When conc. nitric acid is allowed to react with copper,what is the color change of the resulting solution and the ion responsible for this change?

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alienrocketscientist's profile pic

alienrocketscientist | (Level 1) eNoter

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Georgiana, where did you get your info from regarding HNO3?

You were not just "a little" wrong on many things. As someone myself who has synthesized countless liters of RFNA, WFNA, and even HNO4 as exists in the upper atmosphere I have to NIX your explanations.

First, the fuming grades of HNO3 give off vapors due to "vapor pressure" and the fumes seen are the acid vapors and not due to some reaction with (carbon?). Red fuming nitric acid fumes consist of some acid vapors and some NOx vapors...

Which brings me to the next point, the color yellow you see in concentrated nitric acid is because of the amount of dissolved NOx in the acid. It is another means by which HNO3 is graded. Even WFNA although a clear liquid starts to take on the yellow color upon decomposition. 

And of course your initial statement that HNO3 is also called "hard water" is quite wrong. Hard water is by definition simple water with high mineral content, which can precipitate from the water causing problems in water handling/treatment systems. Of course there are no minerals in HNO3 at all.

As far as the reaction with Cu, it raises the oxidative state of Cu to Cu2. This turns HNO3 solution to a green color while changing the undissolved Cu to a brown color which gets precipitated because HNO3 can oxidize way more Cu atoms than it can hold in solution. A reduction of Cu2 to the next lowest possible oxidative state can be done with aqueous ammonia giving off a vibrant blue colored solution (which is typical of most of the copper complex solutions.) Driving off the NH3 will leave the Cu1. Upon heating this will finally turn everything back into elemental Cu. Of course, much of this could have been done with a simply replacement reaction using any metal higher on the reactionary scale. 

So there is no "smoke" from concentrated NHO3, it is not called "hard water", is not yellow because of carbonization or anything like that, and your formulas and processes for synthesizing is all wrong (except the part about NO oxidizing In The Presence Of Oxygen and becoming nitrous dioxide). 

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alienrocketscientist's profile pic

alienrocketscientist | (Level 1) eNoter

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For clarification, H2SO4 combines with nitrate salts to form nitric acid and (metal) bisulfate. In some methods of synthesis there are left over NOx ions which can be passed through water or peroxide to make up to a 68% solution. The primary product though is HNO3 of a concentrated grade. Your method of synthesis determines what you get, how much untreated NOx,s you have, and how much byproduct you get. There is one way to almost double your output of HNO3 in the sulfuric acid/nitrate salt method, and also one way to synthesize liquid phase HNO4 (without UV radiation).

HNO4 also plays a critical role in the maintenance of the ozone layer and reaction cycle with N, NO, H2O2, HNO2, HNO3, H2SO4, and the effects on ice crystals in the troposphere and stratosphere, and can even be measured in the ice coverage in article environments.

HNO4 is experimental and in attempts at reactions such as nitration immediate oxidation occurs, (eg: instead of nitrating substances such as glycerine, toluene, cotton, cellulose, etc, the propencity to decompose initiates oxidation without adding heat or dynamic exhertion. Mass-wise HNO4 is mostly oxygen if you go so far as to figure out the specific gravity. Current focus is on aeronautical propulsion systems;  the benefits to rocket engine impulse measurements, and flexibilities to fuel blends extensive.

This is the current focus in studying the atmosphere today, and can even explain some geological mysteries on our planet. 

mkcapen1's profile pic

mkcapen1 | Middle School Teacher | (Level 3) Valedictorian

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Copper is a metal in color which most people associate with the copper penny or the material used for plumbing and wiring.  When copper interacts with concentrated nitric acid the result is called oxidation. 

"Copper is oxidized by concentrated nitric acid, HNO3, to produce Cu2+ ions"

"Cu(s)  +  4HNO3(aq)  ——>  Cu(NO3)2(aq)  +  2NO2(g)  +  2H2O(l)"

Once the concentrated nitric acid begins to oxidize the copper creates a brownish gas.  Gas bubbles are noted during the chemical reaction.  The reaction between the two is exothermic.  The material then turns a green color and then a brownish green color. When the copper is first oxidized, the solution is very.  The nitrate ions produce this reaction. 

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alienrocketscientist's profile pic

alienrocketscientist | (Level 1) eNoter

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I wish I could have given you more than one thumb's up. Try this for a simplified experiment, (because we all know you don't always have nitric sitting in the freezer):

Nitrate salt


SMALL amount of copper.

Combine 100ml HCl, 50g nitrate salt, and copper in a heatproof VENTED vessel, and watch the reaction.

The reaction progresses upon the production of Cu2, and at first there is little to none. But as the amounts of Cu2 increase it drives the production of more Cu2, exponentially. Although the reaction starts off slow, it soon becomes very exothermic and can only be controlled by temperature reduction. Left by itself it will always be a runaway reaction (as I have made a few inadvertent rocket launches out of laboratory equipment). 

The end result though would be a solution of CuCl and Cu(NO3)2, and precipitated Cu2. What vents out of the vessel would be NOx,s and nitrosyl chloride gas. This is one way to produce dilute HNO3 by passing the NOx,s (mostly NO2) into water, or H2O2 to increase output by transforming any NO into NO2 also in the peroxide.

In the end you get a realtime experiment that answered the initial question. Green solution (Cu [NO3]2), brown residue (Cu2).

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giorgiana1976 | College Teacher | (Level 3) Valedictorian

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Nitric acid is called also hard water.

Nitric acid is found in tiny amounts in the atmosphere and water rain, due to lightning, but it does not remain as such, combining with ammonia in the atmosphere, giving ammonium nitrate. Of the nitrates, the most important are: sodium nitrate (saltpeter of Chile), potassium (saltpeter from India) and calcium (saltpetre of Norway).

In laboratory, nitric acid is prepared from salts (natural nitrate) by treatment with hot concentrated sulphuric acid. The reaction takes place intwo stages:



2NANO+ H SO -> 2HNO + Na SO

In pure state is colorless, pungent-smelling, with density 1.52. It dissolves in water, in any proportions, providing various concentrations of nitric acid. Nitric acid with a concentration of 96-98%, smokes in the air because it's vapors absorb moisture and give birth to a fog consisting of fine drops of liquid.

Concentrated nitric acid is colored yellow because dioxide carbon that is formed after partial decomposition in air, in presence of light and which remains dissolved in it:

4 HNO -> 4 NO + H 2 O + O ^

Therefore it should be stored in dark bottles and in darkness.Concentrated nitric acid, rich in NO, is called fuming nitric acid.

In aqueous solution ionizes as reversible reaction:

HNO + H O = [H O] + NO

This acid is one of the strongest, with a high degree of ionization.

With Nitric acid, copper reacts violently, forming copper nitrate, Cu (NO3) 2,  nitrogen monoxide which oxidizes instantaneously into nitrogen dioxide and  water. In the tube appears a suffocating gas, irritating, light reddish brown, due to nitrogen dioxide formed from the oxidation of nitrogen monoxide.

8HNO3 + 3Cu → 3Cu (NO3) 2 + 2NO + 4HNO3

2NO + O2 → 2NO2

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revolution | College Teacher | (Level 1) Valedictorian

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This process is known as oxidation state and it turns light reddish brown once reacted upon it. This reaction was caused upon by the nitrate ions present in the solution that cause this change.