Hydrogen Sulphide (H2S)

Natural Occurrence of Hydrogen Sulphide

Hydrogen sulphide is found in several natural sources. Understanding where it comes from helps explain its properties and uses.

1. Volcanic gases: When volcanoes erupt or release gases, H₂S is one of the compounds emitted. This is why volcanic areas often have a sulphur smell.

2. Natural gas and crude petroleum: H₂S is present in natural gas deposits and crude oil. Oil companies must remove it before the gas can be used because it is poisonous and corrosive. This is why gas processing plants exist in places like Port Harcourt and Warri.

3. Coal gas: When coal is heated in the absence of air (destructive distillation), the gases produced contain hydrogen sulphide.

4. Decaying organic matter: When proteins in dead plants and animals decompose, bacteria break down sulphur-containing compounds to release H₂S. This is why you smell rotten eggs in:

• Blocked drains and gutters in Lagos or Kano
• Decomposing fish in markets
• Poorly maintained public toilets
• Swamps and stagnant water

The bacteria responsible for this process are called sulphur-reducing bacteria. They use sulphur compounds in the absence of oxygen to get energy, producing H₂S as waste.

Laboratory Preparation of Hydrogen Sulphide

In the laboratory, hydrogen sulphide is prepared by treating a metal sulphide with dilute acid. The most common method uses iron(II) sulphide and dilute hydrochloric acid.

Method Using Iron(II) Sulphide

Equation:
FeS_(s) + 2HCl_(aq) → FeCl_2(aq) + H₂S_(g)
Iron(II) sulphide + Hydrochloric acid → Iron(II) chloride + Hydrogen sulphide

Alternative with sulphuric acid:
FeS_(s) + H₂SO_4(aq) → FeSO_4(aq) + H₂S_(g)
Iron(II) sulphide + Sulphuric acid → Iron(II) sulphate + Hydrogen sulphide

Other metal sulphides that can be used:

• Zinc sulphide: ZnS_(s) + 2HCl_(aq) → ZnCl_2(aq) + H₂S_(g)
• Calcium sulphide: CaS_(s) + 2HCl_(aq) → CaCl_2(aq) + H₂S_(g)

Why use dilute acid? Concentrated acids would oxidize the hydrogen sulphide as soon as it forms, especially concentrated sulphuric acid or nitric acid. You must use dilute hydrochloric acid or dilute sulphuric acid.

Kipp’s Apparatus

Kipp’s apparatus is specially designed for producing gases that require the reaction of a solid with a liquid. It is used for preparing both H₂S and CO₂ in the laboratory.

Advantages of Kipp’s apparatus:

• Allows controlled production of gas (you can start and stop the reaction)
• Gas is produced when needed, avoiding waste
• The apparatus can be reused many times
• Safer than continuous production in a flask

How it works:

1. Iron(II) sulphide pieces are placed in the middle bulb
2. Dilute HCl is added through the top and flows down to contact the FeS
3. H₂S gas is produced and collected
4. When the tap is closed, pressure builds up and pushes the acid away from the solid, stopping the reaction
5. Opening the tap releases pressure, acid contacts solid again, and gas production continues

Important note for WAEC exams: Kipp’s apparatus is used ONLY for gases that:

• Are produced by solid + liquid reaction
• Are produced at room temperature (no heating needed)
• Examples: H₂S, CO₂, H₂ (from Zn + acid)

Collection of Hydrogen Sulphide

H₂S is collected by different methods depending on what you want to do with it.

1. Downward displacement of air: Since H₂S is slightly heavier than air (relative molecular mass 34 vs air 29), it can be collected by downward displacement. The gas pushes air out of the bottom of the collection jar.

2. Over warm water: H₂S is slightly soluble in cold water but less soluble in warm water. Therefore, it can be collected over warm water to minimize loss due to dissolution.

Why not over mercury? H₂S reacts with mercury to form mercury sulphide, so it cannot be collected over mercury.

Physical Properties

Although there is a separate post on physical properties of H₂S, here are the key ones relevant to its preparation and identification:

• Colorless gas
• Offensive smell of rotten eggs (very distinctive)
• Slightly soluble in water (forms weak acid)
• Denser than air (can be collected by downward displacement)
• Highly poisonous (can cause death at high concentrations)

Comparison Table: Methods of Preparing H₂S

Comparison: H₂S vs CO₂ Production in Kipp’s Apparatus

Safety Precautions

Hydrogen sulphide is a dangerous gas. When preparing it in the laboratory, follow these safety rules:

1. Work in a fume cupboard: H₂S is poisonous. Breathing high concentrations can cause unconsciousness and death. Always prepare it in a well-ventilated area or fume cupboard.
2. Use small quantities: Only prepare the amount you need for your experiment.
3. No naked flames: H₂S is flammable and burns in air to form SO₂ and water.
4. Proper disposal: Unused H₂S should be bubbled through sodium hydroxide solution to neutralize it, not released into the air.
5. Emergency response: If someone inhales too much H₂S, move them to fresh air immediately and get medical help.

