Homework Help Forum: Honors Chemistry

Posted by Emily on Wednesday, October 7, 2009 at 12:01am.

We did the gold penny lab, where we bathed a copper penny in a hot zinc-NaOH mixture to turn it a silver color, then we heated that in a flame to turn it a gold color. There are two questions that go with this lab that I don't understand, however: 4. Did the copper and zinc undergo a physical change or a chemical change when the penny was bathed in the zinc-NaOH solution? Explain your answer. - I would put it was a physical change because the zinc pretty much just melted onto the penny to turn it a silver color -- is this right? 5. Did the copper and zinc undergo a physical change or a chemical change when the penny was heated in the flame? Explain your answer. - Wouldn't you also put it was a physical change because the zinc mixed with the copper of the penny to turn a gold color? Or did I get this completely backwards? Any help is GREATLY appreciated! :D

• Honors Chemistry - DrBob222, Wednesday, October 7, 2009 at 12:23am

  Both of your questions should be answered as a chemical change. In the first one, the Zn ions in solution are plated out onto the Cu penny. The silvery color you see is Zn. The reaction is Cu + Zn^+2 ==> Zn + Cu^+2 Basically, you dissolved Cu and plated out Zn. Remember the activity series of metals? Zn is above Cu so Zn^+2 ion comes out of solution and Cu goes in. Second question. When you heated the bare Zn-coated copper penny, you formed brass which is a gold color. Brass is an alloy of Cu and Zn. Here is a site on the web where you can read about the experiment. http://misterguch.brinkster.net/MLX040.pdf

  
  

• Honors Chemistry - Emily, Wednesday, October 7, 2009 at 12:29am

  Ohhhh, I get it now. Thank you so much!! :D

  
  

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