Sulfur (S): Key Reactions & Properties

Chemical Properties Overview

Sulfur (S) is a non-metal belonging to Group 16 (Chalcogens) of the periodic table, located in Period 3.

• Atomic Number: 16
• Electronic Configuration: $[Ne] 3s^2 3p^4$
• Electronegativity (Pauling): 2.58
• Common Oxidation States: -2, 0, +2, +4, +6
• Reactivity: Generally forms covalent compounds. It is less electronegative than oxygen and less reactive than fluorine and oxygen but more reactive than halogens in certain contexts (e.g., forming H₂S). It readily combines with metals and non-metals.

Action of Air and Oxygen

Sulfur burns in air or oxygen with a blue flame, producing sulfur dioxide.

• Combustion in Oxygen: $S_\{(s)\} + O_\{2(g)\} \xrightarrow\{\text\{heat\}\} SO_\{2(g)\}$

• Catalytic Oxidation of Sulfur Dioxide (Contact Process intermediate): Sulfur dioxide can be further oxidized to sulfur trioxide, especially in the presence of catalysts like vanadium(V) oxide ($V_2O_5$) and high temperature, a key step in the Contact Process for sulfuric acid manufacturing. $2SO_\{2(g)\} + O_\{2(g)\} \xrightarrow\{V_2O_5, \text\{ heat\}\} 2SO_\{3(g)\}$

Action of Water and Steam

Elemental sulfur is generally insoluble in water and does not react with water or steam under normal conditions to form new compounds.

Action of Acids and Bases

With Acids

Sulfur is unreactive with non-oxidizing dilute acids. However, it reacts with strong oxidizing acids.

• With Concentrated Hot Sulfuric Acid ($H_2SO_4$): Sulfur is oxidized to sulfur dioxide, and sulfuric acid is reduced. $S_\{(s)\} + 2H_2SO_\{4(conc)\} \xrightarrow\{\text\{hot\}\} 3SO_\{2(g)\} + 2H_2O_\{(l)\}$

• With Concentrated Hot Nitric Acid ($HNO_3$): Sulfur is oxidized to sulfuric acid, and nitric acid is reduced to nitrogen dioxide. $S_\{(s)\} + 6HNO_\{3(conc)\} \xrightarrow\{\text\{hot\}\} H_2SO_\{4(aq)\} + 6NO_\{2(g)\} + 2H_2O_\{(l)\}$

With Bases

Sulfur undergoes disproportionation (simultaneous oxidation and reduction) when heated with concentrated alkalis.

• With Hot Concentrated Sodium Hydroxide ($NaOH$): Sulfur is simultaneously oxidized to sulfite ($SO_3^\{2-\}$) and reduced to sulfide ($S^\{2-\}$). $3S_\{(s)\} + 6NaOH_\{(aq)\} \xrightarrow\{\text\{hot\}\} 2Na_2S_\{(aq)\} + Na_2SO_\{3(aq)\} + 3H_2O_\{(l)\}$

Key Laboratory Test/Identification Reactions

For Sulfide Ions ($S^\{2-\}$):

1. Lead Acetate Test: When treated with lead acetate solution, sulfide ions produce a black precipitate of lead(II) sulfide. $Pb^\{2+\}_\{(aq)\} + S^\{2-\}_\{(aq)\} \rightarrow PbS_\{(s)\} \text\{ (Black precipitate)\}$ (Alternatively, a filter paper moistened with lead acetate solution turns black when exposed to $H_2S$ gas).

2. Sodium Nitroprusside Test: Sulfide ions give a violet coloration with freshly prepared sodium nitroprusside solution. $Na_2S_\{(aq)\} + Na_2[Fe(CN)_5NO]_\{(aq)\} \rightarrow Na_4[Fe(CN)_5NOS]_\{(aq)\} \text\{ (Violet coloration)\}$

3. Dilute Acid Test: On adding dilute HCl or H₂SO₄, sulfide salts evolve hydrogen sulfide ($H_2S$) gas, which has a characteristic rotten egg smell. $S^\{2-\}_\{(aq)\} + 2H^+_\{(aq)\} \rightarrow H_2S_\{(g)\}$

For Sulfite Ions ($SO_3^\{2-\}$):

1. Dilute Acid Test: On adding dilute HCl or H₂SO₄, sulfite salts evolve sulfur dioxide ($SO_2$) gas, which has a suffocating smell. $SO_2$ turns acidified potassium dichromate solution green (due to the formation of $Cr^\{3+\}$) and turns lime water milky. $SO_3^\{2-\}_\{(aq)\} + 2H^+_\{(aq)\} \rightarrow SO_\{2(g)\} + H_2O_\{(l)\}$ $K_2Cr_2O_\{7(aq)\} + H_2SO_\{4(aq)\} + 3SO_\{2(g)\} \rightarrow K_2SO_\{4(aq)\} + Cr_2(SO_4)_\{3(aq)\} \text\{ (Green)\} + H_2O_\{(l)\}$

2. Barium Chloride Test: Sulfite ions react with barium chloride solution to form a white precipitate of barium sulfite, which is soluble in dilute HCl. $Ba^\{2+\}_\{(aq)\} + SO_3^\{2-\}_\{(aq)\} \rightarrow BaSO_\{3(s)\} \text\{ (White precipitate, soluble in dilute HCl)\}$

For Sulfate Ions ($SO_4^\{2-\}$):

1. Barium Chloride Test: Sulfate ions react with barium chloride solution to form a white precipitate of barium sulfate, which is insoluble in dilute HCl and dilute HNO₃. This test distinguishes sulfate from sulfite. $Ba^\{2+\}_\{(aq)\} + SO_4^\{2-\}_\{(aq)\} \rightarrow BaSO_\{4(s)\} \text\{ (White precipitate, insoluble in dilute HCl/HNO\}_3\text\{)\}$

2. Lead Acetate Test: Sulfate ions form a white precipitate of lead(II) sulfate with lead acetate solution. $Pb^\{2+\}_\{(aq)\} + SO_4^\{2-\}_\{(aq)\} \rightarrow PbSO_\{4(s)\} \text\{ (White precipitate)\}$