Why Does Copper and Nitric Acid Make a Dark Blue

[Deposit Photos]

Cop­per is one of the old­est known met­als, which has been used by peo­ple from an­cient times. In Latin, cop­per is known as cuprum, and its atom­ic num­ber is 29. In Mendeleev's pe­ri­od­ic ta­ble, cop­per is lo­cat­ed in the fourth pe­ri­od, in the first group.

Phys­i­cal and chem­i­cal prop­er­ties of cop­per

Nat­u­ral­ly oc­cur­ring cop­per is a heavy met­al of pink-red col­or with a duc­tile and soft struc­ture. The boil­ing tem­per­a­ture is over 1,000 de­grees Cel­sius. Cuprum is a good con­duc­tor of elec­tric­i­ty and heat, and melts at a tem­per­a­ture of 1,084 de­grees Cel­sius. The den­si­ty of the met­al is 8.9 g/cm3, and in na­ture it is en­coun­tered in its ba­sic form. Ac­cord­ing to the elec­tron for­mu­la of the cop­per atom, it has 4 lev­els. In the 4-s va­lence or­bital there is one elec­tron. In a chem­i­cal in­ter­ac­tion with oth­er sub­stances, one to three neg­a­tive­ly charged par­ti­cles (elec­trons) split away from the atom, as a re­sult of which cop­per com­pounds form with a de­gree of ox­i­da­tion of +3, +2, +1. The max­i­mum sta­bil­i­ty is dis­played by di­va­lent de­riv­a­tives of cop­per.

[Deposit Photos]

Cop­per is a sub­stance with a low ca­pac­i­ty to in­ter­act. There are two main de­grees of ox­i­da­tion of the met­al dis­played in com­pounds: +1 and +2. Sub­stances in which these val­ues change to +3 are en­coun­tered rarely. Cop­per in­ter­acts with car­bon diox­ide, air, hy­drochlo­ric acid and oth­er com­pounds at very high tem­per­a­tures. A pro­tec­tive ox­ide film forms on the sur­face of the met­al. This met­al pro­tects the cop­per from fur­ther ox­i­da­tion, makes it sta­ble and gives the met­al a low ac­tiv­i­ty.

Met­al in­ter­acts with sim­ple sub­stances – halo­gens, se­le­ni­um, sul­fur. The met­al is ca­pa­ble of form­ing dou­ble salts or com­plex com­pounds. Al­most all the com­plex com­pounds of this el­e­ment are poi­sonous, apart from ox­ides. Sub­stances that are formed by mono­va­lent cop­per eas­i­ly ox­i­dize to di­va­lent equiv­a­lents.

In chem­i­cal re­ac­tions cop­per acts as a low-ac­tiv­i­ty met­al. The met­al does not dis­solve in wa­ter in or­di­nary con­di­tions. In dry air the met­al does not cor­rode, but when heat­ed the sur­face of cop­per is cov­ered with a black coat­ing of ox­ide. The chem­i­cal sta­bil­i­ty of the el­e­ment is shown in its re­sis­tance to im­pact of car­bon, dry gas­es, sev­er­al or­gan­ic com­pounds, al­co­hols and phe­nol resins. For cop­per, com­plex re­ac­tions are char­ac­ter­is­tic, in which col­ored com­pounds are re­leased. Cop­per has sim­i­lar­i­ties with met­als of the al­ka­line group, as it forms mono­va­lent de­riv­a­tives.

Cop­per — re­ac­tion with ni­tric acid

Cop­per dis­solves in ni­tric acid. This re­ac­tion takes place be­cause the met­al ox­i­dizes with a strong reagent.

