The Daniell cell
Two electrodes, two beakers, one wire — but the voltage isn't fixed. It only equals the textbook 1.10 V under standard conditions. So change the concentrations and watch. Every reading updates by the Nernst equation, E = E° − (0.0591/n)log Q. Electrons even flow the way they really do — anode to cathode.
The ideas you're seeing
Standard electrode potential
E° values (Zn²⁺/Zn = −0.76 V, Cu²⁺/Cu = +0.34 V) are always measured relative to the standard hydrogen electrode (SHE), fixed at exactly 0 V — every half-cell potential is pinned to that one reference point.
Which way electrons flow
The electrode with the more negative E° is the anode — it oxidises and loses electrons. The more positive electrode is the cathode — it reduces and gains electrons. Electrons always flow anode → cathode through the external wire.
The Nernst equation
E = E° − (0.0591/n)log₁₀Q. Cell voltage isn't fixed once you pick the electrodes — it depends on the actual ion concentrations too, and drifts away from E° as the reaction proceeds and Q changes.
Why voltage changes with concentration
Lowering [Zn²⁺] — the anode's own product — makes oxidation "easier" (Le Chatelier), pushing Ecell above 1.10 V. This is exactly a battery's declining voltage over its life, run in reverse: concentrations shift the other way as it discharges.
Part of the Electrochemistry chapter — read the notes, grab the formula sheet and take the quiz. One of Priodemy for School, free with every EduSuite school.
How a galvanic cell turns a reaction into a voltage
Separating the two halves
Drop a zinc strip into copper sulphate solution and the reaction happens immediately, but all you get is heat and a coating of copper. A galvanic cell extracts useful work from the same reaction by physically separating oxidation from reduction. Zinc dissolves in one beaker, copper deposits in the other, and the electrons are forced to travel through an external wire to get from one to the other. That flow is the current.
Zinc is oxidised at the anode, losing two electrons to become Zn²⁺. Copper ions are reduced at the cathode, gaining those electrons to plate out as metal. In a galvanic cell the anode is negative and the cathode positive — the opposite of an electrolytic cell, which is a distinction worth fixing early because it is examined often.
What the salt bridge is for
Without it the cell stops almost at once. As zinc dissolves, the left-hand solution accumulates positive charge; as copper plates out, the right-hand solution is left with excess negative charge. That charge imbalance opposes further electron flow within moments. The salt bridge supplies ions that migrate to cancel the build-up, keeping both solutions electrically neutral so the reaction can continue. It completes the circuit without letting the two solutions mix.
Cell potential and the Nernst equation
Each half-reaction has a standard electrode potential measured against the standard hydrogen electrode, and the cell potential is E°cell = E°cathode − E°anode. For the Daniell cell this gives about 1.10 V. A positive E°cell means the reaction is spontaneous as written, which connects to thermodynamics through ΔG° = −nFE°cell.
Those values assume standard conditions — 1 M solutions at 298 K. Change a concentration and the voltage shifts, as described by the Nernst equation. Raising the concentration of the reactant ion drives the reaction forward and raises the voltage; raising the product ion concentration lowers it. Adjust the sliders here and watch the reading move. As the cell discharges, concentrations drift towards equilibrium, Ecell falls to zero, and the battery is flat — a dead battery is simply a reaction that has reached equilibrium.
Mistakes that cost marks
Multiplying E° when balancing electrons. Doubling a half-equation to balance electrons does not double its potential. Electrode potential is an intensive property, unlike ΔG, which does scale. This is one of the most frequently penalised errors in the chapter.
Mixing up anode and cathode polarity. Oxidation is always at the anode and reduction always at the cathode, in both cell types. What flips between galvanic and electrolytic cells is the sign, not the chemistry.
Writing cell notation backwards. The convention is anode on the left, cathode on the right, with the salt bridge shown as a double vertical line. Reversing it reverses the reported sign of the cell potential.