9.3
Cell Diagrams and IUPAC Conventions
Electrochemical cell notation is a standardized symbolic representation that communicates the structure and reaction pathway of galvanic and electroly…
Electrochemical cell notation compactly represents the components and directionality of a cell, where a vertical bar denotes a phase interface and a dashed or dotted line denotes the boundary between miscible liquids or a liquid junction.
Species in the same phase are separated by a comma, and a double vertical line denotes an interface where the junction potential is neglected to simplify emf measurements.
Using this cell notation, IUPAC conventions define cell electromotive force, E, as the difference of the open-circuit potentials of the right and left terminals.
A positive cell electromotive force in a cell diagram means that the corresponding cell reaction will spontaneously occur when connected to a load.
Oxidation occurs at the left electrode and reduction at the right electrode. The emf equals the potential of the right electrode minus that of the left electrode. Adding the two half-reactions gives the overall cell reaction.
If the right electrode is the cathode, the reaction is spontaneous; if the left electrode is the cathode, the reverse cell reaction is spontaneous.
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Q1: What do the vertical bars mean in electrochemical cell notation?
In electrochemical cell notation, a single vertical bar denotes a phase interface, such as between a solid electrode and an aqueous solution. A double vertical line represents a salt bridge or liquid junction where junction potentials in galvanic cells are assumed to be eliminated. If the junction potential is not negligible, a dotted or dashed line is used instead.
Q2: How is cell electromotive force defined according to IUPAC conventions?
According to IUPAC conventions, cell electromotive force (emf) is defined as E = ΦR − ΦL, where ΦR and ΦL are the open-circuit potentials of the right and left electrodes, respectively. A positive emf indicates that the corresponding cell reaction will spontaneously occur when the cell is connected to an external load.
Q3: Which electrode is the anode and which is the cathode in cell notation?
In electrochemical cell notation, the left electrode is designated as the anode where oxidation occurs, and the right electrode is the cathode where reduction occurs. If the overall cell reaction proceeds in this direction, the emf is positive and the reaction is spontaneous under standard conditions.
Q4: What does a positive cell emf tell you about the spontaneity of a reaction?
A positive cell electromotive force indicates that the corresponding cell reaction will spontaneously occur when the cell is connected to an external load. If the right electrode acts as the cathode and the left as the anode, the reaction proceeds spontaneously. Conversely, a negative emf means the reverse reaction is spontaneous.
Q5: How are multiple species in the same phase represented in cell notation?
When multiple species are present in the same phase, they are separated by commas in electrochemical cell notation. This allows the compact representation of complex electrochemical systems without requiring detailed diagrams, making it easier to communicate cell structure and reaction pathways.
Q6: What is the relationship between electrode position and cell reaction spontaneity?
If the right electrode is the cathode, the cell reaction is spontaneous and emf is positive. If the left electrode is the cathode, the reverse cell reaction is spontaneous and emf is negative. The overall cell reaction is determined by adding the two half-reactions: oxidation at the left electrode and reduction at the right electrode.
Q7: When is a dotted or dashed line used instead of a double vertical line in cell notation?
A dotted or dashed line is used instead of a double vertical line when the junction potential between miscible liquids or at a liquid junction is not negligible. The double vertical line assumes the junction potential is eliminated, simplifying emf measurements, but when this assumption is invalid, the alternative notation indicates that junction effects must be considered.