LỰC LIÊN kết PHÂN tử, TRẠNG THÁI NGƯNG kết của vật CHẤT (hóa đại CƯƠNG)

Intermolecular Forces, Liquids, and Solids Lực liên kết liên phân tử Trạng thái ngưng kết vật chất Copyright 1999, Chapter and 10 Liquids and Solids: A Molecular Comparison Tính chất vật lý chất hiểu theo khái niệm thuyết động học phân tử (kinetic molecular theory) sau: • Chất khí dễ bị nén, có hình dạng thể tích vật chứa : • Các phân tử chất khí cách xa nhau, khơng tương tác với • Chất lỏng khơng bị nén, có hình dạng (nhưng khơng tích) vật chứa : • Các phân tử chất lỏng giữ gần phân tử chất khí, không đến mức chúng trượt qua (slide past each other) • Chất rắn khơng bị nén, có hình dạng thể tích xác định • Các phân tử chất rắn giữ gần Các phân tử chất rắn đến mức chúng trượt qua Copyright 1999, Chapter and 10 A Molecular Comparison of Liquids and Solids Copyright 1999, Chapter and 10 A Molecular Comparison of Liquids and Solids • Converting a gas into a liquid or solid requires the molecules to get closer to each other/ Để biến đổi chất khí thành lỏng hay rắn: – cool or compress/ làm lạnh hay nén • Converting a solid into a liquid or gas requires the molecules to move further apart:/Để biến đổi chất rắn thành lỏng hay khí: – heat or reduce pressure/ gia nhiệt hay giảm áp suất • The forces holding solids and liquids together are called intermolecular forces ( lực giữ chất lỏng rắn lai với gọi lực liên phân tử) Copyright 1999, Chapter and 10 Intermolecular Forces • The covalent bond holding a molecule together is an intramolecular forces (lục nội phân tử) • The attraction between molecules is an intermolecular force (lực liên phân tử) • Intermolecular forces are much weaker than intramolecular forces (e.g 16 kJ/mol vs 431 kJ/mol for HCl) • When a substance melts (nóng chảy) or boils (sơi) the intermolecular forces are broken (not the covalent bonds) • When a substance condenses (ngưng tụ) intermolecular forces are formed Copyright 1999, Chapter and 10 Intermolecular Forces Copyright 1999, Chapter and 10 Intermolecular Forces Ion-Dipole Forces (lực ion-lưỡng cực) • Interaction between an ion (e.g Na+) and a dipole (e.g water) • Strongest of all intermolecular forces: F =k Q1Q2 d2 – Since Q1 is a full charge and Q2 is a partial charge, F is comparatively large • F increases as Q increases and as d decreases: – the larger the charge and smaller the ion, the larger the ion-dipole attraction Copyright 1999, Chapter and 10 Intermolecular Forces Copyright 1999, Chapter and 10 Intermolecular Forces Dipole-Dipole Forces (lực lưỡng cực-lưỡng cực) • Interaction between a dipole (e.g water) and a dipole (e.g water) • Dipole-dipole forces exist between neutral polar molecules (giữa phân tử phân cực trunghồ) • Polar molecules need to be close together • Weaker than ion-dipole forces: F =k Q1Q2 – Q1 and Q2 are partial charges Copyright 1999, d Chapter and 10 Intermolecular Forces Dipole-Dipole Forces • There is a mix of attractive (hút) and repulsive (đẩy) dipoledipole forces as the molecules tumble • If two molecules have about the same mass and size, then dipole-dipole forces increase with increasing polarity Copyright 1999, Chapter and 10 10 Structures of Solids X-Ray Diffraction Copyright 1999, Chapter and 10 54 Structures of Solids X-Ray Diffraction • X-ray diffraction (X-ray crystallography): – X-rays are passed through the crystal and are detected on a photographic plate – The photographic plate has one bright spot at the center (incident beam) as well as a diffraction pattern – Each close packing arrangement produces a different diffraction pattern – Knowing the diffraction pattern, we can calculate the positions of the atoms required to produce that pattern – We calculate the crystal structure based on a knowledge of the diffraction pattern Copyright 1999, Chapter and 10 55 Bonding in Solids • There are four types of solid: – Molecular (formed from molecules) - usually soft with low melting points and poor conductivity – Covalent network (formed from atoms) - very hard