Electrode Potential
Definition of E: Electrode potential is a way to measure how much a chemical species, such as a metal atom or an ion, wants to lose or gain electrons when placed into a solution. It helps us know if the substance is more likely to be oxidized (which means losing electrons) or reduced (which means gaining electrons) during a chemical reaction.
Quantifying redox tendency: Electrode potential gives a specific number that tells us how easily a chemical species will join in a redox reaction. Redox reactions involve one substance losing electrons (oxidation) and another gaining electrons (reduction). The higher or lower the value, the easier it is for the reaction to happen.
Unit of measurement: Electrode potential is measured using volts (V), which is the same unit used to measure the electricity that powers lights, computers, and phones. The voltage tells us how much “force” is pushing the electrons to move.
High E value: If a substance has a high (positive) electrode potential, it means it is not willing to lose its electrons easily. It would prefer to keep its electrons and might behave as an oxidizing agent, which means it helps other substances lose electrons while it gains them.
Low E value: If a substance has a low (negative) electrode potential, it gives away its electrons easily. This makes it a strong reducing agent, because it donates electrons to other substances during a reaction.
Standard Electrode Potential (E°)
Definition of E°: Standard electrode potential, written as E°, is the value of electrode potential measured under special, standard conditions. These conditions allow scientists to fairly compare the electron-gaining or losing tendencies of different substances.
Standard conditions: The standard conditions are a temperature of 298 Kelvin (which is about 25°C, or room temperature), a solution concentration of 1.0 molar (mol dm⁻³), and a gas pressure of 1 atmosphere (atm) if gases are involved.
Measurement method: To find a substance’s E° value, scientists connect it to a reference device called the standard hydrogen electrode (SHE) and measure how much voltage is produced.
The Standard Hydrogen Electrode (SHE): The Reference
Role of SHE: The standard hydrogen electrode (SHE) is used as the universal reference or “zero point” for measuring and comparing the electrode potentials of other substances.
Structure of SHE: The SHE setup has a platinum metal electrode placed into a solution that contains hydrogen ions (H⁺) at 1.0 mol dm⁻³ concentration. Hydrogen gas (H₂) is passed over the electrode at a pressure of 1 atm, all kept at 25°C.
Defined potential: The electrode potential of the SHE is exactly defined as 0.00 volts. It is like setting the “starting line” for all measurements.
Half-reaction for SHE: The chemical reaction at the SHE can be shown as: 2H⁺(aq) + 2e⁻ ⇌ H₂(g), which means two hydrogen ions gain two electrons to form one molecule of hydrogen gas.
Importance of SHE: Using the SHE makes it possible for scientists all over the world to have a consistent way to compare different materials’ tendencies to gain or lose electrons.
Electrode Potentials and Redox Strength
Predicting redox strength: By knowing the electrode potential, we can predict whether a substance will act as a strong oxidizing agent (which accepts electrons) or a strong reducing agent (which gives away electrons).
Positive E°: A substance with a more positive standard electrode potential is a stronger oxidizing agent. It is more eager to accept electrons from other materials.
Example of strong oxidiser: For example, chlorine gas (Cl₂) turning into chloride ions (Cl⁻) has an E° value of +1.36 V, showing that chlorine is a very powerful oxidizer.
Negative E°: A substance with a more negative standard electrode potential is a stronger reducing agent. It loses electrons easily to other materials.
Example of strong reducer: For instance, zinc metal (Zn) turning into zinc ions (Zn²⁺) has an E° value of -0.76 V, meaning it is very good at giving away electrons.
General trend: Overall, the higher (more positive) the E° value, the better a substance is at gaining electrons. The lower (more negative) the E° value, the better a substance is at losing electrons.
Electrochemical Series and Standard Electrode Potentials
Definition of series: The electrochemical series is a ranked list that arranges different substances based on their standard electrode potentials, starting from the highest values down to the lowest.
Predicting spontaneity: We can use the electrochemical series to predict which redox reactions will occur on their own without needing extra energy. This helps in designing batteries and other devices.
Location of strong oxidisers: In the electrochemical series, half-reactions with high positive E° values are found lower on the list. These substances are strong oxidizers because they easily gain electrons.
Location of strong reducers: Half-reactions with negative E° values are placed higher on the list. These substances are strong reducers because they easily give away electrons.
Anode and cathode determination: In an electrochemical cell, the electrode with the lower E° becomes the anode (where oxidation happens), and the electrode with the higher E° becomes the cathode (where reduction happens).
Calculating Cell Potential
Formula for E°cell: The standard cell potential, E°cell, is found using the formula: E°cell = E°cathode – E°anode. This helps us figure out the overall voltage produced by a redox reaction.
Cathode function: The cathode is the electrode where the reduction reaction occurs. Here, the material gains electrons.
Anode function: The anode is the electrode where the oxidation reaction happens. Here, the material loses electrons.
Positive E°cell: If the E°cell value is positive, it means the redox reaction will happen naturally and produce electricity without needing any outside energy.
Negative E°cell: If the E°cell value is negative, it means the reaction will not happen on its own and will require energy from an external source to proceed.
Factors Affecting Electrode Potential
Influence of conditions: Electrode potential values can change if the surrounding conditions (temperature, pressure, or concentration) are different from the standard values.
Ion concentration: If the concentration of ions in the solution increases, it usually makes the electrode potential more positive because there are more ions ready to accept or donate electrons.
Temperature effects: Raising or lowering the temperature can speed up or slow down the electron transfer reactions, which in turn affects the measured electrode potential.
Electrode material: The type of material used as the electrode itself can also change the electrode potential. Different materials have different abilities to interact with ions.
Examples of Standard Electrode Potentials
Highly negative E° examples: Lithium (Li⁺/Li, E°= -3.05 V), potassium (K⁺/K, E°= -2.92 V), and sodium (Na⁺/Na, E°= -2.71 V) are metals that strongly prefer to lose electrons and act as very powerful reducers.
Moderately negative E° examples: Magnesium (Mg²⁺/Mg, E°= -2.38 V) and aluminum (Al³⁺/Al, E°= -1.66 V) are also good reducers, but not as strong as lithium or potassium.
Moderately positive E° examples: Copper (Cu²⁺/Cu, E°= +0.34 V) and silver (Ag⁺/Ag, E°= +0.80 V) have moderate tendencies to gain electrons, making them good oxidizers.
Highly positive E° examples: Chlorine gas (Cl₂/Cl⁻, E°= +1.36 V) and the permanganate ion (MnO₄⁻/Mn²⁺, E°= +1.52 V) are very strong oxidizers that readily accept electrons.
Trend in E° values: In general, the more negative the E° value, the better the metal is at acting as a reducer. The more positive the E° value, the better the substance is at acting as an oxidizer.
Application of Standard Electrode Potentials
Cell design: Knowing the standard electrode potentials helps engineers design batteries and fuel cells that can create electricity efficiently and reliably.
Predicting electron flow: By looking at the E° values, scientists can figure out which way electrons will naturally move in a redox system, always flowing from the anode to the cathode.
Voltage prediction: By using the electrode potentials, we can predict the total voltage or electrical energy that an electrochemical cell can produce.
Industrial processes: In industry, electrode potentials guide important activities like extracting pure metals from ores or coating items with metals through a process called electroplating.
Research and development: Scientists use their understanding of electrode potentials to invent new kinds of batteries, improve fuel cells, and create cleaner and more efficient chemical processes.