# Sulfur and its compounds

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u11-sulfur-and-its-compounds/

This module covers the occurrence, extraction, chemical properties and reactions of sulfur and its key compounds, including sulfur dioxide, sulfur trioxide and sulfuric acid. We also cover the environmental impact of sulfur oxides.

**Prerequisites:** [Oxidation states and redox reactions](https://www.owlsprep.com/study/cie-9701-u02-redox-reactions/); [Le Chatelier's principle and equilibrium](https://www.owlsprep.com/study/cie-9701-u07-equilibria/)

## Learning objectives

- Describe the occurrence and extraction of sulfur via the Frasch process
- Recall the chemical properties of sulfur and common sulfur compounds
- Explain the Contact process for sulfuric acid manufacture using equilibrium principles
- Discuss the environmental impact of sulfur oxides and methods of pollution control

## Occurrence and Extraction of Sulfur

**Frasch Process** — The industrial method used to extract elemental sulfur from underground deposits. Superheated water melts sulfur, which is forced to the surface by compressed air.

*Example:* This method was the main source of sulfur for most of the 20th century.

Sulfur occurs naturally as elemental sulfur around volcanic regions and underground salt domes. It is also found in sulfide ores (e.g. pyrite $FeS_2$) and sulfate minerals (e.g. gypsum $CaSO_4·2H_2O$).

**Worked example:** A student claimed the Frasch process uses cold water to dissolve sulfur for extraction. Identify the error and correct this description.

1. Sulfur is insoluble in water, so it cannot be extracted by dissolution. Solid sulfur also cannot be pumped out of underground deposits.
2. The process uses superheated water (160°C under high pressure) pumped into the deposit, which melts sulfur because sulfur has a low melting point of ~115°C.
3. Compressed air is then pumped in through a second pipe to force the molten sulfur up a third pipe to the surface, where it solidifies on cooling.

## Chemical Properties of Sulfur and Sulfur Oxides

**Allotropy of Sulfur** — Sulfur has two common solid allotropes: rhombic sulfur (stable at room temperature) and monoclinic sulfur (stable above 96°C). Allotropes are different structural forms of the same element in the same physical state.

Sulfur burns in excess oxygen to form only sulfur dioxide. Sulfur has a range of common oxidation states, from -2 in sulfides, +4 in sulfur dioxide and sulfites, to +6 in sulfur trioxide and sulfates.

$$S(s) + O_2(g) \rightarrow SO_2(g)$$

**Worked example:** Write a balanced redox equation for the reaction of sulfur with hot concentrated sulfuric acid. Identify the oxidizing agent.

1. Sulfur is oxidized from 0 to +4, while sulfur in $H_2SO_4$ is reduced from +6 to +4. Write the half equations:
2. $$\text{Oxidation: } S(s) + 2H_2O(l) \rightarrow SO_2(g) + 4H^+(aq) + 4e^-$$
3. $$\text{Reduction: } H_2SO_4(l) + 2H^+(aq) + 2e^- \rightarrow SO_2(g) + 2H_2O(l)$$
4. Multiply the reduction half equation by 2, add to the oxidation equation, and cancel common terms:
5. $$S(s) + 2H_2SO_4(l) \rightarrow 3SO_2(g) + 2H_2O(l)$$
6. Concentrated sulfuric acid is the oxidizing agent, as it accepts electrons and is reduced.

## The Contact Process and Sulfuric Acid Properties

Sulfuric acid is one of the most widely produced industrial chemicals, manufactured via the Contact process. The process has three core stages: 1) combustion of sulfur to make $SO_2$, 2) catalytic oxidation of $SO_2$ to $SO_3$, 3) absorption of $SO_3$ and dilution to concentrated $H_2SO_4$.

**Contact Process** — Industrial process for producing concentrated sulfuric acid, named for the contact of $SO_2$ and oxygen with the solid vanadium(V) oxide catalyst.

**Worked example:** Use Le Chatelier's principle to explain why 450°C is the preferred temperature for the Contact process, rather than 200°C or 800°C.

1. The key equilibrium reaction is:
2. $$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \quad \Delta H = -196 \text{ kJ mol}^{-1}$$
3. The forward reaction is exothermic. At 200°C, equilibrium shifts far to the right, giving a very high yield, but the rate of reaction is too slow for industrial production.
4. At 800°C, the rate is very fast, but equilibrium shifts far to the left, resulting in a very low yield of $SO_3$, which is uneconomic.
5. 450°C is a compromise temperature that gives an acceptable yield (~95% conversion) and a fast enough rate to be profitable.

