Sciencemadness Discussion Board - Why Nitric acid doesn't react with Al?

Why Nitric acid doesn't react with Al?

AsocialSurvival - 18-10-2014 at 12:02

See this video. Why? Is it because the oxide layer can't dissolve in water, because there's only 30% water in 15.6M (68.8%) Nitric acid? Would it work with more dilute Nitric acid?

diddi - 18-10-2014 at 15:22

Vladimir M. PETRUŠEVSKI, Miha BUKLESKI,
Marina STOJANOVSKA
Ss Cyril & Methodius University, Skopje, Republic of Macedonia (2010) state:

In the course of the reinvestigation of the reactions of aluminium with strong
inorganic acids (concentrated and diluted), one fact soon became obvious: aluminium
is readily dissolved in practically all strong acids, providing the solution contains chloride
anions (obviously acting as a catalyst). Traces of Cu2+ ions further catalyze the
process. The product thus obtained is practically pure hydrogen, in all cases except
when one deals with nitric acid. The product of the reaction with diluted nitric acid is,
again, a gas (depending on the quantity of the added Cu2+ ions the reaction may be
somewhat vigorous). In all instances but one, where a minor puff occurred, the product
gas was nonflammable, thus suggesting that hydrogen is far from being the principal
component present.

DrMario - 19-10-2014 at 00:31

The reason is that HNO3 does not etch alumina (aluminium oxide), and aluminium is covered with a thin layer of native alumina. Moreover, any exposed bits of aluminium will be immediately covered by alumina, when placed in HNO3.

EDIT: even the video you linked to mentions: "aluminium does not react with nitric acid due to a passivation layer that forms on the surface". NurdRage doesn't say that the passivation layer is alumina, but now you know

[Edited on 19-10-2014 by DrMario]

vmelkon - 22-10-2014 at 11:06

Quote: Originally posted by diddi

Vladimir M. The product of the reaction with diluted nitric acid is,
again, a gas (depending on the quantity of the added Cu2+ ions the reaction may be
somewhat vigorous). In all instances but one, where a minor puff occurred, the product
gas was nonflammable, thus suggesting that hydrogen is far from being the principal
component present.

I'm sorry but I didn't catch that. They add Al to HNO3 with some Cu2+ ions present and it reacts and produces a gas that is non flammable? What is the gas? NO? NO2?

Amos - 22-10-2014 at 11:12

Quote: Originally posted by vmelkon

Quote: Originally posted by diddi

I'm sorry but I didn't catch that. They add Al to HNO3 with some Cu2+ ions present and it reacts and produces a gas that is non flammable? What is the gas? NO? NO2?

Sorry, but wouldn't this whole reaction just be aluminum replacing copper in the solution, precipitating copper which then reacts with nitric acid? It's not a reaction between nitric acid and aluminum at all. And yes, assuming it is dilute nitric acid in this case, NO and possibly some NO2 would be evolved, depending on what they mean by dilute.

diddi - 22-10-2014 at 22:12

http://www.khimiya.org/pdfs/KHIMIYA_19_3_PETRUSEVSKI.pdf

blogfast25 - 23-10-2014 at 05:55

Quote: Originally posted by vmelkon

I'm sorry but I didn't catch that. They add Al to HNO3 with some Cu2+ ions present and it reacts and produces a gas that is non flammable? What is the gas? NO? NO2?

NO (NO2 only forms when the NO hits the air: NO + 1/2 O2 ===> NO2). But there will be hydrogen too, just not enough to make it flammable.

With electropositive elements dilute acids oxidise them with H3O+(aq). But nitric acid also contains nitrate ions which oxidise stuff as follows:

NO3-(aq) + 4 H+ + 3 e- ===> NO(g) + 2 H2O(l)

With electropositive metals and dilute NA, the first oxidation tends to prevail. With increasing concentration the oxidation via nitrate kicks in.

Non-electropositive metals like Cu need the nitrate route, as, H3O+(aq) cannot oxidise them.

