Metals and Non-Metals - Part XIV

Copper
Copper has chemical symbol Cu. Cu has many isotopes but Cu with atomic mass 64 is most abundant. It has 29 protons and 35 neutrons. An atom of Cu is represented as ⁶⁴₂₉Cu. The symbol of copper is derived from its Latin name cuprum. Cu is one of the transition metals, just like Fe. In transition metals, electrons from few of the outer shells participate in chemical reactions. It is placed in the periodic table after Fe, Co and Ni, as their properties are similar. Cu is non magnetic.

Since Cu has 29 protons, it also has 29 electrons. The electronic configuration of Cu is K-shell – 2 electrons, L-shell – 8 electrons, M-shell - 18 and N-shell has 1 electron. Cu shows metallic properties and gives off its 1 electron in the outermost shell easily to achieve a stable electronic configuration Cu¹⁺. Sometimes Cu gives off 2 or even 3 electrons from the M-shell to become Cu²⁺  or Cu³⁺ (variable valence).

Extraction of Cu from pyrites : The ore is concentrated by breaking the ore in smaller particles. Froth floating procedure is used where pine oil is added to a tank full of powdered ore and water. The mixture is agitated by passing compressed air. Sulphide particles rise to surface with the froth. Sand, clay and other gangue particles are removed from the bottom of the tank. After this the ore is heated or roasted in sufficient air so that water is removed and pure oxide of the metal remains behind. The metal-oxide is then reduced by heating further but this time in the absence of air.  

The above process is called “bessemerization” of Cu. Cu metal obtained after “bessemerization” is in a molten state. The sulphur dioxide bubbles through the molten copper and forms blisters on the surface. This remains as an impurity in the Cu metal extracted. The Cu thus has to be purified further; this is done by electrorefining or electrolytic refining.

In an electrolytic tank, acidified copper sulphate (CuSO₄ + dil H₂SO₄) solution forms the electrolyte. A block of impure copper is made into an anode by connecting the positive terminal of a power supply (battery). A thin strip of highly pure copper metal is the cathode of the cell. The negative terminal of the power supply is connected to it. A small electric current is passed through the cell.  Atoms from the anode enter the electrolyte. The copper from the anode gets converted into copper sulphide. An equal number of copper atoms from the solution get deposited on the cathode. This is to keep the concentration of the solution constant. Impurities from the anode block either remain in solution or collect below the anode, as they are unable to displace copper form the sulphate solution. The impurities remain insoluble in the electrolyte and they are called anode mud. Pure copper is scraped or removed from the cathode. Anode becomes thinner as the electrolysis process proceeds.

Physical properties of copper : Cu  is a reddish brown metal. Its surface may appear dull due to the formation of an oxide layer.  Its density is high (8.94 gm/cc). Its melting point is 1083°C. Cu is a very ductile and malleable metal. It is a very good conductor of heat and electricity.

Chemical properties of copper :
1. Valence : Cu shows variable valence. Since Cu has 1 electron in its N-shell, it gives it off easily. Monovalent Cu has valence = 1+. Cu is able to show 2+ valance by emitting one electron from the M-shell. Divalent Cu has valence = 2 +. Cu forms cuprous Cu¹⁺ and cupric Cu²⁺ compounds. Cu does not react easily and is the least reactive of transition metals.

2. Action of air : Cu is an unreactive metal. Cu does not react with dry air at ordinary temperatures.

When exposed to damp air Cu forms greenish coating of CuCO₃.Cu(OH)₂. When Cu is heated in air or oxygen, it forms cuprous Cu (II) oxide.

3. Action of water : Cu does not react with water to form a hydroxide at ordinary temperatures. Even  steam has no reaction with Cu. We can therefore conclude that Cu is resistant to water.  

4. Reaction with acids : Cu is below H in the metal reactivity series. It therefore does not replace hydrogen in an acid and form salt, releasing hydrogen gas. Cu has no reaction with acids, especially dilute acids.

Strong acids are oxidizing agents. Cu reacts with concentrated H₂SO₄ and dilute as well as concentrated H₂NO₃. The reactions are shown below.

With hot concentrated sulphuric acid, Cu reacts by releasing sulphur dioxide and water.  

With dilute nitric acid, Cu reacts by releasing nitric oxide and water.  

With concentrated nitric acid, Cu reacts by releasing nitrogen dioxide and water.  

5. Action of alkalis : Pure Cu does not react with NaOH and KOH. It is unable to replace Na or K from the hydroxides.

6. Reaction with chlorine : Cuprous chloride is formed when Cu is heated in chlorine atmosphere. The reaction is shown below.  

7. Reducing action :  Cu does not show any reducing action.

8. Reaction with sulphur : When Cu is heated with sulphur powder, CuS, that is cupric sulphate is formed. The reaction is shown below.  

9. Test for copper : If Cu is present as a cupric (II) salt form in a solution, it can be tested by pouring ammonium hydroxide. A pale blue precipitate of cupric (II) hydroxide separates out and ammonium chloride solution remains behind. This is a practical test for the presence of Cu (II).  

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