martes, 29 de mayo de 2007

Reacción entre el ácido nítrico y los metales

En el vídeo se muestran cuatro matraces que contienen unos 50 ml de ácido nítrico en los que se deposita: oro, magnesio y cinc.

El oro no reacciona con el ácido nítrico, si queremos disolver oro tenemos que utilizar agua regia, una mezcla de ácido nítrico y ácido clorhídrico en proporción 1:3. Las limaduras de cobre y magnesio reaccionan rápidamente liberando un gas marrón. El cinc reacciona, después de un breve instante, de la misma manera.
El ácido nítrico ataca a la mayor parte de los metales, incluso a la plata y al cobre. Las únicas excepciones son los metales nobles: oro, platino e iridio. Algunos metales, como el hierro, el cromo y el aluminio no son atacados porque se pasivan, es decir, se forma una fina capa de óxido que protege al metal del ataque del ácido. Con el cinc pasa algo similar, pero finalmente la capa de óxido es atacada por el ácido nítrico.
En todos los casos tienen lugar reacciones redox: el ácido nítrico es un potente oxidante:
3 Mg + 2 NO3- + 8 H3O+ -> 3 Mg2+ + 2NO + 12 H2O
El metal se oxida y el nitrógeno del se reduce. El monóxido de nitrógeno es incoloro, pero reacciona con el oxígeno del aire y forma NO2 de color marrón.

14 comentarios:

Anónimo dijo...

A lo peor es una tontería de pregunta, pero ¿la “pasivación” del hierro, cuando se trata con ácido nítrico, protege a aquél de la oxidación? ¿Tiene esto algo que ver con el pavonado del hierro para protegerlo de la acción oxidante ambiental?

Anónimo dijo...

El pavonado es una forma de pasivación del hierro y del acero, existen varios métodos (ácidos y alcalinos).Se utlilizan distintas mezclas para obtener tonos negros o azulados. En la página http://tiroalcorcon.com/parmas.htm#1p
tienes información al respecto.

Anónimo dijo...

Muchas gracias, Lola.
Por cierto, el blog... ¡cada vez mejor!

Anónimo dijo...

muy bueno el blog! me hizo recordar que era una redox !! :D Chechula!

Anónimo dijo...

porque el oro literalmente no reacciona con los acidos

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