in solution. The standard electrode potential is measured under standard conditions (298 K, 1 M concentration, 1 atm pressure).
A positive value indicates a strong tendency to gain electrons (reduction).
A negative value indicates a strong tendency to lose electrons (oxidation).
The standard electrode potentials of electrodes are used to calculate the cell potential:
Applications of Redox Reactions and Electrochemistry
1.Batteries: Devices like lithium-ion and lead-acid batteries rely on redox reactions to store and deliver energy.
2. Electroplating: The process of coating a metal object with another metal using electrolysis.
3. Corrosion Prevention: Redox helps in developing methods to prevent rusting of metals.
4. Industrial Processes: Production of chemicals like chlorine and hydrogen are redox and electrode processes.
Key points :-
Redox couple:- It is defined as having together the oxidised and reduced forms of a substance taking part in an oxidation or reduction half reaction. For example, in Daniell cell it is represented as Zn2+ / Zn and Cu2+ / Cu a metal dipped in the solution of its own ions.
Salt bridge :– It completes the circuit by providing an electric contact between two solution without allowing them to mix with each other and used to maintain electrical neutrality.
Standard electrode potential (E0) :- If the concentration of each species taking part in the electrode reaction is unity and further the reaction is carried out at 298 K, then the potential of each electrode is called standard electrode potential.
To be remember:-
Lower the electrode potential, stronger is the reducing agent:
Li > Mg > Al > Zn > Fe > Co > Ni > Sn > Pb > Cu > I2 > Fe2 > Hg > Ag > Br2 > Cl2 > F2 .
Higher the electrode potential, stronger is the oxidizing agents : F2 > Cl2 > Br2 > I
Fluorine is the strongest oxidizing agent.
Li is the strongest reducing agent due to its high hydration energy.
Only those electrolytes for which cations and anions have nearly have the same ionic mobilities are used as electrolytes in the salt bridge.
Equivalent mass of oxidizing / reducing agent = Equivalent mass / Change in oxidation state per mole.
Note:-
Redox reactions and electrode processes are central concepts with significant theoretical and practical implications. From powering electronic devices to industrial applications, these principles highlight the interconnection between chemical and electrical energy.
A redox reaction is a chemical reaction, the transfer of electrons, where one substance is oxidised (loses electrons) and another is reduced (gains electrons).
An oxidising agent gains electrons and is reduced, while a reducing agent loses electrons and is oxidised.
Redox reactions can be balanced using either the oxidation number method or the ion-electron (half-reaction) method.
Electrodes serve as the site for redox reactions, where oxidation occurs at the anode and reduction occurs at the cathode.
Standard electrode potential is the potential of an electrode measured under standard conditions, indicating its tendency to gain or lose electrons.