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Galvanic cells: identify an unknown metal

Put metal strips into 0.100 M solutions of their salts, join pairs with a salt bridge and read the voltage; build a series of cells against reference metals, rank the metals, then compute the standard potential of an unknown metal X and identify it.

Difficulty ●●○ ≈ 25 min

Aim Rank metals by galvanic-cell voltages and find the standard potential of an unknown metal from the measured EMF.
Virtual experiment · 0.100 M solutions · high-impedance meter

Voltaic cell bench: identify an unknown metal

electrons →+1.104 Vsalt bridge KNO₃Zn | Zn²⁺0.100 Manode (−)Cu | Cu²⁺0.100 Mcathode (+)

Lab notebook

Nothing recorded yet. Build a cell and press “Record reading”.

Hint: with Cu on the right and X on the left, the reading ≈ E°(Cu²⁺/Cu) − E°(X²⁺/X); both ions are 2+ at the same concentration, so the concentration terms cancel.

Readings are computed from CRC standard potentials with the Nernst equation (25 °C, every metal ion 0.100 M, concentrations for activities); the unknown metal is chosen from the sample number by a deterministic hash. Real measurements also carry junction potentials, oxide films and activity coefficients — typically a few to tens of millivolts.

Procedure

  1. Build Zn | Zn²⁺ ‖ Cu²⁺ | Cu first (zinc on COM, copper on V), record the reading and compare it with E°(Cu²⁺/Cu) − E°(Zn²⁺/Zn) = 1.104 V.
  2. Keep copper on the right and change the left electrode to Mg, Fe, Ni, Sn, Pb and Ag in turn, recording each. A negative reading means the left side is really the positive electrode.
  3. Order the metals from the largest reading to the smallest and compare with the activity series in your textbook.
  4. Put the unknown metal X on the left and measure it against copper; then measure it against zinc, and check the two results against each other.
  5. Compute E°(X²⁺/X) = E°(Cu²⁺/Cu) − E, look it up in the table of potentials to name X, then press Check. Repeat with another sample number.

What to notice

  • The sign of the reading says which electrode is positive; swapping the sides flips the sign but not the size.
  • The larger the voltage against copper, the more active the metal: Mg > Zn > Fe > Ni > Sn > Pb; silver reads negative against copper — it is less active.
  • With two 2+ ions at the same concentration, the concentration terms cancel in the Nernst equation and the reading equals the difference of E° values; silver (1+) against copper reads about 30 mV less than the E° difference.
  • Electrons always run through the wire from the negative electrode to the positive one, while anions in the bridge drift toward the negative side.

In a real lab

  • Sand the strips to remove oxide films first, or magnesium, aluminium and chromium read far too low and drift; let each reading settle for tens of seconds.
  • Use a high-impedance voltmeter: an ammeter or a low-resistance load draws current, and internal resistance and polarisation lower the reading.
  • Real readings include junction potentials and activity effects, typically a few to tens of millivolts from the calculation; magnesium also slowly releases hydrogen in water.
  • Lead and cadmium salts are toxic and silver nitrate stains skin black: wear gloves and collect the waste as heavy-metal waste.

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