Intermolecular Interactions and Chemical Potential Energy PDF

Intermolecular Interactions and Chemical Potential Energy Strong forces: Ion-dipole Salts, e.g. NaCl are readily soluble in water 360 gL−1 at room temperature … But is sparingly soluble in C2H5OH - 0.65g/L How to bring salts...

Intermolecular Interactions and Chemical Potential Energy Strong forces: Ion-dipole Salts, e.g. NaCl are readily soluble in water 360 gL−1 at room temperature … But is sparingly soluble in C2H5OH - 0.65g/L How to bring salts to organic solvents for chemical reactions? ---Host-guest chemistry ---Supramolecular chemistry, (Nobel prized) ---Fundamental technique for nanomaterials  Hydrogen bonding (a type of dipole-dipole interaction or electrostatic interactions) Hydrogen bond donor: H atom with partially positive charges. Usually, the H directly bonds to a large EN atom (e.g. O, N, F, etc). Hydrogen bond acceptor: highly electronegative atom (like O, N, or F) with with a lone pair of electrons and larger partially negative charges Is it right that every H-containing molecule can act as an H-bond donor? *** Not every H atom can undergo H-bonding! And not every molecule with O, N, and F atoms can act as H-bond an acceptor. Can methyl ether, ammonium ion (NH4+), amine (NH3, NMe3) act as H- bond donors? Only NH3 possess hydrogen bonds, why? (try to figure the reason out by reviewing the definition) Which one can act as H-bond acceptor? Methyl ether, NH3, NMe3 Which one can act as H-bond donor? NH4+ ,NH3 Why ammonium ion (NH4+) can not act as an H-bond acceptor? Intermolecular H-bonds, intramolecular H-bonds H-bonds for materials Explain the behavior using hydrogen bonds of water Why does oil not have this behaviour?  How many intermolecular interactions are in the process of dissolving NaCl in water? Na+ Cl- interaction (Ionic interactions) Water-ion interaction (ion-dipole) Water-water interaction (hydrogen bond) Dipole-dipole Polar vs apolar molecules Electronegativity and Molecular shape. HCN is a polar molecule Do HCN molecules have H-bonds? No Do HCN molecules have dipole-dipole interaction? Yes When HCN is dissolved in water, what kinds of intermolecular interactions existed in the solution? London dispersion Force (induced dipole-induced dipole), is named after the German physicist Fritz London A nonpolar molecule or an atom, can be momentarily polarized. Its electron cloud is distorted only for an instant. This temporary dipole will induce a dipole in its neighbour. The strength of interaction depends on polarizability. Larger atoms are “soft” and have a larger polarizability; Aromatic structures have a higher polarizability; Electron-rich structures have a higher polarizability. ***Present in ALL ATOMS/MOLECULES (polar and nonpolar)  Summary of intermolecular interactions (all related to the polarization of electrons, so electronegativity, charges, sizes (“softness”), molecular shapes (symmetries) are considered) the van der Waals Named after Dutch physicist Johannes Diderik van der Waals, the van der Waals force plays a fundamental role in fields as diverse as supramolecular chemistry, structural biology, nanotechnology, etc.. The van der Waals forces are usually described as a combination of the London dispersion forces between "instantaneously induced dipoles", Dipoles-induced dipoles, Dipole-dipole interactions. Ethanol (CH3CH2OH), b.p. = 78 oC vs Dimethyl Ether (CH3OCH3), b.p. = -25 oC Napthalene (shown here, C10H8) has a melting point of MP = 80 oC. A decane with a very similar molar mass (C10H22) has MP = -30 oC. Which has the highest boiling point? Explain. CCl4, CF4, CBr4 (Mass and intermolecular interactions) Which has the lowest vapor pressure? Explain.CH3-CH2-O-H (ethanol) CH3-CH2-CH3 (propane) Explain why n-propylamine, CH3CH2CH2NH2, melts at-83 oC while trimethylamine, (CH3)3N, melts at -117 oC. NaCl(s) dissolves readily in water. Describe the intermolecular forces made and broken. What is driving this dissolution? How do the molecules organize in water when there are hydrophobic and hydrophilic regions (these are surfactants)?  Metal: metallic bonds Thermal plastic: Intermolecular interaction, chain entanglement, Thermoset, e.g. tire, crosslinked macromolecules, the molar mass is infinite. Aspirin powder: Hydrogen bonds, London dispersion interactions; pi-pi polar-polar interactions, London interactions dispersion interactions, pi-pi Colloids: particles in water No H bonding; no polar-polar interaction; no interactions Surfactants; amphiphilic molecules ion-polar interactions Ion-polar interactions What is hydrophobic interactions (Oil/water repulsion) Nanotechnology: Adhesive inspired by Climbing Gecko  Water-Steam conversion Releasing heat to the environment Reverse osmosis Steam Ion exchange System absorbs energy Higher energy Electrolysis (heat) via the boiler … All energy consumed Energy The system releases released Liquid water energy (heat) via a radiator or Spontaneously Lower energy absorbed Release heat (energy) The intermolecular interactions lower the chemical energy The “downhill” processes (though no chemical reactions) release energy and the products are relatively stable, so they are favoured. Is it possible to get an “uphill” process (unfavored) to occur? Where does the (absorbed or released) energy come from? The total energy, also called internal energy (given symbol U), is the sum of the kinetic and potential energy for a sample (chemical). Potential energy: a form of stored energy due to its position, condition, or internal structure. This stored energy has the potential to do work when the object changes its position, condition, or configuration. Chemical Potential Energy: This is the energy stored in the chemical bonds of molecules, e.g. explosives, food, fuel. Gravitational Potential Energy, e.g. rocks on a hill Elastic Potential Energy, e.g. compressed spring or a stretched rubber Etc. Kinetic energy: the energy of an object due to its motion. Any object that is moving has kinetic energy that is proportional to the mass of the object and the square of its velocity.  Gravitational potential energy Chemical potential energy B 0 Reference, e.g. atoms state without bonds Positive A A-B A B The height The strength of chemical bonds A=B A-B or intermolecular interactions Ground A=B 0 Negative Bonding energy vs System energy. Formation of bonds release energy, generating molecules with lower system energy.  Single bond (less strong, so less energy is required to break it): Relatively higher chemical potential energy reference level for chemical energy Relatively Less stable Double bond (stronger and needs more energy to break it). So, Relatively low chemical potential energy A B More stable (larger bonding energy) III Assuming that both A-B and A=B can separately undergo various reactions A-B 1 generating products A and B. 2 Which reactions release energy, which reactions absorb energy? 3 A B Which reaction is the most favoured? Does it release or absorb energy? 4 II Which reaction is the most unfavoured? A=B 5 One of the six reactions is explosive; which one is possible? 6 If products are stabler relative to reagents, the reaction is “downhill” in energy and will release energy. I A B Blue vs red objects, which one stores more potential energy?  Non-covalent interactions between molecules lower energy, so they usually occur spontaneously, so-called self-assembly, such as hydrogen bonding between water molecules. The interaction is reversible (energy input is usually required for the systems with strong intermolecular interactions). Fundamental for supramolecular chemistry and nanomaterial synthesis (bottom-up) Understanding energy conversion is one requirement to explain the behaviour and design self-assembled nanomaterials. But energy-favored processes do not always automatically occur. why? Will be taught later.