Nitric acid molecule [Deposit Photos]

Ni­tric acid (di­lut­ed and con­cen­trat­ed) dis­plays ox­i­diz­ing prop­er­ties, with the dis­so­lu­tion of cop­per. In the re­ac­tion of the met­al with di­lut­ed acid, cop­per ni­trate and ni­tro­gen di­va­lent ox­ide form in the ra­tio of 75% and 25%. The equa­tion of the re­ac­tion is

8H­NO₃ + 3Cu → 3Cu(NO₃)₂ + 2NO + 4H₂O

In the re­ac­tion process, 1 mole of cop­per and 3 moles of con­cen­trat­ed ni­tric acid take part. When the cop­per is dis­solved, the so­lu­tion heats up in­tense­ly, the ther­mal break­down of the ox­i­diz­er takes place, and ad­di­tion­al ni­tric ox­ide is re­leased. The equa­tion of the re­ac­tion is

4H­NO₃ + Cu → Cu(NO₃) + 2NO₂ + 2H₂O

This method of dis­solv­ing cop­per has its draw­backs – in the re­ac­tion of cop­per with ni­tric acid, a large amount of ni­tric ox­ide is re­leased. To cap­ture or neu­tral­ize ni­tric ox­ide, spe­cial equip­ment is re­quired, so this process is too ex­pen­sive. The dis­so­lu­tion of cop­per in ni­tric acid is con­sid­ered com­plete when volatile ni­tric ox­ides stop be­ing pro­duced. The re­ac­tion tem­per­a­ture is from 60 to 70 de­grees Cel­sius. The next stage is drain­ing the so­lu­tion from the chem­i­cal re­ac­tor. Pieces of cop­per re­main at the bot­tom of the re­ac­tor, which did not en­ter into the re­ac­tion. Wa­ter is added to the liq­uid ob­tained, and it is fil­tered. Click here for learn­ing prop­er­ties of cop­per il­lus­trat­ed in in­ter­ac­tions with oth­er sub­stances.

The re­ac­tion of ni­tric acid and cop­per il­lus­trat­ed by an ex­per­i­ment

The en­tire re­ac­tion of ni­tric acid and cop­per can be fol­lowed with the help of an ex­per­i­ment: place a piece of cop­per in con­cen­trat­ed ni­tric acid. A brown gas is re­leased – first slow­ly, then more in­tense­ly. The so­lu­tion turns green. If you add plen­ty of cop­per in the re­ac­tion process, the so­lu­tion grad­u­al­ly turns blue. The re­ac­tion of cop­per with ni­tric acid takes place with the re­lease of heat and tox­ic gas, which has an acrid odor. The re­ac­tion of cop­per and con­cen­trat­ed ni­tric acid is an ox­ida­tive-re­duc­tive re­ac­tion. The re­duc­er sis the met­al, and the ox­i­diz­er is ni­tric acid. The equa­tion of the re­ac­tion is

Cu + 4H­NO₃ = Cu(NO₃)₂ + 2NO₂↑ + 2H₂O

The re­ac­tion is exother­mic, so in the spon­ta­neous heat­ing of the mix­ture it ac­cel­er­ates. The re­ac­tion of cop­per with ni­tric acid starts at room tem­per­a­ture. The met­al is cov­ered with bub­bles, which start to rise to the sur­face and fill the test tube with brown gas – NO₂ (tox­ic poi­sonous ni­tro­gen diox­ide with an acrid odor). This gas is 1.5 times heav­ier than air. The re­ac­tion of cop­per with ni­tric acid takes place in two stages: at the first stage, the acid ox­i­dizes the cop­per to cop­per ox­ide, re­leas­ing ni­tro­gen diox­ide; at the sec­ond stage, cop­per ox­ide re­acts with new por­tions of acid, form­ing cop­per ni­trate Cu(NO₃)₂. The mix­ture heats up, and the re­ac­tion ac­cel­er­ates.

Copper(II) nitrate trihydrate sample [Wikipedia]

The re­sult is that the met­al dis­solves, and a so­lu­tion of cop­per ni­trate forms. The cop­per ni­trate gives the so­lu­tion a green or blue col­or (this will de­pend on the amount of wa­ter used).

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Source: https://melscience.com/US-en/articles/characteristics-copper-and-reaction-metal-nitric-a/

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