[Edited on 23-10-2014 by blogfast25]

AJKOER - 23-10-2014 at 13:28

As an interesting experiment (I am currently out of HNO3), try an react the HNO3 with Aluminum foil that was heated to a glowing red and grind to a powder.

This is reported to be Alumimum with a weakened coating of gamma Al2O3. I have found the latter to be, in general, more chemically reactive. Here is a link to a white paper (link https://www.google.com/url?sa=t&source=web&rct=j&... ) detailing reaction of Aluminum and water with promoters to address the protective Al2O3 layer. To quote from page 7:

"It has been shown that mixtures of aluminum and aluminum oxide (Al2O3) powders are reactive with water in the pH range of 4-9 (11-13) and at temperatures of 10-90 oC. These Al-Al2O3 powder mixtures must be heavily ball-milled together in order to produce hydrogen reactions. Hydrogen can be evolved at room temperature using essentially neutral water, although the hydrogen evolution rate increases with increasing temperature. "

Also, to quote page 8: "The aluminum oxide may be in the form of bayerite (Al(OH)3), boehmite (AlO(OH)), gamma alumina (γ-Al2O3), or alpha alumina (α-Al2O3). Alpha alumina powder was reported to give the maximum hydrogen evolution. It has been speculated that the milling of aluminum and aluminum oxide powders together helps to mechanically disrupt the adherent and coherent oxide layers present on the aluminum powder, and that this is the reason for the enhanced hydrogen generation in pH neutral water (11-13).
However, recent research has suggested that the enhancing effect of aluminum oxide on the reactivity of aluminum with water may also be mechanochemical in nature (14). Aluminum powders that were reacted with fine boehmite powders at elevated temperatures produced a layer of fine-grained, mechanically weak gamma alumina on the surfaces of the aluminum powders. "

I suspect dilute Nitric acid (pH 4) probably will still behave the same, but it would be interesting if it doesn't (the point being not all protective Al2O3 may act equally depending on circumstances).

[Edited on 23-10-2014 by AJKOER]

DrMario - 23-10-2014 at 14:06

Quote: Originally posted by AJKOER

As an interesting experiment (I am currently out of HNO3), try an react the HNO3 with Aluminum foil that was heated to a glowing red and grind to a powder.

This is reported to be Alumimum with a weakened coating of gamma Al2O3. I have found the latter to be, in general, more chemically reactive. Here is a link to a white paper (link https://www.google.com/url?sa=t&source=web&rct=j&... ) detailing reaction of Aluminum and water with promoters to address the protective Al2O3 layer. To quote from page 7:

"It has been shown that mixtures of aluminum and aluminum oxide (Al2O3) powders are reactive with water in the pH range of 4-9 (11-13) and at temperatures of 10-90 oC. These Al-Al2O3 powder mixtures must be heavily ball-milled together in order to produce hydrogen reactions. Hydrogen can be evolved at room temperature using essentially neutral water, although the hydrogen evolution rate increases with increasing temperature. "

Also, to quote page 8: "The aluminum oxide may be in the form of bayerite (Al(OH)3), boehmite (AlO(OH)), gamma alumina (γ-Al2O3), or alpha alumina (α-Al2O3). Alpha alumina powder was reported to give the maximum hydrogen evolution. It has been speculated that the milling of aluminum and aluminum oxide powders together helps to mechanically disrupt the adherent and coherent oxide layers present on the aluminum powder, and that this is the reason for the enhanced hydrogen generation in pH neutral water (11-13).
However, recent research has suggested that the enhancing effect of aluminum oxide on the reactivity of aluminum with water may also be mechanochemical in nature (14). Aluminum powders that were reacted with fine boehmite powders at elevated temperatures produced a layer of fine-grained, mechanically weak gamma alumina on the surfaces of the aluminum powders. "

I suspect dilute Nitric acid (pH 4) probably will still behave the same, but it would be interesting if it doesn't (the point being not all protective Al2O3 may act equally depending on circumstances).

[Edited on 23-10-2014 by AJKOER]

Oh, hey, that's some neat info, there! Thanks!