Galvanic Cells
HSC Chemistry Syllabus
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Conduct investigations to measure and compare the reaction potential of galvanic half-cells
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Predict the spontaneity of redox reactions using the value of cell potentials (ACSCH079, ACSCH080)
Galvanic Cell Explained
What is a Galvanic Cell?
A galvanic cell (also known as a voltaic cell) is a type of electrochemical cell that generates electrical energy from spontaneous redox (reduction-oxidation) reactions. It often consists of two different metals or metal compounds, each placed in separate solutions called half-cells.
Components of a Galvanic cell
A galvanic cell comprises several key components
- Electrodes: Electrodes are conductive materials which facilitate the oxidation and reduction reactions of a galvanic cell. The anode and cathode are relatively negatively and positively charged respectively in the beginning. Oxidation occurs at the anode while reduction occurs at the cathode.
- Electrolyte Solutions: Each electrode is submerged in a solution with ions that participate in the redox reaction (see examples below). An electrode and its respective solution are either involved in oxidation or reduction, and is termed a half-cell.
- Voltmeter: A device that measures the potential between the anode and cathode or between the two half-cells.
- Wires connecting the two electrodes to the voltmeter, which allow electrons to flow between the two half-cells when the galvanic cell is active.
- Salt Bridge: A crucial component of a galvanic cell is the salt bridge which maintains electrical neutrality through allowing ions to flow and balance charges during the reaction. It also completes the circuit
In the example of a galvanic cell above, copper and silver act as the electrodes while the salt bridge contains sodium nitrate solution.
Galvanic cells efficiently convert chemical energy (redox reaction) into electrical energy through metal-ion reactions. Sometimes, inert electrodes like graphite or platinum are used when redox components cannot serve as electrodes themselves.
Displacement Reactions and Electrode Processes
When a more reactive metal is submerged in a solution of a less reactive metal's ions, a displacement reaction occurs, leading to the deposition of the less reactive metal. This process is integral to the functioning of a galvanic cell.
For example, copper (more reactive), placed into a silver ion solution (less reactive), leads to copper oxidisation and silver reduction – observable by the development of a characteristic blue colour from the copper ions and the deposition of silver on the copper electrode.
In galvanic cells, oxidation and reduction reactions occur simultaneously while separated into two different beakers connected by a salt bridge. Oxidation occurs at the anode and reduction occurs at the cathode.
Calculating Electrical Potential of Galvanic Cells
The electrical potential of a cell (`E`) is the combined potential of the oxidation and reduction reactions. Most data sources including the NESA HSC Chemistry data sheet provide standard reduction potentials, that is potentials of various reduction reactions under standard conditions.
What are standard conditions?
In electrochemistry, standard conditions refer to a set of specific conditions under which the properties of electrochemical cells, such as electrode potentials, are measured. These conditions are standardised to ensure consistency and comparability of results across different experiments and systems.
Standard conditions refer to:
- Temperature: The temperature is set at 25°C (298.15 K).
- Pressure: For any gases involved in the reaction or galvanic cell, the pressure is set at 1 atmosphere (atm) or approximately 100 kPa.
- Concentration: The concentration of all aqueous solutions is 1 mol/L (1 M).
The potential of an electrochemical cell measured under these conditions are called standard electrode potentials (denoted as `E^{\circ}`).
Using the standard potential table which is provided to you on the NESA data sheet:
The copper (Cu) oxidation potential: `E^{\circ} = -0.34 \text{ V}`
The silver ion (Ag+) reduction potential: `E^{\circ} = +0.80 \text{ V}`
The total standard potential of this cell is therefore 0.80 – 0.34 = +0.46 V, indicating a spontaneous redox reaction. Calculation of electrochemical potentials is discussed in greater detail in here.
Flow of Electrons in Galvanic Cells
The total potential from oxidation and reduction reactions drives electron flow, which is always from the anode to the cathode in a galvanic cell. For this reason, the initial relative charges of anode and cathode are negative and positive respectively.
Anode equation:
$$Cu(s) \rightarrow Cu^{2+}(aq) + 2e^–$$
The oxidation of copper leads to loss of electrons, and formation of copper ions.
