Silver

Silver is not affected by dry air, but air contains traces of hydrogen sulphide. Silver reacts slowly with it (in presence of oxygen and moisture) and a black layer of silver sulphide, \(Ag_2S\), is formed on the surface. This is called tarnishing:

Silver chloride is photosensitive. In light it decomposes into silver and chlorine. The tiny particles of finely divided silver which are formed make the white salt look grey (or violet):

Silver nitrate is photosensitive, and in presence of light its crystals and its solution darken slowly because of the decomposition to black metallic silver. A dark (brown) bottle cuts off the light, so the salt remains stable and pure.

The Mac-Arthur-Forrest process, also called the cyanide process, is a hydrometallurgical method of extraction of a metal in which the powdered and concentrated ore is treated with a dilute solution of sodium cyanide in presence of air and water. The metal goes into the solution as a soluble complex cyanide, and then it is precipitated out with a more electropositive metal such as zinc:

Type of element: it is used for the extraction of noble (unreactive) metals which form stable soluble cyanide complexes, namely silver (from argentite) and gold.

Lunar caustic is silver nitrate, \(AgNO_3\), a colourless crystalline solid, highly soluble in water (it is also soluble in alcohol, pyridine and acetone). It is photosensitive and caustic.

Preparation: by dissolving silver in nitric acid and crystallising (in a fume cupboard, as the oxides of nitrogen are toxic):

Chemical properties:

1. Action of heat: above \(200^\circ C\) it gives silver nitrite and oxygen, and at red heat metallic silver:

2. Action of alkali: brown precipitate of silver oxide, \(2AgNO_3+2NaOH\rightarrow Ag_2O\downarrow+2NaNO_3+H_2O\). With ammonia solution the same precipitate forms, and it dissolves in excess of ammonia to give diamminesilver(I) nitrate, \(Ag_2O+2NH_4OH+2NH_4NO_3\rightarrow2[Ag(NH_3)_2]NO_3+3H_2O\).

3. Action on organic matter: skin, wood, paper and cloth are stained black, as silver is deposited (used in indelible ink).

4. With potassium cyanide: \(AgNO_3+KCN\rightarrow AgCN\downarrow(\text{white})+KNO_3\).

5. Precipitation: \(Ag^++Cl^-\rightarrow AgCl\downarrow\) (white), \(Ag^++Br^-\rightarrow AgBr\downarrow\) (pale yellow), \(Ag^++I^-\rightarrow AgI\downarrow\) (yellow), \(2Ag^++SO_4^{2-}\rightarrow Ag_2SO_4\downarrow\) (white), \(3Ag^++PO_4^{3-}\rightarrow Ag_3PO_4\downarrow\) (yellow).

6. Displacement: metals above silver displace it, \(Cu+2AgNO_3\rightarrow Cu(NO_3)_2+2Ag\).

Uses: as a laboratory reagent, to prepare indelible ink, Tollens’ reagent and photographic film, and in silvering mirrors.

Horn silver is silver chloride, \(AgCl\), an ore of silver. It is so called because the native mineral is soft and tough and can be cut like animal horn. It is a white crystalline solid, sparingly soluble in water.

Preparation: by the action of silver nitrate on a soluble chloride: \(AgNO_3+NaCl\rightarrow AgCl\downarrow+NaNO_3\).

Properties:

1. Action of light: it decomposes to silver and chlorine, so it turns grey or violet in sunlight: \(2AgCl\xrightarrow{\text{light}}2Ag+Cl_2\).

2. Action of ammonia: it dissolves in ammonia solution to form the soluble complex diamminesilver(I) chloride: \(AgCl+2NH_4OH\rightarrow[Ag(NH_3)_2]Cl+2H_2O\). On adding nitric acid to this solution, silver chloride is precipitated again.

3. It dissolves in sodium cyanide solution: \(AgCl+2NaCN\rightarrow Na[Ag(CN)_2]+NaCl\).

Uses: in photography because of its light sensitivity, as an antimicrobial agent, in photochemical lenses which darken in sunlight, and to prepare the silver-silver chloride electrode.

Prediction: a displacement reaction takes place. Copper lies above silver in the electrochemical series, so it is more reactive and it displaces silver from the solution of its salt.

Observations: a grey shining deposit of silver (needle-like crystals) forms on the copper wire, and the colourless solution slowly turns blue because of the copper(II) nitrate. The copper wire is eaten away and the silver nitrate is wasted.

Modification: do not keep any metal which lies above silver (such as \(Cu\), \(Pb\) or \(Zn\)) in contact with the solution. Handle the solution with a glass rod and keep it in a glass or plastic container, preferably in a dark coloured bottle because silver nitrate is also photosensitive.

Silver is dissolved in nitric acid and the solution is evaporated and crystallised. In a fume cupboard, because oxides of nitrogen are formed:

The skin contains organic matter (proteins). When silver nitrate touches the skin, the silver ions are reduced to finely divided metallic silver, which is black, and the organic matter is oxidised (the process is helped by light): \(Ag^++e^-\rightarrow Ag\). The black silver is deposited on the tissue and gives a stain, which does not wash away until the dead skin is shed. This property is used in the indelible ink.

