A copper penny was added to a sodium hydroxide solution with granulated zinc in it and turned silver then turned gold after heating and i need to know what is the chemical equation of this reaction?

Answer 1

Metallic zinc dissolves in the alkaline solution, producing zincate ions and releasing hydrogen: Zn + 2OH– → ZnO2-- + H2 (visible as tiny bubbles on the surface of the zinc) You can get the same solution by adding zinc oxide to sodium hydroxide. (The zincate is probably hydrated with a couple of water molecules, but they only clutter up our equations, so we'll ignore them here.) Here's where it gets interesting: When copper is in contact with zinc* (bear with me for a few moments), in a conducting solution, an electrolytic cell is produced -- you have a battery. (In fact, you have Prof. Volta's original battery!) Electrons flow from the zinc* to the copper. At the surface of the copper coin, these electrons reduce the zincate: ZnO2-- + 2H2O + 2e- → Zn + 4OH– This produces the zinc plating you see on the coin. At the surface of the zinc*, the electrons that are being sent to the copper metal are generated by zinc dissolving to produce more zincate: Zn + 4OH– → ZnO2-- + 2H2O + 2e- Here's the cool thing: There is NO NET REACTION, but there is a net motion of zinc! The "zinc*" referred to above can be left-over zinc powder from the first step, or it can be zinc that's already built into the coin. Post-1981 US pennies, which are in fact mostly zinc with a thin copper cladding, work fine without an external zinc supply. It would seem that microscopic flaws in the cladding are necessary to let the zinc core contact the solution -- so perhaps "mint" condition pennies do need an external zinc source. (Science project, anyone?) The reaction seems at first glance to run against the electrochemical potentials of the metals, which perplexes many people. What makes it run, however, is the unseen dissolution of the metallic zinc*, which is particularly invisible in the case of a new US penny. (This is the same reaction that allows "sacrificial anodes" of zinc or magnesium to protect ship hulls against corrosion in salt water.) The experiment is more impressive, actually, with a true copper coin. Clean an old penny (1981 or earlier) by soaking it in hot vinegar until it's shiny, and drop it into the plating solution. Nothing will happen unless the penny is in contact with some zinc metal. Separate the penny from the zinc, and the plating will dissolve -- the penny returns to its original copper color. Touching the penny with a bit of zinc recreates the battery, and the zinc plating re-appears within seconds! Heating the zinc=plated penny, or just letting it sit around for several months, results in copper atoms dissolving into the zinc to produce brass - the "gold" coin is actually a brass-plated coin. This explanation, with more details and some great photos, can be found at http://woelen.scheikunde.net/science/chem/exps/copper+zinc/index.html -Jim Demers (9/4/2008)