Cathode equation:
$$2Ag^{+}(aq) + 2e^– \rightarrow 2Ag(s)$$
The reduction of silver leads to the gaining of electrons, and formation of silver atoms.
Oxidation reaction at the oxidation half-cell (e.g. copper electrode in the above example), releases electrons. These electrons then move towards the relatively positively charged cathode (e.g. silver electrode in the above example).
Recall that:
- Oxidation is loss of electrons (electrons are released from anode)
- Reduction is gain of electrons (electrons are received by cathode)
When electrons arrive at the cathode (e.g. silver), the ions in the reduction half-cell solution (e.g. silver ions) will be reduced by gaining these electrons. In the example above, silver ions are reduced to form silver metal deposits around the silver cathode.
The observations that can be made in a copper-silver galvanic cell is the same as the metal displacement reaction between copper and silver ion. The solution in the oxidation half-cell becomes more blue due to an increase in copper ion concentration. At the same time, silver deposits will form around the existing silver cathode, leading to a greater mass. The copper electrode will also decrease in mass throughout the operation of the galvanic cell.
The Role of Salt Bridge in Galvanic Cell
A salt bridge is a component of galvanic cells that connect oxidation and reduction half cells. It contains an inert ionic solution (cation and anion) such as sodium nitrate.
As electrons flow from the anode to the cathode, the anode becomes more positive (or less negative) due to the loss of electrons; the cathode becomes more negative due to the accumulation of electrons. The accumulation of these charges will gradually decrease the electrical potential between the half-cells. Ultimately, the potential will reduce to zero, at which point electrons will stop flowing. This is a limitation of galvanic cells as the generation of electrical energy stops whenever the potential becomes zero.
The use of a salt bridge overcomes the above limitation. The positive and negative ions from the salt bridge enter the solution to neutralise build-up of positive and negative charges due to the movement of electrons. Specifically, cations move to the negatively charged cathode while anions move to the positively charged anode.
In the example above, positively charged `Na^+` ions will move into the oxidation half-cell (copper ion solution) to neutralise the excess negative charge, and negatively charged `NO_3^–` ions will move into the reduction half-cell (silver ion solution) to neutralise the excess positive charge. The movement of these free ions from the salt bridge maintains the original charge difference between the half-cells and therefore keeps the electrical potential constant.
Therefore, the role bridge serves to maintain electrical neutrality in both solutions of the two half-cells. Without a salt bridge, the flow of electrons eventually stops, and so does the redox reaction.
Galvanic Cell Notation
Galvanic cells have a specific shorthand notation that succinctly represents the setup of the electrochemical cell. This is a quick way to describe what a galvanic cell consist of without drawing a diagram.
Understanding the symbols:
- Single Vertical Line (|): This symbol is used to separate different states of matter within the same half-cell. For example, a solid electrode from its aqueous ions or an aqueous ion from a gas. A comma is used instead to separate species of the same state in the same half cell. For example, when a half cell contains two ions both involved in the redox reaction, the two ions are separated by a comma.
- Double Vertical Lines (||): These lines represent the salt bridge within the cell.
For instance:
$$Cu(s) | Cu^{2+}(aq) || Ag^{+}(aq) | Ag(s)$$
The left-hand side always demonstrates the anode (copper) where oxidation occurs. the single vertical line separates the solid electrode from its aqueous ions. The double vertical lines indicate the presence of a salt bridge between the two half-cells, allowing for ion exchange.
The right-hand side indicates components of the reduction half cell. The silver ions in aqueous state and the single vertical line next to it similarly indicates the separation of the silver ions from the solid silver electrode (different states).
If the concentration of aqueous species is known, it should be included in brackets. This is because the concentration of aqueous species that are involved in the redox reaction affects the potential.
For example:
$$Cu(s) | Cu^{2+}(1 M) || Ag^{+}(1M) | Ag(s)$$
Example
Let's consider an electrochemical cell consisting of nickel and zinc electrodes in their respective metal ion solutions.
(a) Identify the anode & cathode and direction of electron flow.
(b) Calculate the total standard potential.
(c) Identify the direction of flow of ions in the salt bridge.
(d) Write the short-hand notation.