The Copper Cycle Experiment:
NO 2 Cu
Mg HNO 3
Cu(NO ) 3 2 D
H SO 2 4
H O 2
In this experiment we will explore chemical reactions of copper metal. You’ll start with approximately 40 mg (0.040 g) of copper and do the following chemical reactions: 1. Dissolve the copper metal and make copper (II) ion: When nitric acid is added to copper metal, each copper atom gives 2 electrons to the 2 nitrogens in nitric acid: Molecular equation: Cu(s) + 4HNO3(aq) à 2NO2 (g) + Cu(NO3)2 (aq) + 2H2O(l) Ionic Equation: Cu(s) + 4H+(aq) + 4NO3 (aq) à 2NO2 (g) + Cu2+ (aq) +2NO3 (aq) + 2H2O(l)
Net ionic equation:
Cu(s) + 4H+(aq) + 4NO3 (aq) à 2NO2 (g) + Cu2+ (aq) + 2NO3 (aq) Cu(s) + 4H+(aq) + 2NO3 (aq) à 2NO2 (g) + Cu2+ (aq) + 2H2O(l
↑ blue copper (II) ion, aka cupric ion
A note on the superscripts, subscripts, etc., in balanced equations: (aq) ≡ aqueous, dissolved in water. Ions are dissociated. NaOH(aq) means that solid NaOH dissolved in water and broke apart (dissociated) into sodium ions (Na+(aq) ) and hydroxide ions (OH(aq). Each ion is hydrated, that is, surrounded by a cage of water molecules. (s) ≡ solid, insoluble in water. Cu(s) and Cu(OH)2(s) don’t dissolve in water, they are solid. (g) ≡ gas ↓ ≡ indicates that a precipitate forms. Soluble ions such as Cu2+(aq) and OH(aq) react and make an insoluble product, Cu(OH)2(s) which settles to the bottom of the test tube. Δ ≡ heat is used to make the reaction proceed.
Oxidation: The loss of electrons. Oxidizing agents get reduced.
Copper loses electrons and is oxidized to blue copper (II) ion. Nitrogen gains the electrons copper loses, and is reduced to reddishbrown nitrogen dioxide gas. Nitric acid is an oxidizing agent. This type of reaction, in which electrons are lost and gained, is called an oxidationreduction reaction or redox reaction.
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2. Make insoluble copper (II) hydroxide: Copper (II) ion will react with hydroxide ion, OH, and make insoluble copper (II) hydroxide, Cu(OH)2. Copper (II) hydroxide looks like light blue Jello. 6 M sodium hydroxide, NaOH, is the source of hydroxide ions. This is a metathesis, or displacement, reaction. It is not a redox reaction; no electrons are being lost or gained. In this reaction, the cages of water around copper (II) ions are being displaced by hydroxide ions, and cages of water around hydroxide ions are displaced by copper (II) ions. Molecular equation: Cu(NO3)2 (aq) + 2NaOH3(aq) à Cu(OH)2(s) ↓ + 2NaNO3(aq) Net Ionic Equation: Cu2+ (aq+ 2OH(aq) à Cu(OH)2(s) ↓
3. Dehydrate Copper (II) Hydroxide. Make Copper (II) Oxide When Copper (II) hydroxide, Cu(OH)2, is heated, it will lose a molecule of water and form black CuO(s) copper (II) oxide: This, by the way, is neither a redox reaction nor a metathesis reaction. It is a decomposition reaction. 4. Dissolve the Copper (II) Oxide and Make Copper (II) Sulfate: Copper oxide is insoluble in water, but will dissolve nicely in sulfuric acid (H2SO4). A rather nice colored solution of copper (II) sulfate will result: This is a metathesis reaction, in which oxide ion (O=) is replaced by sulfate ion (SO4=). There is also an acidbase reaction going on: 2H+ (an acid) + O2 (a base) à H2O 5. Get the Copper Metal Back: Copper metal can be made from copper (II) ion if a source of electrons is supplied. For example, you can plate copper metal out of a copper (II) sulfate solution in contact with the cathode of an electrolytic cell, and in fact we’ll do that next semester. In today’s case we will use another element as a source of electrons. Magnesium will work nicely. Copper metal will reform, and the solid magnesium will dissolve and make magnesium sulfate. Aluminum metal will accomplish the same thing. Copper metal will reform, and the solid aluminum will dissolve and make aluminum sulfate. This is another...