Common Exam Mistakes

Based on WAEC Chief Examiner reports, students commonly make these errors:

1. Wrong acid concentration: Writing “concentrated HCl” instead of “dilute HCl”. Concentrated acids will oxidize H₂S, so you must use dilute acid.
2. Wrong metal sulphide: Writing sodium sulphide or potassium sulphide. These are soluble in water and will not work well in Kipp’s apparatus. Use insoluble sulphides like FeS, ZnS, or CaS.
3. Missing state symbols: Not including (s), (g), (aq) in equations. Always show states.
4. Confusing apparatus: Saying “Kipp’s apparatus is used to prepare oxygen or nitrogen.” No! It is only for gases produced from solid + liquid at room temperature (H₂S, CO₂, H₂).
5. Wrong collection method: Saying “collect over mercury.” H₂S reacts with mercury, so this is wrong.
6. Incomplete equation balancing: Writing “FeS + HCl → FeCl₂ + H₂S” without the coefficient 2 before HCl.
7. Confusing natural occurrence with preparation: Describing natural occurrence when asked about laboratory preparation, or vice versa. Read the question carefully.

Practice Questions

Multiple Choice Questions

1. Hydrogen sulphide is prepared in the laboratory by the action of:
a) Concentrated HCl on FeS
b) Dilute HCl on FeS ✓
c) Concentrated H₂SO₄ on FeS
d) Dilute HNO₃ on FeS

2. The apparatus used for the controlled production of hydrogen sulphide is:
a) Liebig condenser
b) Kipp’s apparatus ✓
c) Desiccator
d) Separating funnel

3. Hydrogen sulphide gas can be collected by:
a) Upward displacement of air
b) Over mercury
c) Downward displacement of air ✓
d) Over concentrated H₂SO₄

4. The smell of hydrogen sulphide resembles:
a) Vinegar
b) Ammonia
c) Chlorine
d) Rotten eggs ✓

5. Which of the following is NOT a natural source of H₂S?
a) Volcanic gases
b) Decaying proteins
c) Photosynthesis ✓
d) Crude petroleum

Essay/Theory Questions

1. (a) Write a balanced equation for the laboratory preparation of hydrogen sulphide. (3 marks)
(b) State two precautions that must be observed during the preparation. (2 marks)
(c) How is the gas collected? Give a reason for your answer. (3 marks)

Examiner’s Tip: For (a), include state symbols. For (b), mention safety (fume cupboard) and use of dilute acid. For (c), explain why the method works (density or solubility).

2. (a) Draw a labeled diagram of Kipp’s apparatus used for preparing hydrogen sulphide. (5 marks)
(b) Explain how the apparatus allows controlled production of the gas. (3 marks)
(c) Name one other gas that can be prepared using this apparatus. (1 mark)

Examiner’s Tip: Your diagram must show all three bulbs, the tap, delivery tube, and labels for FeS position and acid entry point. For (b), describe what happens when tap is opened vs closed.

3. (a) State three natural sources of hydrogen sulphide. (3 marks)
(b) Explain why hydrogen sulphide is described as a poisonous gas. (2 marks)
(c) Why is dilute acid used instead of concentrated acid in the preparation of H₂S? (2 marks)

Examiner’s Tip: For (a), give specific sources like volcanic gases, not just “natural sources.” For (c), explain that concentrated acid oxidizes H₂S.

4. A student prepared hydrogen sulphide by adding dilute hydrochloric acid to iron(II) sulphide in a conical flask.
(a) Write the equation for the reaction. (3 marks)
(b) State two observations the student would make. (2 marks)
(c) Suggest one improvement to make the preparation safer. (2 marks)

Examiner’s Tip: For (b), mention the smell (rotten eggs) and appearance (colorless gas with bubbles). For (c), suggest using Kipp’s apparatus or working in fume cupboard.

Memory Aids

Remember the equation – “2 Fans”:
FeS + 2HCl → FeCl₂ + H₂S
The number 2 appears twice in the equation

Kipp’s apparatus gases – “H-H-C”:

• Hydrogen sulphide (FeS + acid)
• Hydrogen (Zn + acid)
• Carbon dioxide (marble + acid)

Natural sources – “V-D-C-P”:

• V – Volcanic gases
• D – Decaying organic matter
• C – Coal gas
• P – Petroleum and natural gas

Why dilute acid? Remember: “Concentrated acid is too STRONG – it will DESTROY the H₂S by oxidizing it”

Collection method: “H₂S is HEAVIER than air (34 > 29), so it goes DOWN”

Related Topics

• Physical Properties of H₂S (ID: 3308)
• Chemical Properties of H₂S (ID: 3310)
• Test for H₂S (ID: 3312)
• Uses of Hydrogen Sulphide (ID: 3314)
• Chemical Properties of Sulphur (ID: 3302)