Laboratory Preparation of Hydrogen

Principle of Preparation

The laboratory preparation of hydrogen is based on a simple chemical principle: reactive metals displace hydrogen from dilute acids. Metals that are more reactive than hydrogen can push hydrogen out of acid solutions.

The general equation for this reaction is:

Metal + Dilute acid → Salt + Hydrogen gas

For the reaction to work, the metal must be above hydrogen in the electrochemical series (also called the reactivity series or activity series). Metals like zinc, magnesium, iron, and aluminum work well. Metals below hydrogen like copper, silver, and gold cannot displace hydrogen from acids.

Choice of Materials

Selecting the Metal

Not all metals are suitable for preparing hydrogen in the laboratory. The best metal is zinc because:

• It reacts at a moderate, controllable rate – not too fast, not too slow
• It is readily available and relatively cheap in Nigerian markets
• It produces pure hydrogen without side reactions
• The reaction is easy to start and stop

Magnesium reacts too vigorously with acids. The reaction is explosive and difficult to control in a school laboratory. However, magnesium can be used if you need hydrogen quickly.

Iron reacts too slowly with dilute acids at room temperature. You would wait a long time to collect enough hydrogen.

Aluminum has an oxide coating that prevents it from reacting readily with acids. Though aluminum is above hydrogen in the reactivity series, it needs special treatment.

Selecting the Acid

The choice of acid is also important. Use only dilute hydrochloric acid (HCl) or dilute sulfuric acid (H₂SO₄).

Why not concentrated acids? Concentrated sulfuric acid is a strong oxidizing agent. It oxidizes hydrogen to water instead of releasing it as a gas. Concentrated hydrochloric acid releases hydrogen chloride gas mixed with hydrogen, giving an impure product.

Why not nitric acid? Nitric acid (HNO₃) is an oxidizing acid, even when dilute. Instead of producing hydrogen, it produces nitrogen oxides (NO, NO₂) and water. The reaction is:

3Zn + 8HNO₃(dilute) → 3Zn(NO₃)₂ + 4H₂O + 2NO

Notice no hydrogen appears in this equation!

Laboratory Setup and Procedure

Apparatus Required

• Conical flask (flat-bottomed flask)
• Thistle funnel or dropping funnel
• Delivery tube
• Trough filled with water
• Gas jar or test tube for collection
• Retort stand and clamp
• Zinc granules
• Dilute hydrochloric acid or dilute sulfuric acid

Step-by-Step Procedure

Step 1: Set up the apparatus. Place zinc granules in the conical flask. Insert the thistle funnel through the cork stopper. The stem of the thistle funnel should reach below the level of zinc to prevent hydrogen from escaping through the funnel.

Step 2: Connect the delivery tube from the flask to the collection jar. The collection jar should be filled with water and inverted in a water trough.

Step 3: Pour dilute hydrochloric acid through the thistle funnel into the flask. The acid contacts the zinc, and the reaction starts immediately.

Step 4: Allow the first few bubbles of gas to escape. This removes air from the apparatus. After about 30 seconds, start collecting the gas.

Step 5: The hydrogen gas displaces water in the collection jar. When the jar is full of gas, remove it while still inverted to prevent hydrogen from escaping.

Step 6: Test the collected gas with a lighted splint. Pure hydrogen burns with a “pop” sound.

Chemical Equations

The specific equations depend on which acid you use:

With Hydrochloric Acid

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

With Sulfuric Acid

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

Fe(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂(g)

In all cases, the acid provides H⁺ ions, and the metal provides electrons. The H⁺ ions gain electrons to become H₂ gas.

Methods of Gas Collection

Collection Over Water (Most Common)

This is the standard method in Nigerian secondary schools. The gas jar is filled with water and inverted in a trough. As hydrogen bubbles through the delivery tube, it rises and displaces the water downward. When all water is displaced, the jar is full of hydrogen.

Advantage: Easy to see when collection is complete. The setup is simple.

Disadvantage: The hydrogen collected is slightly moist because water vapor mixes with it. For most school experiments, this doesn’t matter.

Downward Displacement of Air

Since hydrogen is lighter than air, it can displace air downward. Hold a dry gas jar upside down over the delivery tube. Hydrogen fills the jar from top to bottom, pushing air out.

Advantage: Produces dry hydrogen gas.

Disadvantage: You cannot see when the jar is full. You must estimate the time based on the rate of gas production.

Upward Delivery

Point the delivery tube upward into a gas jar held right-side up. Hydrogen rises into the jar.

Why this works: Hydrogen is less dense than air (density = 0.09 g/dm³ compared to air at 1.29 g/dm³). This is rarely used because it is less efficient than the other methods.

Industrial Production Methods

While the laboratory method works for small amounts, industry needs large quantities of hydrogen. Two main industrial methods exist:

Bosch Process (Steam-Iron Process)

This process passes steam over red-hot iron at about 700°C:

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

The iron oxide formed is reduced back to iron by passing carbon monoxide over it:

Fe₃O₄(s) + 4CO(g) → 3Fe(s) + 4CO₂(g)

The iron can be reused repeatedly. This process is economical for large-scale production.

Electrolysis of Water

Passing electric current through acidified water splits it into hydrogen and oxygen:

2H₂O(l) → 2H₂(g) + O₂(g)

Hydrogen collects at the negative electrode (cathode) and oxygen at the positive electrode (anode). The volume of hydrogen is twice the volume of oxygen – this proves water is H₂O.

This method produces very pure hydrogen but requires a lot of electricity. Companies use it when they need extremely pure hydrogen for special purposes, like making computer chips.

