Pennies in 1982?

Question

We did a lab on pennies post-1982 and pre-1982.

Pre-1982: contain 95% copper, 5% zinc.
Post-1982: purpose of this lab is to determine this.

We added hydrochloric acid solution. After 1 day, there were bubbles coming out from the notches that we carved in. After 2 days, the pennies turned a little black and the pre-1982 penny was harder than the post-1982 penny.

Questions:
Why did the pennies turn a little black?
What can you conclude about the two different metals and their reactivity with strong acids?
Would this have practical/consumer applications?

By the way, I really don't get this lab. Can someone also explain what's the point of this lab?

Answer 1

American pennies made after 1982 contain much less copper (around 97.5% zinc with just 2.5% copper plating) compared to pennies made before 1982.

The pennies turned black because the zinc reacted with the hydrochloric acid.

Copper is unreactive to HCl (If anything, I would expect the pre-1982 pennies to lighten somewhat, as the HCl would eat through any gunk covering the penny). This is also why the post-1982 pennies turned softer (because the zinc core was "dissolving") compared to the pre-1982 pennies.

Not sure about commercial applications, except that if you need to get rid of a lot of zinc, HCl will do the job.

The point of the lab may be to show you how different metals react to the same substance in drastically different ways, and also to show you that the chemical properties of items can be drastically different, even when they are similar in appearance. Mostly, though, it's an attempt to get you interested in chemistry through a real-world example.

Answer 2

what you are doing is etching the pennies with acid, that is, allowing the acid to eat away at the first 10 to 20 millionths of an inch of the surface, reacting with it, turning it into tiny black particles. this would reveal any surface defects that may not otherwise be visable.

this is a form of non destructive testing (NDT). if you etch it too long it can corrode through the entire metal, making it just a DT.

The two metals should etch differently, because one is more corrosion resistant then the other. The difference in hardness can suggest that the softer contains more zinc, a les corrosion resistant metal.

a little bit into 1982 pennies became mostly zink, with a copper plating. there are much easier ways to distinguish them:

1) you can weigh them, the copper ones weigh more.

2) you can flip the coins, the copper one will go "ringggggg"
and the zink one will go "clink"

Answer 3

Zinc is much more reactive than copper is, and the contact between the two metals in an acidic solution generates an electric current that makes the zinc react that much more rapidly to form zinc chloride.

The older cents (a penny is a British coin) contain a Cu.Zn alloy. The newer ones (I think) contain a zinc core plated with copper alloy. The alloy is fairly unreactive, because it's mostly copper.

Cupric oxide is black, but it's unlikely that's where the color comes from, because the copper doesn't react. It could be from minute holes where the zinc was leached out as it reacted and dissolved.

Answer 4

this lab shows that around that year the government changed the metal composition of pennies. Do a search for the US Mint or something. It is common for metals to turn black after reaction with a strong acid. In the case of the newer pennies, less copper content and more coatings with zinc and other metals provide better corrosion resistance (typical knowledge from a Materials engineering course)

Answer 5

the post 82s are 95% zinc and 5% copper. 1944-82 95%copper. The earlier ones were actually bronze, they used steel 1 year, before that copper-nickel, and the first 60 years were almost all copper and 3 times the size. For most part it comes down to production cost. The post 82s are a little lighter, cheaper,softer metal, and a lower melting point(bunsen burner might be enough).

Answer 6

I think the zinc percentage was increased in the post-1982 pennies. Zinc is sufficiently electropositive that it will displace hydrogen ion from acid, forming H2 gas and zinc ion in HCl solution. That's why it bubbled, and that the post-1982 pennies were less hard (more of the metal had dissolved).

Answer 7

I have 2 1882 pennies. Any idea what they are worth?

Answer 8

Yeah, the goverment is using cheaper materials to make coins! More money for them!