When it is heated above \(200^\circ C\) it decomposes to silver nitrite and oxygen. If the heating is continued up to red hot, metallic silver is obtained with the release of reddish brown nitrogen dioxide and oxygen:

The reaction is reversible, so air is blown through to oxidise the sodium sulphide and shift the equilibrium forward: \(Na_2S+2O_2\rightarrow Na_2SO_4\).

(Silver hydroxide, \(AgOH\), is unstable and loses water at once to give silver oxide.)

The chief ore of silver is argentite (silver glance), \(Ag_2S\). Silver is extracted from it by the cyanide process (Mac-Arthur Forrest process), which is hydrometallurgy. The steps are:

1. Crushing and pulverization: big lumps are broken by a jaw crusher or hammer and powdered in a ball mill.

2. Concentration: by froth flotation. The powdered ore is mixed with water and pine oil in a tank and agitated by compressed air. The sulphide ore particles stick to the froth and float, and the earthy impurities (gangue) settle at the bottom.

3. Formation of complex cyanide: the concentrated ore is treated with a dilute sodium cyanide solution in presence of air and water, and a soluble complex, sodium argentocyanide, forms:

Air is blown to remove \(Na_2S\) as \(Na_2SO_4\) (\(Na_2S+2O_2\rightarrow Na_2SO_4\)), so the reaction moves forward.

4. Reduction by precipitation (cementation): the solution is filtered and treated with zinc, which is more electropositive. Silver is precipitated as a dark spongy powder, which is filtered and dried:

5. Purification: (i) Cupellation: if lead is the impurity, the silver is heated in a special furnace and a blast of air blows the lead away as \(PbO\), until bright shining silver is left. (ii) Electrorefining: the modern method. The impure silver is the anode, a thin strip of pure silver is the cathode and acidified \(AgNO_3\) solution is the electrolyte. At the anode \(Ag\rightarrow Ag^++e^-\) and at the cathode \(Ag^++e^-\rightarrow Ag\). The impurities settle below the anode as anode mud, and silver of 99.9% purity is deposited on the cathode.

Frosting of silver: it is the process of giving a dull white (matt, frosted) look to the surface of silver articles. The article, which contains some copper, is heated in air so that copper on the surface changes to black copper oxide, and then it is dipped in dilute sulphuric acid. The acid dissolves the copper oxide and leaves a porous surface of pure white silver.

The metal \(Q\) is silver (\(Ag\)): atomic number 47, atomic mass 107.87 amu, electronic configuration \([Kr]\,4d^{10}5s^1\), a d-block metal of period 5 and group 11 (coinage metals: \(Cu\), \(Ag\), \(Au\)). It is a white shining soft and malleable metal, called Chandi in Nepali, with a melting point of \(956^\circ C\), boiling point of \(1955^\circ C\) and specific gravity of 10.52.

The chief ore is argentite (silver glance), \(Ag_2S\).

By the cyanide (Mac-Arthur Forrest) process:

1. The ore is crushed and powdered, and concentrated by froth flotation using pine oil, water and compressed air.

2. The concentrated ore is leached with a dilute sodium cyanide solution in presence of air, giving a soluble complex: \(Ag_2S+4NaCN\rightleftharpoons2Na[Ag(CN)_2]+Na_2S\). Air oxidises \(Na_2S\) to \(Na_2SO_4\) (\(Na_2S+2O_2\rightarrow Na_2SO_4\)) and drives the reaction forward.

3. The filtered solution is treated with zinc, which precipitates spongy silver: \(2Na[Ag(CN)_2]+Zn\rightarrow2Ag\downarrow+Na_2[Zn(CN)_4]\).

4. The silver is refined by cupellation (removal of lead as \(PbO\) in a blast of air) or by electrorefining, with impure silver as the anode, pure silver as the cathode and acidified \(AgNO_3\) as the electrolyte, to get 99.9% pure silver.

Silver is not affected by dry air, but it is tarnished when the air contains traces of hydrogen sulphide. The metal gets covered with a black layer of silver sulphide, and this is why silver coins, ornaments and utensils turn black:

(The textbook writes this change as \(2Ag+H_2S\rightarrow Ag_2S+H_2\).)

Silver is not oxidised by oxygen even when it is strongly heated. When it is heated to melting, molten silver dissolves about 20 times its volume of oxygen from air. On cooling, the dissolved oxygen is suddenly expelled, and it throws out small drops of the metal from the surface. This is called the spitting of silver. When it is heated with sulphur or chlorine, it forms \(Ag_2S\) or \(AgCl\) (\(2Ag+S\rightarrow Ag_2S\), \(2Ag+Cl_2\rightarrow2AgCl\)).

1. Silver is used for making jewelry, coins, utensils, decorative items and electrodes.

2. Silver compounds such as \(AgBr\) and \(AgCl\) are used in photography. It is also used in dental fillings, in alloys, and in silvering mirrors.