Steam Reforming (Petrochemical Method)

Natural gas (methane) reacts with steam at high temperature over a nickel catalyst:

CH₄(g) + H₂O(g) → CO(g) + 3H₂(g)

This is the cheapest industrial method and produces about 95% of commercial hydrogen worldwide. Oil companies in Nigeria could use this method.

Drying Hydrogen Gas

Hydrogen collected over water contains water vapor. To dry it, pass the gas through:

• Fused calcium chloride (CaCl₂): This absorbs water vapor efficiently
• Concentrated sulfuric acid: This removes moisture but must be used carefully
• Silica gel: A safe, reusable drying agent

Never use alkaline drying agents like quicklime (CaO) or sodium hydroxide (NaOH) for hydrogen. These substances do not react with hydrogen, but they are harder to handle safely.

Common Exam Mistakes

WAEC examiners report these frequent errors:

• Using wrong metals: Students write “copper and dilute acid produce hydrogen.” Wrong! Copper is below hydrogen in the reactivity series and cannot displace it from acids.
• Using nitric acid: Many candidates write equations showing hydrogen from nitric acid. This never happens. Nitric acid produces nitrogen oxides, not hydrogen.
• Forgetting to balance equations: Writing Zn + HCl → ZnCl₂ + H₂ is wrong. The correct equation is Zn + 2HCl → ZnCl₂ + H₂
• Wrong collection method: Stating “hydrogen is collected over mercury” or other impossible methods. Hydrogen is collected over water or by downward displacement of air only.
• Confusing concentrated and dilute acids: Not stating “dilute” when writing about the acid. You must specify dilute HCl or dilute H₂SO₄.
• Poor diagram drawing: Forgetting to show the thistle funnel stem below the zinc level. If the stem is above the zinc, hydrogen escapes through the funnel!
• Incomplete procedure: Not mentioning that first bubbles should be discarded to remove air from the apparatus.

Practice Questions

Multiple Choice Questions

1. Which of the following metals is most suitable for laboratory preparation of hydrogen?
a) Copper
b) Zinc ✓
c) Silver
d) Gold

2. Hydrogen cannot be prepared using dilute nitric acid because:
a) Nitric acid is too weak
b) Nitric acid is an oxidizing agent ✓
c) Nitric acid does not dissolve in water
d) Nitric acid is too expensive

3. In the laboratory preparation of hydrogen, the thistle funnel stem must:
a) Be above the level of zinc
b) Reach below the level of zinc ✓
c) Touch the bottom of the flask
d) Be removed after adding acid

4. Hydrogen is collected over water because it is:
a) Soluble in water
b) Denser than water
c) Only slightly soluble in water ✓
d) Heavier than air

5. The industrial Bosch process produces hydrogen by:
a) Heating zinc with acid
b) Passing steam over red-hot iron ✓
c) Electrolyzing brine
d) Heating hydrogen peroxide

Essay/Theory Questions

1. (a) Draw a labeled diagram of the apparatus used for laboratory preparation of hydrogen. (5 marks)
(b) Write a balanced equation for the reaction between zinc and dilute sulfuric acid. (2 marks)
(c) Describe how you would test the gas produced. (2 marks)

Tips: For (a), show conical flask, thistle funnel (stem below zinc level), delivery tube, water trough, inverted gas jar. Label all parts. For (b): Zn + H₂SO₄ → ZnSO₄ + H₂. For (c): bring a lighted splint to the mouth of the jar – it burns with a pop sound.

2. (a) Explain why nitric acid is not used in the preparation of hydrogen. (3 marks)
(b) Why is zinc preferred to magnesium for preparing hydrogen in the laboratory? (3 marks)
(c) State two methods of collecting hydrogen gas. (2 marks)

Tips: For (a), explain that nitric acid is an oxidizing agent that oxidizes hydrogen to water and produces nitrogen oxides instead. For (b), mention that magnesium reacts too vigorously and is difficult to control. For (c): collection over water and downward displacement of air.

3. (a) Describe briefly how hydrogen is produced industrially by the Bosch process. (4 marks)
(b) Write equations for the reactions involved in the Bosch process. (4 marks)
(c) Why is the Bosch process more economical than electrolysis for large-scale production? (2 marks)

Tips: For (a), mention passing steam over red-hot iron at 700°C, formation of iron oxide, regeneration of iron using carbon monoxide. For (b), write both equations clearly. For (c), explain that electrolysis requires expensive electricity while Bosch process reuses iron repeatedly.

4. (a) State three precautions to observe when preparing hydrogen in the laboratory. (3 marks)
(b) Explain why the first few bubbles of gas are not collected. (2 marks)
(c) How would you obtain dry hydrogen from hydrogen collected over water? (2 marks)

Tips: For (a): ensure no naked flames nearby, thistle funnel stem below zinc level, use dilute acids only. For (b): to remove air from the apparatus. For (c): pass through fused calcium chloride or concentrated sulfuric acid or silica gel.

Memory Aids

For suitable metals: “MZIA Can’t” – Magnesium (too fast), Zinc (just right), Iron (too slow), Aluminum (oxide coating), Copper (Can’t react)

For acids to avoid: “Never Concentrated Nitric” – Never use Concentrated acids or Nitric acid

For thistle funnel position: “Funnel Goes Deep Below Zinc” – Stem must go below zinc level

For collection methods: “Water DOWN” – collected over Water or by DOWNward displacement of air (because hydrogen is lighter)

For the reaction: “Metal Acid → Salt Hydrogen” – General equation pattern

For industrial methods: “BSE” – Bosch (steam + iron), Steam reforming (methane), Electrolysis (water)

Related Topics

• Hydrogen (Properties and Uses)
• The Reactivity Series of Metals
• Acids and Their Properties
• Oxidation and Reduction
• Collection of Gases in the Laboratory