Chemistry · Class 11 · Hydrogen · Term 3

Hydrides: Ionic, Covalent, and Metallic

Classify the binary compounds of hydrogen, known as hydrides, into ionic (saline), covalent (molecular), and metallic (interstitial) types based on their bonding and properties.

TL;DR:Let's explore the dual personality of hydrogen! We will investigate how this simple element forms three completely different types of compounds, called hydrides, depending on its partner from the periodic table.

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

About This Topic

This topic, 'Hydrides: Ionic, Covalent, and Metallic', is a fundamental part of the 'Hydrogen' chapter in the Class 11 NCERT syllabus. It builds directly upon students' prior knowledge of chemical bonding and the periodic table. The core concept is to understand that hydrogen, despite its simple structure, exhibits remarkable versatility in its bonding behaviour. The classification of its binary compounds, hydrides, is based on the electronegativity of the element it combines with. This framework helps in predicting the properties of these compounds systematically.

For the Indian curriculum, it is crucial to connect these classifications to the position of elements in the periodic table. Ionic or saline hydrides are formed with highly electropositive s-block elements, leading to salt-like properties. Covalent or molecular hydrides are formed with p-block elements, resulting in discrete molecules whose properties are governed by intermolecular forces like van der Waals forces and hydrogen bonding. The anomalous boiling point of water is a classic example emphasised in exams. Metallic or interstitial hydrides, formed with d- and f-block elements, introduce the advanced concept of non-stoichiometric compounds, which is important for understanding materials science and catalysis. This topic serves as a bridge, connecting basic bonding principles to more complex concepts of solid-state chemistry and reaction mechanisms.

Key Questions

1. Compare the properties of ionic hydrides like NaH and covalent hydrides like CH4.
2. Explain why metallic hydrides are often non-stoichiometric.
3. Analyse the trend in the boiling points of Group 16 hydrides (H2O, H2S, H2Se).

Learning Objectives

• Classify binary hydrides as ionic, covalent, or metallic based on the periodic position of the other element.
• Compare the characteristic physical and chemical properties of the three main types of hydrides.
• Explain the anomalously high boiling points of NH3, H2O, and HF by describing intermolecular hydrogen bonding.
• Define and provide an example of a non-stoichiometric hydride.
• Write balanced equations for the reactions of different types of hydrides, particularly their reaction with water.

Key Vocabulary

Hydride	A binary compound containing hydrogen and another element.
Ionic (Saline) Hydride	A salt-like hydride formed with s-block elements, containing the hydride ion (H-).
Covalent (Molecular) Hydride	A hydride consisting of discrete molecules, formed between hydrogen and p-block elements.
Metallic (Interstitial) Hydride	A compound formed when hydrogen atoms occupy the spaces within the crystal lattice of d- and f-block metals.
Non-stoichiometric Compound	A chemical compound in which the ratio of the elements cannot be represented by small whole numbers.

Watch Out for These Misconceptions

Common Misconception

All compounds with hydrogen are acids.

What to Teach Instead

Hydrogen's chemical nature depends on the element it is bonded to. In ionic hydrides like NaH, hydrogen exists as the hydride ion (H-), which is a strong base and reacts with water to produce H2 gas and a basic solution.

Common Misconception

Boiling points of hydrides must always increase down the group.

What to Teach Instead

While boiling points generally increase down a group due to stronger van der Waals forces from more electrons, the hydrides of N, O, and F (NH3, H2O, HF) have exceptionally high boiling points for their size. This is due to strong intermolecular hydrogen bonding, which is much stronger than the van der Waals forces in the hydrides below them.

Common Misconception

Metallic hydrides are just simple mixtures of a metal and hydrogen.

What to Teach Instead

Metallic hydrides are interstitial compounds where hydrogen atoms occupy the empty spaces (interstices) in a metal's crystal lattice. They are not true compounds with fixed ratios, which is why they are often non-stoichiometric, like PdH0.6, and can alter the properties of the parent metal.

Active Learning Ideas

Concept Mapping

Hydride Property Sorting Challenge

Students in small groups receive cards with names of hydrides (e.g., LiH, CH4, H2S, TiH1.7) and property descriptions (e.g., 'gas at room temperature', 'conducts electricity in molten state'). They must match the properties to the correct hydride and then sort them into the three categories: ionic, covalent, and metallic.

20 min·Small Groups

Concept Mapping

Plot the Boiling Point Anomaly

Provide students with a data table of the boiling points for Group 15 and 16 hydrides. In pairs, they plot this data on a graph. The visual anomaly of NH3 and H2O leads to a guided discussion about hydrogen bonding.

25 min·Pairs

Concept Mapping

Predict the Reaction

Present students with reaction scenarios, such as 'What happens when CaH2 is added to water?' or 'What happens when methane is burned in air?'. Students predict the products and write balanced chemical equations, reinforcing the chemical properties of different hydride types.

15 min·Individual

Real-World Connections

• Metallic hydrides (e.g., LaNi5H6) are studied for safe and efficient hydrogen storage for use in hydrogen fuel cell cars.
• Ammonia (NH3), a covalent hydride, is the foundation of the fertiliser industry, crucial for feeding the global population.
• Methane (CH4), the simplest covalent hydride, is the primary component of natural gas and CNG used for cooking and transport.
• Complex hydrides like sodium borohydride (NaBH4) are essential reducing agents used in the pharmaceutical industry to synthesise medicines.
• Water (H2O), the most vital covalent hydride, is the universal solvent and the basis for all life on Earth.

Assessment Ideas

Exit Ticket

Use an 'exit slip' where students must classify LiH, SiH4, and VH0.56 into their respective hydride types and list one defining property for each.

Quick Check

A section in a unit test could ask students to draw and explain a graph of the boiling points of Group 16 hydrides, and to write the reaction for calcium hydride with water, identifying the products.

Quick Check

Students complete a 'Know-Want to know-Learned' (KWL) chart for the topic of hydrides before and after the lesson to reflect on their learning journey.

Frequently Asked Questions

Why are ionic hydrides also called saline hydrides?

They are called 'saline' because it means 'salt-like'. They share many properties with common salts like NaCl, such as having a crystalline solid structure, high melting and boiling points, and conducting electricity when molten.

What is the difference between an electron-rich and an electron-deficient covalent hydride?

An electron-rich hydride has enough valence electrons to form normal covalent bonds and also has at least one lone pair of electrons on the central atom (e.g., NH3, H2O). An electron-deficient hydride does not have enough valence electrons to form the expected number of normal covalent bonds, leading to unique structures like the 'banana bonds' in diborane (B2H6).

Can metallic hydrides be used for anything practical?

Yes, they are very important. Their ability to absorb and release hydrogen makes them excellent candidates for hydrogen storage materials, which could be used to power fuel-cell vehicles in the future. They are also used as catalysts in many industrial chemical reactions.