Activation of Iron in Nitric Acid
Iron is dissolved in nitric acid by first adding zinc and copper metals. The 24 thumbnail images summarize the content of the video. Click an image to see the image gallery.
There is no narration for this video.
In this demonstration, nitric acid is poured into a dish and copper and zinc metal pieces are added. An immediate and visible reaction takes place as the metals react with the acid, producing a blue-green solution and brown fumes. An iron nail is added, and bubbles can be seen as the iron begins to react with the acid solution. When the nail is removed from the solution, it is significantly smaller than before the reaction.
If iron is added to nitric acid, no reaction occurs due to the passivation of iron in nitric acid. This layer can be disrupted by adding copper metal and either zinc or platinum metal to the acid. The reaction doesn't occur unless both copper and zinc or platinum metal is present in solution.
The entire mechanism of the reaction is unclear; however, some facets of the reaction are recognizable. This reaction is certainly a redox reaction, as the iron nail is visibly reduced in size by the end of the reaction. Copper is important to the reaction as a complexation agent to the iron; when the reaction was run with less than excess nitric acid, the nail was smaller and plated with copper metal, suggesting the transformation
Cu2+(aq) → Cu(s) → Cu2+(aq)
is occurring during the full reaction. Zinc or platinum is important to the reaction. An interesting observation is that platinum wire quenches the reaction if the wire is wrapped all the way around the iron nail, rather than just wrapped around the nail at one end. The gas produced at the nail is probably hydrogen, and the iron probably becomes a iron(III) ion with the production of hydroxide ions which are then neutralized in the acidic environment.
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