Mada za sehemu hiiElectrocemistryMada 3
- Oxidation and Reduction
- The Nernst Equation
- Electrolytes in Solutions
Electrode potential
Metals have a small tendency to dissolve in a solution of their ions, producing cations and leaving their valency electrons on the metal rod. The metal acquires a negative potential, which prevents further release of cations, and equilibrium is established.
Reaction: ⇒ number of electrons (s).
As a result, the region of the solution very close to the rod suffers an increase in charge, while the rod carries a layer of negative charge (electrons). An electric double layer is set up, known as the "Helmholtz double layer."
Whenever there is a separation of negative and positive charges, we can measure the voltage between the electrode and surrounding solution, known as the Electrode Potential.
Definition
Electrode Potential: The potential difference formed between an electrode and its hydrated ions.
Magnitude of electrode potential
This depends on the position of equilibrium in a reversible reaction. The further to the right the equilibrium lies, the greater the electron density on the surface of the metal and the larger the potential difference (P.D.) between the metal and the solution. The opposite is also true.
The position of equilibrium depends on the concentration of the solution the electrode is immersed in. If the concentration is high, the equilibrium will shift to the left, decreasing the tendency of the zinc rod to dissolve, and vice versa.
Electrode potential involves the following stages
- Atomization of electrode
- Ionization of gaseous atoms
- Hydration of ions
For example:
If the total energy is low, the electrode dissolves more easily, and its equilibrium moves forward, resulting in a large electrode potential value. Metals with large electrode potentials release electrons easily and are good reducing agents.
Types of electrodes found in galvanic cells
- Metal – metal ion electrodes (metal dipped in its soluble salts)
- Gaseous electrodes (platinum dipped in gas at 1 atm and 25°C, and its ions at a given concentration)
- Metal – insoluble salt electrodes (metal dipped in its insoluble salts)
- Redox electrodes (platinum dipped in cations having different oxidation states at a given concentration at 25°C)
Measurements of electrode potential
Electrode potential is measured in combination with a standard electrode, the Standard Hydrogen Electrode (SHE). It is conventionally agreed that the electrode potential for hydrogen is zero.
The electrode potential of any element is measured against the hydrogen electrode at 1 atm and 1M concentration and is called the standard electrode potential.
Salt bridge
A salt bridge is an inverted U-tube that contains an electrolyte (e.g., ) and connects the solution of two half-cells. It balances the charge and completes the circuit.
Note
The standard redox potential is actually a reduction potential. All elements below hydrogen have a negative reduction potential and positive oxidation potential, making them strong reducing agents. All elements above hydrogen have positive reduction potential and negative oxidation potential, making them strong oxidizing agents.
Functions of salt bridge
- To complete the circuit
- To balance the charge
Application of standard electrode potential
- Construction of electrochemical cells
- Prediction of the occurrence of chemical reactions
- Determination of concentration of a solution without using a meter
- Replacement of elements in the electrochemical series
Electromotive force (EMF) of a cell
The difference between the electrode potential of the two electrodes constituting an electrochemical cell is known as the electromotive force (EMF) or cell potential. This acts as a driving force for a cell reaction and is expressed in volts.
Calculation of EMF
The EMF of a cell is calculated by subtracting the standard electrode potential of the left electrode from that of the right electrode.
For example, in the Daniel cell:
Alternatively, since standard electrode potentials are in reduced form, for the oxidation half-reaction, the sign of the electrode potential should be reversed:
The Nernst equation is a fundamental equation in electrochemistry that relates the electrode potential of a half-cell to the concentrations of the chemical species involved in the redox reaction. It is especially useful for calculating the potential under non-standard conditions, where concentrations of reactants and products are not at their standard values.
The Nernst equation can be expressed as:
Where:
is the electrode potential at non-standard conditions (in volts).
is the standard electrode potential (in volts),
is the number of electrons involved in the redox reaction.
is the reaction quotient, which is the ratio of concentrations of products to reactants.
is a constant used at 25°C (298 K).
In galvanic (voltaic) cells, the Nernst equation is used to calculate the potential under non-standard conditions. The equation can be used to determine the cell potential, which will change depending on the concentrations of the ions involved in the half-reactions.
For a general redox reaction:
the reaction quotient, , is given by:
If the concentrations of the ions are different from standard conditions, the Nernst equation helps in determining the potential difference of the electrochemical cell.
Consider the following redox reaction in a galvanic cell:
The standard electrode potential for the half-reaction is .
If the concentration of is 0.10 M, we can use the Nernst equation to find the cell potential at this concentration.
Using the Nernst equation:
Substituting the values:
Simplifying the logarithmic part:
Thus, the cell potential under the given conditions is 0.37 V.
In concentration cells, the Nernst equation helps calculate the potential difference between two half-cells that have the same electrode but different concentrations of ions. The equation simplifies as the same half-reaction occurs at both electrodes.
For a concentration cell:
The cell potential is calculated using the Nernst equation:
If the concentrations are 1 M on one side and 0.1 M on the other side, the potential will be driven by the concentration difference.
The Nernst equation can be applied to determine the pH of a solution by using the hydrogen ion concentration. The hydrogen electrode potential can be related to pH using:
If the concentration of hydrogen ions () is known, the potential of the hydrogen electrode can be calculated, and from that, the pH can be derived.
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