Chemistry · Class 11 · Hydrogen · Term 3

Preparation of Dihydrogen (H2)

Learn the various methods for preparing dihydrogen gas, including laboratory methods like the reaction of metals with acids and commercial methods like the electrolysis of water.

TL;DR:Let's unlock the power of the universe's most common element! Today, we will explore how we can produce dihydrogen gas, a clean fuel of the future, right here in our lab and on a massive industrial scale.

• CBSE Learning Outcomes
• NCERT Class 11 Chemistry: Unit 9 - Hydrogen

About This Topic

This topic, 'Preparation of Dihydrogen (H2)', is a fundamental part of the 'Hydrogen' chapter in the Class 11 NCERT chemistry syllabus. It builds upon students' prior knowledge of chemical reactions and the reactivity series from Class 10. The core focus is on understanding the principles behind isolating the most abundant element in the universe from its combined state, primarily from water and hydrocarbons. The curriculum requires a clear distinction between small-scale laboratory methods and large-scale commercial production, emphasising the different requirements for purity, cost, and volume in each context.

For the Indian context, it is crucial to connect these preparation methods to their real-world applications within the country. This includes the production of ammonia for fertilisers via the Haber-Bosch process, which is vital for our agricultural economy, and the hydrogenation of vegetable oils. Furthermore, introducing the concept of 'green hydrogen' production via electrolysis powered by renewable energy directly links the topic to India's National Hydrogen Mission and its ambitious goals for a sustainable energy future. The topic thus serves as a bridge between foundational chemical principles like redox reactions and electrolysis, and their large-scale industrial and environmental significance.

Key Questions

1. Explain the principle behind the laboratory preparation of dihydrogen from zinc and dilute acid.
2. Compare the industrial production of dihydrogen by electrolysis of water and from hydrocarbons.
3. Analyse the Bosch process for the commercial production of hydrogen.

Learning Objectives

• Describe the laboratory preparation of dihydrogen from metals and acids with a labelled diagram.
• Explain the commercial production of dihydrogen by electrolysis, steam reforming, and the Bosch process.
• Write balanced chemical equations for the key reactions involved in dihydrogen preparation.
• Compare the different methods of preparation based on cost, purity of the product, and environmental impact.
• Analyse the significance of dihydrogen as an industrial chemical and a future fuel source.

Key Vocabulary

Dihydrogen	The diatomic molecule, H2, which is the most common molecular form of the element hydrogen.
Electrolysis	The process of decomposing a chemical compound, particularly an ionic compound in a fused state or in solution, by passing a direct electric current through it.
Steam Reforming	An industrial process for producing hydrogen from hydrocarbons, especially natural gas, by reaction with steam over a nickel catalyst at high temperatures.
Water-gas shift reaction	A chemical reaction in which carbon monoxide reacts with water vapour to form carbon dioxide and hydrogen. It is a key step in the Bosch process.
Reactivity Series	A series in which metals are arranged in order of their reactivity, which determines their ability to displace hydrogen from acids or water.

Watch Out for These Misconceptions

Common Misconception

Any metal will react with any acid to produce hydrogen gas.

What to Teach Instead

Only metals that are more reactive than hydrogen (placed above it in the electrochemical series) can displace it from dilute non-oxidising acids like HCl and H2SO4. Less reactive metals like copper, silver, and gold will not produce hydrogen.

Common Misconception

Hydrogen is readily available in the air for us to collect.

What to Teach Instead

Although hydrogen is the most abundant element in the universe, it is extremely light and rare in its free gaseous form in Earth's atmosphere. It is almost always found in compounds like water (H2O) and hydrocarbons, from which it must be extracted.

Common Misconception

Electrolysis of water is the main way hydrogen is produced commercially because it is so clean.

What to Teach Instead

While electrolysis produces very pure hydrogen, it is an energy-intensive and expensive process. Currently, over 95% of commercial hydrogen is produced by steam reforming of natural gas, which is more economical but produces carbon dioxide as a by-product.

Active Learning Ideas

Experiential Learning

The Classic 'Pop' Test Demonstration

The teacher demonstrates the reaction of granulated zinc with dilute sulphuric acid in a flask, collecting the evolved gas over water. The collected gas is then tested with a burning splint to produce the characteristic 'pop' sound, confirming the presence of hydrogen.

20 min·Whole Class

Experiential Learning

DIY Water Electrolysis

In small groups, students build a simple electrolyser using a 9V battery, two graphite pencil leads as electrodes, and a beaker of salt water. They observe bubbles forming at each electrode and can test the collected gases.

30 min·Small Groups

Experiential Learning

Industrial Process Flowchart Challenge

Students work in pairs to create a comparative flowchart for two commercial methods: steam reforming of methane and the Bosch process. The flowchart should highlight reactants, catalysts, temperatures, and by-products for each process.

35 min·Pairs

Real-World Connections

• Production of ammonia for fertilisers using the Haber-Bosch process, which is critical for India's food security.
• Hydrogenation of vegetable oils to manufacture vanaspati ghee and margarine.
• Used as a high-energy, clean rocket fuel by organisations like ISRO for satellite launches.
• Potential use in fuel cell electric vehicles (FCEVs) as a clean alternative to petrol and diesel, aligning with the goals of India's National Hydrogen Mission.
• In metallurgy, it is used as a reducing agent to extract pure metals like tungsten from their oxides.

Assessment Ideas

Exit Ticket

An 'exit ticket' activity where students must write down one laboratory method and one commercial method for preparing H2, including the main reactants for each.

Quick Check

A section in the unit test with short answer questions requiring students to write balanced equations and compare the economic and environmental aspects of electrolysis versus steam reforming.

Quick Check

Students use a 'traffic light' system (red, yellow, green) to rate their confidence in explaining each of the preparation methods discussed in class.

Frequently Asked Questions

Why is granulated zinc used in the lab instead of a solid block of zinc?

Granulated zinc has a much larger surface area compared to a solid block of the same mass. This increases the rate of reaction with the acid, allowing for a faster and more consistent production of hydrogen gas.

What is the difference between 'green', 'blue', and 'grey' hydrogen?

These terms describe the method of production. 'Grey' hydrogen is made from fossil fuels (like natural gas), releasing CO2. 'Blue' hydrogen is also from fossil fuels, but the CO2 is captured. 'Green' hydrogen is made by electrolysis of water using renewable energy (like solar or wind), making it the most environmentally friendly.

Why can't we use concentrated nitric acid to prepare hydrogen from metals?

Concentrated nitric acid is a powerful oxidising agent. Instead of liberating hydrogen gas, it oxidises the hydrogen produced to form water (H2O), while the nitric acid itself gets reduced to nitrogen oxides.