CHEM1910 Inorganic lecture 3 Hydrogen Part 2 (4)

CHEM1910 Block 1: Chemistry of the Main Group Elements

Lecture Overview

Date: January 30th, 2025Instructor: Dr. Nickeisha StephensonFocus: Hydrogen and its properties.

Topics Covered in the Lecture

Element Hydrogen

• Placement in the Periodic Table: Hydrogen is positioned at the top of Group 1, although it is a nonmetal, and displays unique behavior compared to alkali metals.

Physical Properties

• Hydrogen exists as a diatomic gas (H2) under standard conditions.

• Isotopes: Hydrogen has three isotopes: Protium (¹H), Deuterium (²H), and Tritium (³H), each differing in neutron count, influencing their physical properties.

Uses of Hydrogen

• Hydrogen's applications range from being a fuel source in hydrogen fuel cells to its role in chemical synthesis, ammonia production via the Haber process, and as a reducing agent in metallurgy.

Industrial Production

• Methods Used:

  • Electrolysis: Splitting water into hydrogen and oxygen using electric current.

  • Reactions of Dilute Acids with Metals: Reaction with metals such as zinc or magnesium releases hydrogen gas.

Hydrogen Economy

• Hydrogen is pivotal to future energy systems due to its potential to replace fossil fuels, reduce greenhouse gas emissions, and provide clean energy through fuel cells.

Compounds of Hydrogen

• The chemistry of hydrogen compounds, particularly with nonmetals in periods 2 and 3, showcases varying bonding modes and reactivity.

Hydrogen Bonding

• Understanding hydrogen bonding is crucial as it significantly influences the physical properties of compounds like water and organic molecules (e.g., alcohols).

Reactivity of Hydrogen

Reactants and Reactions

• Hydrogen Reactivity with Various Elements:

  • Group 1 Metals: 2 M(s) + H2(g) → 2 MH(s)

  • Group 2 Metals (excluding Be or Mg): M(s) + H2(g) → MH2(s)

  • D-Block Metals: 2 M(s) + H2(g) → 2 MH(s)

  • With Oxygen: O2(g) + 2 H2(g) → 2 H2O(l)

  • With Nitrogen: N2(g) + 3 H2(g) → 2 NH3(g)

  • Halogens: X2(g,l,s) + H2(g) → 2 HX(g)

Compounds of Hydrogen - Binary Hydrides

• Formation: Hydrogen readily combines with most periodic table elements to form binary hydrides, significant in various chemical applications.

• Definition: Any compound of hydrogen with one other element is termed a binary hydride, showcasing diverse properties based on the electron configuration of the accompanying element.

Electronegativity

• Importance: Understanding electronegativity helps explain bond characteristics in hydride chemistry, with hydrogen’s electronegativity value noted as cP = 2.20, defining its bonding nature.

Bonding in Binary Hydrides

• Electronegativity and Bond Types:

  • Comparison of expected vs. actual bond types for Period 2 elements reveals the transition from polar covalent to ionic characteristics.

Characteristics of Binary Hydrides

Ionic vs. Covalent Hydrides

• Hydride Classifications:

  • Saline/Ionic Hydrides: Formed with group 1 and 2 metals, displaying characteristic ionic properties.

  • Covalent Hydrides: Primarily formed with p-block elements, exhibiting covalent bonds.

• Protic vs. Hydridic:

  • Protic when bonded to electronegative elements (e.g., H-F).

  • Hydridic when bonded to electropositive elements (e.g., LiH).

Stability and Formation

• Thermodynamic Favorability: A negative Gibbs Free Energy (∆fG°) indicates the feasibility and stability of hydride formation, impacting its practical applications.

• Bond Strength: The stability of E–H bonds is critical; stronger bonds often result from favorable atomic properties across different elements.

Specific Binary Hydrides of Period Two

• List of Binary Hydrides: Includes LiH, BeH2, B2H6, HF, CH4, NH3, H2O, highlighting diversity in bonding and properties.

• Properties of Saline Hydrides: Salt-like, with high melting points, often white crystalline solids.

  • Formation: Typically arise from reactions involving hydrogen and metal elements, frequently conducted at elevated temperatures.

Ionic Hydrides

• Properties and Reactivity: Highly reactive, commonly stored under oil to prevent rapid oxidation; react with water, generating H2 gas.

• Ionic Character: The ionic character tends to increase down the group in the periodic table, with these hydrides often acting as effective reducing agents.

Covalent Hydrides Classification

• Classification based on hydrogen's role as H+, H-, and H in complex structures.

• Examples: Encompasses boron compounds and ammonium derivatives, demonstrating the variety of hydrogen's covalent interactions.

Summary of Strong Covalent Bonds

• Hydrogen Bonds:

  • Essential for forming strong covalent bonds, competing with electronegative elements like oxygen and fluorine.

  • Hydrogen’s unique ability to form stable molecular orbitals leads to complex interactions in various compounds.

Conclusions on Hydrogen Compounds

• The complexity of bonding in diborane exemplifies unique properties of boron hydrides.

• Use of Lewis Structures: Vital for visualizing structures and bonding in hydrides, aiding in understanding their reactivity and interactions.