Copper reacts in two ways with nitric acid. But this reaction is different from typical metal - acid reaction because nitric acid is an oxidizing acid.
According to the concentration of HNO3 acid solution, products given by the reaction with copper are different. In this tutorial, we discuss following topics.
Copper with dilute nitric acid reaction
Copper with concentrated nitric acid reaction
How to balance those reactions
Questions and safety
Both reaction are redox reactions and have to consider about their oxidation numbers when reactions are being balanced.
Now, we will study briefly about reactants, copper and HNO3.
HNO3 is an oxoacid of nitrogen and a strong monobasic acid. HNO3 can behave as an oxidizing acid.
Dilute nitric acid reacts with copper and produce copper nitrate ( Cu(NO3)2 ), nitric oxide (NO) and water as products. In this reaction, copper is oxidized while nitric acid is reduced to nitric oxide.
After the reaction, solution which contains Cu(NO3)2 is blue color.
3Cu(s) + 8HNO3(aq) → 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)
Nitric oxide is a colorless gas and oxidation number of nitrogen is +2. So oxidation number of nitrogen is reduced from +5 to +2 while oxidation number of copper is increased from 0 to +2.
Concentrated nitric acid reacts with copper and produce copper nitrate ( Cu(NO3)2 ), nitrogen dioxide (NO2) gas and water as products. In this reaction, copper is oxidized to its +2 oxidation state while nitric acid is reduced to nitrogen dioxide.
After the reaction, color of the solution which contains Cu(NO3)2 is blue.
Cu(s) + 4HNO3(aq) → Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)
In this reaction too, copper is oxidized to its +2 oxidation state. But nitrogen in nitric acid is reduced from +5 to +4 by producing nitrogen dioxide which is a brown color and acidic gas.
You are advised to read material safety data sheets (MSDS) of Cu, HNO3, NO, NO2 before you do this experiment.
When Cu reacts with nitric acid, NO or NO2 can be given as products according to the temperature and concentration of the acid solution. However, oxygen converts produced NO to NO2.
NO2 is very toxic gas and can cause many health issues including deaths if highly amount is inhaled. You can identify NO2 gas very easily because it is brown color and have a very unpleasnat smell.
So try to prevent as much as exposing to this hazardous gas in this experiment and anywhere when you work.
Now, we are going to learn how to balance two reactions of copper and nitric acid. Already we have identified which atoms are oxidized and reduced in the reactions. Lets take the reaction of copper with dilute nitric acid.
Nitric acid can behave as an acid or an oxidizing acid. It depends on the concentration and temperature of the nitric acid solution. If very dilute nitric acid is mixed with a metal, it can show typical acid - base reaction by evolving hydrogen gas. But, if concentration and temperature of nitric acid is high, nitric acid can behave as an oxidizing acid and will form nitrogen dioxide or nitric oxide.
Yes. Nitrogen dioxide gas can be evolved. But, you have to use concentrated nitric acid. Otherwise, nitric oxide gas is evolved.
In typical acid metal reaction, hydrogen gas emits. But, with copper, while copper is oxidized, nitrogen of nitric acid is reduced to +4 or +2 oxidation states instead of reduction of hydrogen +1 ion to hydrogen gas (0 oxidation number).
HCl acid is not a oxidizing acid because chlorine atom in the HCl is -1 oxidation state. Therefore that chlorine atom does not reduce to show oxidizing acid characteristics. But in HNO3, nitrogen is at +5 and it can be reduced to lower oxidation numbers.
Nitric acid is a colorless solution. (but if nitric acid was decomposed, there can be yellow color). When copper reacts with nitric acid solution become blue color. Also high concentration and and high temperature nitric acid was used, you can see a brown color gas emits during the reaction.
Silver reacts with nitric acid and emit nitrogen dioxide and produce silver nitrate and water.
Ag(s) + 2HNO3(aq) → AgNO3(aq) + NO(g) + H2O(l)