> **Exam tip:** Examiners always require you to mention both yield and rate when explaining compromise conditions in industrial processes. You will lose marks if you only discuss one.

## Environmental Impact of Sulfur Oxides

Burning sulfur-containing fossil fuels (like coal) for power generation releases large amounts of sulfur dioxide into the atmosphere. This is the main cause of acid rain, which damages forests, aquatic ecosystems, and buildings made of limestone.

**Flue Gas Desulfurization** — A process used to remove sulfur dioxide from power station flue gases before they are released to the atmosphere, reducing acid rain formation.

**Worked example:** Write a balanced equation for the removal of sulfur dioxide by calcium carbonate in flue gas desulfurization, and explain how this works.

1. Flue gases are passed through a slurry of crushed calcium carbonate (limestone). Sulfur dioxide reacts with calcium carbonate to form solid calcium sulfite.
2. $$CaCO_3(s) + SO_2(g) \rightarrow CaSO_3(s) + CO_2(g)$$
3. The solid calcium sulfite is filtered out and disposed of, so the sulfur dioxide cannot escape into the atmosphere to form acid rain. This process removes over 90% of sulfur dioxide from flue gases.

## Common pitfalls

- **Wrong:** Claiming sulfur burns in excess air to form sulfur trioxide directly
  - Why it fails: Burning sulfur only produces sulfur dioxide, even with excess oxygen. Oxidation to sulfur trioxide requires a catalyst.
  - Correct: State that sulfur burns to form $SO_2$, which is then catalytically oxidized to $SO_3$ in the Contact process.
- **Wrong:** Writing that sulfur trioxide is absorbed directly into water in the Contact process
  - Why it fails: The reaction of $SO_3$ with water is highly exothermic, producing an uncontrollable mist of sulfuric acid that cannot be collected.
  - Correct: $SO_3$ is absorbed into concentrated sulfuric acid to form oleum, which is then safely diluted to concentrated sulfuric acid.
- **Wrong:** Forgetting that concentrated sulfuric acid acts as an oxidizing agent, not just an acid
  - Why it fails: Students often only remember sulfuric acid as a strong acid, so miss the redox half reaction when it reacts with metals, halides or other reducing agents.
  - Correct: When concentrated sulfuric acid reacts, check if it is acting as an acid (forming salts) or an oxidizing agent (being reduced to $SO_2$).
- **Wrong:** Assuming all sulfur compounds have an oxidation state of +6
  - Why it fails: Sulfur has a full range of oxidation states from -2 to +6, and students often misidentify oxidation states in sulfides or sulfites.
  - Correct: Always calculate the oxidation state of sulfur from the known oxidation states of other elements in the compound.

## Cheatsheet

| Process/Concept | Key Details | Key Equation |
| --- | --- | --- |
| Frasch Process | Superheated water melts sulfur, compressed air brings to surface | N/A |
| Contact Process Conditions | 450°C, 2 atm, $V_2O_5$ catalyst | $2SO_2 + O_2 \rightleftharpoons 2SO_3 \quad \Delta H=-196$ kJ/mol |
| Acid Rain Formation | $SO_2$ dissolves in water to form sulfurous acid, oxidized to $H_2SO_4$ | $2H_2SO_3 + O_2 \rightarrow 2H_2SO_4$ |
| Flue Gas Desulfurization | Reacts $SO_2$ with $CaCO_3$ to form solid $CaSO_3$ | $CaCO_3 + SO_2 \rightarrow CaSO_3 + CO_2$ |

## What's next

Sulfur and its compounds link to many core topics in CIE A-Level Chemistry, from redox and equilibrium to environmental and industrial inorganic chemistry. The properties of sulfuric acid also connect to organic chemistry, where it acts as a catalyst for esterification and a dehydrating agent in elimination reactions. Mastery of this sub-topic is essential for both multiple-choice and extended response questions, which frequently ask for explanations of Contact process conditions and environmental impacts of sulfur oxides.

- [Nitrogen and its Compounds](https://www.owlsprep.com/study/cie-9701-u11-nitrogen-and-its-compounds/)
- [Introduction to organic chemistry](https://www.owlsprep.com/study/cie-9701-u12-overview/)
- [Nomenclature](https://www.owlsprep.com/study/cie-9701-u12-nomenclature/)

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