Penny reactions

According to Wikipedia, recent Canadian pennies have a composition of 94% steel, 1.5% nickel, 4.5% copper plated zinc. Pure copper metal is bright and shiny, but the oxide is dull. Pennies get dull over time because the copper in the pennies slowly reacts with air to form copper oxide.

This is copper oxide

Cleaning pennies

When you place copper coins, such as pennies, in a NaCl, tablesalt, and vinegar solution, the acetic acid from the vinegar dissolves the dull cupric oxide, leaving behind pennies with a shiny clean copper surface. The copper from the copper oxide stays in the liquid as Cu²⁺ ions.

Remove the cleaned penny from the liquid and let it sit for an hour. The residue that is on the copper surface promotes a reaction between the copper and the oxygen in the air. The resulting blue-green copper oxide is a tarnish that forms called patina. These are copper minerals that are observed in nature.

Green patina forms when a shiny copper penny is removed from the salt/vinegar solution.

This is the same green that is seen on the rooftop of architecture such as the Sun Tower and Vancouver Hotel, and the Statue of Liberty in New York.

Cleaning pennies when the zinc is exposed

Time lapsed photos of two pennies immersed in a salt/vinegar solution. The penny on the right has been scored to expose the zinc metal. What is happening to the penny?

Rub the penny against a hard surface so that it exposes the shiny silvery metal under the copper layer. The shiny silvery metal is zinc.

Immerse two pennies, side-by-side, in the salt/vinegar solution. The penny with the zinc exposed (on the right) will never get shiny.

What is happening to the penny on the right?

Zinc reacts with the H⁺ ions in solution to form H₂ (g), which bubbles up on the surface of the penny.

Zn (s) + 2 H⁺ (aq)   Zn²⁺ (aq) + H₂ (g)   .... Reaction (1)

Why does the penny on the right darken?

The Cu²⁺ in the salt/vinegar solution reacts with the zinc metal and causes zinc metal to dissolve. When zinc dissolves, zinc goes into solution as Zn²⁺ ions. This leaves a negative charge on the surface of the penny. As opposite charges attract, the copper ions that are in solution are more strongly attracted to the penny than the positive zinc ions, so a copper coating forms on the penny. The penny turns darker as a copper coating covers the surface.

The reaction is

Zn (s)  + Cu²⁺ (aq)    Zn²⁺ (aq) + Cu (s)    .... Reaction (2)

Check with the Table of Activity of Metals:

Reaction 1 is observed because zinc is higher up on the activity series of metal than hydrogen.
Reaction 2 is observed because zinc is higher up on the activity series of metal than copper.

Zinc metal is more reactive than the copper metal. Zinc is capable of displacing Cu²⁺ ions in solution.

Coating copper on nickel

We can take the solution that we have been cleaning the copper pennies and coat a nickel nail with copper metal.

The reaction is:

Ni (s) + Cu²⁺ (aq)  Ni²⁺ (aq) + Cu (s)  ...... Reaction(3)

Check with the Table of Activity of Metals:

Reaction 3 is observed because nickel is higher up on the activity series of metal than copper.

Nickel metal is a more active metal than copper metal. Nickel is capable to displacing Cu²⁺ ions in solution.

Reaction of copper immersed in HCl

According to the Table of Activity of Metals, copper is lower on the activity series of metal than hydrogen. We would not expect a reaction to occur.

Cu (s) + HCl (aq)   No reaction ....... Reaction (4)

By dropping a piece of copper into HCl, we confirm no reaction occurs.

By the same argument, if we drop magnesium metal into an HCl solution, we observe

Mg (s) + 2 H⁺ (aq)   Mg²⁺ (aq) + H₂ (g)   .... Reaction (5)

This reaction occurs because according to the Table of Activity of Metals, magesium is higher on the activity series of metal than hydrogen. Magnesium is the more active metal than hydrodgen, and is capable of displacing H⁺ ions in solution.