with very high melting points and poor conductivity – Ions (formed form ions) - hard, brittle, high melting points and poor conductivity – Metallic (formed from metal atoms) - soft or hard, high melting points, good conductivity, malleable and ductile Copyright 1999, Chapter and 10 56 Bonding in Solids Copyright 1999, Chapter and 10 57 Bonding in Solids Molecular Solids • Intermolecular forces: dipole-dipole, London dispersion and H-bonds • Weak intermolecular forces give rise to low melting points • Room temperature gases and liquids usually form molecular solids at low temperature • Efficient packing of molecules is important (since they are not regular spheres) Copyright 1999, Chapter and 10 58 Bonding in Solids Covalent Network Solids • Intermolecular forces: dipole-dipole, London dispersion and H-bonds • Atoms held together in large networks • Examples: diamond, graphite, quartz (SiO2), silicon carbide (SiC), and boron nitride (BN) • In diamond: – – – – each C atom has a coordination number of 4; each C atom is tetrahedral; there is a three-dimensional array of atoms Diamond is hard, and has a high melting point (3550 °C) Copyright 1999, Chapter and 10 59 Bonding in Solids Covalent Network Solids Copyright 1999, Chapter and 10 60 Bonding in Solids Covalent Network Solids • In graphite – each C atom is arranged in a planar hexagonal ring; – layers of interconnected rings are placed on top of each other; – the distance between C atoms is close to benzene (1.42 Å vs 1.395 Å in benzene); – the distance between layers is large (3.41 Å); – electrons move in delocalized orbitals (good conductor) Copyright 1999, Chapter and 10 61 Bonding in Solids Ionic Solids • Ions (spherical) held together by electrostatic forces of attraction: F =k Q1Q2 d2 – The higher the charge (Q) and smaller the distance (d) between ions, the stronger the ionic bond • There are some simple classifications for ionic lattice types: Copyright 1999, Chapter and 10 62 Bonding in Solids Ionic Solids Copyright 1999, Chapter and 10 63 Bonding in Solids Ionic Solids – NaCl Structure • Each ion has a coordination number of • Face-centered cubic lattice • Cation to anion ratio is 1:1 • Examples: LiF, KCl, AgCl and CaO – CsCl Structure • Cs+ has a coordination number of • Different from the NaCl structure (Cs+ is larger than Na+) • Cation to anion ratio is 1:1 Copyright 1999, Chapter and 10 64 Bonding in Solids Ionic Solids – Zinc Blende Structure • Typical example ZnS • S2- ions adopt a fcc arrangement • Zn2+ ions have a coordination number of • The S2- ions are placed in a tetrahedron around the Zn 2+ ions • Example: CuCl – Fluorite Structure • Typical example CaF2 • Ca2+ ions in a fcc arrangement • There are twice as many F- per Ca2+ ions in each unit cell • Examples: BaCl2, PbF2 Copyright 1999, Chapter and 10 65 Bonding in Solids Metallic Solids • Metallic solids have metal atoms in hcp, fcc or bcc arrangements • Coordination number for each atom is either or 12 • Problem: the bonding is too strong for London dispersion and there are not enough electrons for covalent bonds • Resolution: the metal nuclei float in a sea of electrons • Metals conduct because the electrons are delocalized and are mobile Copyright 1999, Chapter and 10 66 Bonding in Solids Metallic Solids Copyright 1999, Chapter and 10 67 Intermolecular Forces, Liquids, and Solids End of Chapter 11 Copyright 1999, Chapter and 10 68 ... Comparison Tính chất vật lý chất hiểu theo khái niệm thuyết động học phân tử (kinetic molecular theory) sau: • Chất khí dễ bị nén, có hình dạng thể tích vật chứa : • Các phân tử chất khí cách xa... tác với • Chất lỏng khơng bị nén, có hình dạng (nhưng khơng tích) vật chứa : • Các phân tử chất lỏng giữ gần phân tử chất khí, khơng đến mức chúng trượt qua (slide past each other) • Chất rắn... lực giữ chất lỏng rắn lai với gọi lực liên phân tử) Copyright 1999, Chapter and 10 Intermolecular Forces • The covalent bond holding a molecule together is an intramolecular forces (lục nội phân