TOPIC 8: SULPHUR

Overview of the Process

• Sulfur Dioxide Production:

  • Obtained by burning sulfur or roasting sulfide ores (main sources: volcanic deposits from regions like Iran, Iraq, Poland, Mexico).

  • Chemical Equation:

  • S(s) + O2(g) ⇌ SO2(g) (burning sulfur)

  • Metal sulfide + O2 ⇌ metal oxide + SO2 (roasting sulfides)

Raw Materials

• Sulfur:

  • Mined from various locations; significant source from fossil fuel impurities (desulfurization).

• Oxygen:

  • Produced from the fractional distillation of liquefied air.

Step 1: Conversion of Sulfur to Sulfur Dioxide

• Equations:

  • 2ZnS + 3O2 → 2ZnO + 2SO2 (example of roasting).

Step 2: Formation of Sulfur Trioxide

• Process:

  • SO2 is cleaned, mixed with O2, and passed over vanadium(V) oxide (V2O5) catalyst in a tower.

• Reaction Equation:

  • 2SO2(g) + O2(g) ⇌ 2SO3(g) (Reversible reaction)

• Yield: 98%.

Reaction Conditions

• Temperature: 450°C

• Pressure: 2 atm (200 kPa)

• Catalyst: Vanadium (V) oxide, V2O5

Step 3: Conversion of Sulfur Trioxide to Oleum

• Reaction:

  • SO3(g) + H2SO4(l) → H2S2O7(l) (Oleum formation)

• Important Note:

  • SO3 is NOT added to water directly due to the highly exothermic reaction, which can cause acid mist and explosions.

Step 4: Dilution to Produce Sulfuric Acid

• Reaction:

  • H2S2O7 + H2O → 2H2SO4 (always add oleum to water, not vice versa).

Equilibrium and Reaction Conditions Effects

• Reversible Reaction: 2SO2 + O2 ⇌ 2SO3 + Energy

1. Temperature:

   • Decreasing temperature favors exothermic reaction leading to higher yield, but slower reaction rate.

2. Pressure:

   • Increasing pressure shifts equilibrium right, favoring fewer gaseous moles (higher yield and reaction rate), but involves high costs and explosion risks.

3. Catalyst:

   • Increases reaction rate without affecting yield.

Chemical Reaction Details

Catalysed Reaction Equation

• Equation for Reaction:

  • 2SO2 + O2 ⇌ 2SO3

Effects of Temperature on Equilibrium Mixture

• Increasing Temperature:

  • Decreases percentage of SO3 in the mixture (shifts equilibrium left).

Exothermic vs Endothermic Reactions

• Forward Reaction (2SO2 + O2 ⇌ 2SO3):

  • Exothermic: Heat is released as SO3 is formed.

Temperature Selection in the Contact Process

• 450°C rationale:

  • Achieves a balance between reaction rate (adequate for industrial processing) and yield (not maximizing yield when too high).

Converting Sulfur Trioxide to Concentrated Sulfuric Acid

1. SO3 + H2SO4 → H2S2O7 (formation of oleum).

2. H2S2O7 + H2O → 2H2SO4 (dilution to sulfuric acid).

Discussion Questions (CQD)

CQD 1

1. a. List of raw materials:

   • Sulfur, Oxygen.
     
     b. Conditions:

   • Temperature: 450°C, Pressure: 2 atm, Catalyst: V2O5.
     
     c. Catalysed reaction equation:

   • 2SO2 + O2 ⇌ 2SO3

CQD 2

1. a. i. Effect of temperature on SO3 percentage:

   • Decreases with increased temperature.
     
     ii. Effects of temperature and pressure on equilibrium:

   • Higher temperature shifts left, higher pressure shifts right.
     
     b. Forward reaction is exothermic as heat is released.
     
     c. Temperature of 450°C chosen for balance of yield and rate of reaction.

CQD 3

1. a. Redox Reaction Identification:

   • SO2 + ½ O2 → SO3 is redox.
     
     b. Alternative formation of acid rain equations:

   • SO2 + H2O → H2SO3 (first),

   • H2SO3 + ½ O2 → H2SO4 (second).

CQD 4

1. a. Chemical equation for SO2 and air forming SO3:-
   (2SO2 + O2 ⇌ 2SO3).
   
   b. Conditions: Pressure: 2 atm, Temperature: 450°C.
   
   c. Catalyst used: Vanadium(V) oxide (V2O5).

CQD 5

1. a. Definition of Catalyst:

   • A substance that increases the rate of a reaction without being consumed.

   • Used in Reaction 2: V2O5.
     
     b. Balanced equation for Reaction 2:

   • 2SO2 + O2 ⇌ 2SO3.
     
     c. Balanced equation for Reaction 4:

   • H2S2O7 + H2O → 2H2SO4.
     
     d. Reaction 2 is exothermic:

   • It releases heat, thus producing less SO3 at higher temperatures due to the shift in equilibrium.