Electrochemical Cells, Notation & Salt Bridge

CHEGalvanic Cells & Electrode Potential

This subtopic distinguishes galvanic (electrochemical) cells from electrolytic cells, sets out the conventions for writing cell notation, and explains the salt bridge. The Daniell cell is the standard example for notation and electrode identification.

Daniell cell notation

Zn\,|\,Zn^{2+}(aq)\,||\,Cu^{2+}(aq)\,|\,Cu

anode (oxidation) on the left, cathode (reduction) on the right;

|

= phase boundary,

||

= salt bridge.

Cell EMF from electrode potentials

E^\circ_{cell} = E^\circ_{cathode} - E^\circ_{anode}

both as standard reduction potentials; a positive value means the cell as written is spontaneous.

Galvanic (voltaic) cell converts chemical energy to electrical energy via a spontaneous redox reaction (e.g. Daniell cell); electrolytic cell uses electrical energy to drive a non-spontaneous reaction (e.g. electrolysis of molten $NaCl$ ).
Sign convention differs: in a galvanic cell the anode is negative and cathode positive; in an electrolytic cell the anode is positive and cathode negative. Oxidation is always at the anode, reduction always at the cathode (in both).
Cell notation: anode (oxidation) on the left, cathode (reduction) on the right; single bar $|$ = phase boundary, double bar $||$ = salt bridge. Daniell cell: $Zn\,|\,Zn^{2+}(aq)\,||\,Cu^{2+}(aq)\,|\,Cu$ .
In the Daniell cell, $Zn$ is the anode ( $Zn \rightarrow Zn^{2+} + 2e^-$ ) and $Cu$ is the cathode ( $Cu^{2+} + 2e^- \rightarrow Cu$ ); electrons flow externally from $Zn$ to $Cu$ .
Inside the cell, current is carried by ion migration; in the external wire, conventional current flows cathode to anode while electrons flow anode to cathode.
The anode of a galvanic cell is the negative electrode because oxidation releases electrons there, building up negative charge that pushes electrons into the external circuit.
Salt bridge (e.g. $KCl$ or $KNO_3$ in agar) functions: (i) completes the internal circuit and allows ion flow, (ii) maintains electrical neutrality of the two half-cells, (iii) prevents the two solutions from mixing while keeping the liquid-junction potential negligible.
The salt bridge uses ions of nearly equal mobility (e.g. $K^+$ and $NO_3^-$ or $K^+$ and $Cl^-$ ) so that anions and cations diffuse at comparable rates, minimising the junction potential.
As the cell runs, the anode compartment accumulates positive ions ( $Zn^{2+}$ ) and the cathode compartment loses positive ions; the salt bridge supplies anions to the anode side and cations to the cathode side to keep both neutral.
$EMF = E^\circ_{cathode} - E^\circ_{anode}$ (both as reduction potentials); a positive EMF means the cell as written is spontaneous.
Without a salt bridge (or porous partition) charge separation quickly builds up and stops the reaction, so no sustained current flows.

Where the marks go

Reversing anode/cathode polarity between cell types — in a galvanic cell the anode is negative; in an electrolytic cell the anode is positive (oxidation is still at the anode in both).
Writing the cathode on the left in cell notation — by convention the anode (oxidation) is always on the left.
Saying electrons flow through the salt bridge — electrons travel through the external wire; only ions move through the salt bridge.
Forgetting that single bar $|$ marks a phase boundary while double bar $||$ marks the salt bridge — swapping them is a common notation error.
Claiming the salt bridge 'carries the main current' — it completes the circuit and preserves neutrality; the cell reaction (not the bridge) is the source of EMF.

How the board asks it

Structure / namingdaniell cell notation conventions
Write the cell notation (representation) for a galvanic cell made of a $Zn$ electrode dipped in $1\,M\,ZnSO_4$ and a $Cu$ electrode dipped in $1\,M\,CuSO_4$ , clearly indicating the anode, cathode, and salt bridge.
Distinguishgalvanic vs electrolytic cell
Distinguish between a galvanic cell and an electrolytic cell with respect to energy conversion and the sign (polarity) of the anode.
Identify / classifyelectrode reactions and electron flow
For the cell $Zn\,|\,Zn^{2+}(aq)\,||\,Cu^{2+}(aq)\,|\,Cu$ , identify the anode and cathode, write the half-cell reaction at each electrode, and state the direction of electron flow in the external circuit.
Define / statesalt bridge composition and role
State any two functions of a salt bridge in an electrochemical cell, and name one electrolyte commonly used in it.
Give reasonscharge build-up without a salt bridge
Give a reason: in a galvanic cell, no sustained current flows if the salt bridge is removed, even though a spontaneous redox reaction is possible.
Numerical $EMF = E^\circ_{cathode} - E^\circ_{anode}$
Given $E^\circ_{Zn^{2+}/Zn} = -0.76\,V$ and $E^\circ_{Cu^{2+}/Cu} = +0.34\,V$ , calculate the standard EMF of the Daniell cell and state whether the cell reaction as written is spontaneous.

Practise this topic

NextElectrode Potential, SHE & Electrochemical Series

Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.