Native Metals

Posted by Brian Lee, 2025-09-25

Problem 38 of the 2025 USNCO National, Part 1 asks:

For a voltaic cell Zn|Zn+2(1.0 M)‖Ag+ (1.0 M)|Ag, which is correct?

(A) The zinc electrode is the anode.
(B) The concentration of Ag+ increases as the cell operates.
(C) The mass of the zinc electrode increases as the cell operates.
(D) Electrons flow through the external circuit from the silver electrode to the zinc electrode.

ChatGPT gives the following answer:

We’re looking at the voltaic cell:

Zn|Zn+2(1.0 M)‖Ag+ (1.0 M)|Ag

Step 1. Write half-reactions and standard potentials.

$\ce{Zn^2+ + 2e- -> Zn (s)}$ E = -0.76V

$\ce{Ag+ + e- -> Ag (s)}$ E = 0.80V

Step 2. Determine cathode/anode.

More positive reduction potential: silver. So Ag is reduced at the cathode.

Zinc undergoes oxidation at the anode. So (A) “The zinc electrode is the anode” is correct.

While correct, there’s a big problem with this answer: a table of standard potentials is not given on the USNCO. Unless you’re in the habit of memorizing standard potentials (don’t, this is a bad idea), you’ll need a different way to solve this problem.

What are native metals?

A native metal is a metal found in pure or nearly pure metallic form in nature (not combined as oxides, sulfides, etc.). Only a few metals occur natively in amounts large enough to be practically useful: gold, silver, copper, and the platinum group metals (mostly palladium and platinum).

Before smelting (which began ~6500 BC), humans lacked the ability to extract metals from ores, so native metals were essentially the only source of metal. Gold and silver are too soft to be practically useful and platinum too rare, so this leaves copper. In a nutshell, this is why humans “started” with the Copper Age. The Bronze Age, which came afterwards, features an alloy of copper and tin (another native metal).

Most other metals are found as combinations of oxides, sulfides, silicates, sulfates, and more. There’s a lot more fascinating chemistry behind metallurgy, the study of how to extract pure metals from these ores, as well as understanding which metals appear in which ores/forms – but that’ll have to wait for a different blog post.

Back to the USNCO question: silver is more stable as a metal than zinc. Therefore, if given a chance to equilibrate, silver metal will form at the expense of zinc metal. Electrons flow from anode to cathode, so zinc must be the anode.