Chapter 14: Liquids and Solids PDF

Chapter 14: Liquids and Solids (Section 14.3-14.4)  Interactions between Molecules The particles in a gas are separated by large distances so the attractive forces between particles in a gas are very weak. The particles are free to move about at random and take up what ever sp...

Chapter 14: Liquids and Solids (Section 14.3-14.4)  Interactions between Molecules The particles in a gas are separated by large distances so the attractive forces between particles in a gas are very weak. The particles are free to move about at random and take up what ever space is available in the container. This is not true for solids and liquids! The attractive forces in liquids are strong enough to hold the particles in close contact while letting them slip and slide over one another. Water molecules are free to move around and will flow to take the shape of the container. 2  Interactions between Molecules The attractive forces in solids are so strong that they hold the particles rigidly in place and prevent their movement. They do vibrate in place however. In this chapter, we will look at the nature of the attractive forces responsible for the properties of liquids and solids and some of the properties related to this. 3  Attractive Forces The nature and structure of a substance determine the type and strength of molecular forces that operate within it. Intramolecular forces operate within the molecules or fundamental units of a substance. These are the covalent bonds within a molecule of a molecular substance, the forces between ions in an ionic compound, and the forces between atoms in a metal. Intermolecular forces operate between the molecules of a covalent substance; the atoms of a monatomic element; or the ions of one substance and the molecules of another. As a rule, intramolecular forces are much stronger than intermolecular forces. 41kJ to break intermolecular forces in 1 mole of water to boil it 930kJ to break all the H-O-H covalent bonds in 1 mole of water 7  Intramolecular Forces Attraction between positive and negative ions in a crystal of an ionic compound. Covalent bonds linking atoms in a network structure, such as that of silicon dioxide (sand). Covalent bonds in molecular substances like H2O and CO2. Metallic bonds like copper and magnesium. H O intramolecular H H O H force O H H H O O H H H H O Structure of ice Structure of diamond 9 H  Intermolecular Forces Attractions exerted by one molecule H of a molecular substance on another, such as the force of attraction H O H H between water molecules in ice. O O Attractions between atoms of the H H noble gas elements, helium through O H O radon. H H H H Attractions between molecules of one O substance and molecules of another, H as when two liquids are mixed, or a molecular solid such as sugar is Intermolecular force dissolved in a liquid. Attraction between molecules of one https://www.youtube.com/ substance and ions of another, as watch?v=BqQJPCdmIp8 when an ionic compound dissolves in a liquid. 10  Liquids: Intermolecular Forces https://www.youtube.com/watch?v=BqQJPCdmIp8 Start 2:20  Geckos and Intermolecular Forces https://www.youtube.com/watch?v=YeSuQm7KfaE  How Do Geckos Defy Gravity? https://www.youtube.com/watch?v=YeSuQm7KfaE Miscibility—a liquid’s ability to mix with another liquid without separating into two phases. In general, polar liquids are miscible with other polar liquids but are not miscible with nonpolar liquids. “like dissolves like” rule For example, water, a polar liquid, is not miscible (soluble) with pentane (C5H12), a nonpolar liquid. Similarly, water and oil (nonpolar) do not mix. Consequently, oily hands or oily stains on clothes cannot be washed away with plain water. 15  Intermolecular Forces in Action: Surface Tension and Viscosity The most important manifestation of intermolecular forces is the very existence of liquids and solids. Without intermolecular forces, solids and liquids would not exist and all matter would be gaseous. In liquids, we can observe several other manifestations of intermolecular forces including surface tension and viscosity. 16  Surface Tension Surface tension is a property of the surface of a liquid that allows it to resist an external force. A paper clip will float on water if it is carefully placed on the surface of the water (even though it is more dense). It is held up by surface tension. A water strider can walk picture by Danica Centracchio (former student) on the surface of water without falling in because of surface tension. You can’t float a paper clip on gasoline because the intermolecular forces among the molecules composing gasoline are weaker than the intermolecular forces among water molecules. 17  Origin of surface tension In a liquid, each molecule is pulled equally in every direction by neighboring liquid molecules through intermolecular forces, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area. 18  Everyday Chemistry Why Are Water Drops Spherical? Water drops are spherical because of the surface tension caused by the attractive forces between water molecules. On the space shuttle, the complete absence of Water droplets on Gore-Tex® gravity results in floating spheres of water. http://www.youtube.com /watch?v=o8TssbmY-GM 19  Surface Tension http://www.youtube.com/watch?v=o8TssbmY-GM  Viscosity Viscosity is the resistance of a liquid to flow. Liquids that are viscous flow more slowly than liquids that are not viscous. Motor oil is more viscous than gasoline. Maple syrup is more viscous than water. Viscosity is greater in substances with stronger intermolecular forces because molecules cannot move around each other as freely, hindering flow. Long molecules, such as the hydrocarbons in motor oil, tend to form viscous liquids because of molecular entanglement. 21  Viscosity and Motor Oils Motor oils are made to have a precise viscosity to reduce engine friction, but still move the parts. As temperature increases, viscosity decreases. The lower the viscosity when cold, the more fluid the oil is. The higher the viscosity when hot, Equal weight steel balls were dropped the more viscous the oil is. into different motor oils. The more viscose the motor oil, the slower the rate the steel ball will fall.  Molecular Polarity In order to predict the types of intermolecular forces in a sample, we need to be able to determine the polarity of a molecule. In order to determine the polarity of a molecule as a whole, we first need to review some ideas from Chapter 12: 1) Bond polarity (polar or nonpolar?) 2) Dipole moment 3) Molecular geometry or shape 23  Bond Polarity A chemical bond can be classified as nonpolar covalent, polar covalent or ionic by determining the difference in electronegativity values between each atom in the bond. We can use ranges of electronegativity values as a guide for classification. While these values are not absolute, they will accurately classify most bonds. DEN Value Type of Bond 0 - 0.4 pure or nonpolar covalent 0.5 - 1.9 polar covalent ≥ 2.0 Ionic (metal and nonmetal) 24 Electronegativity 25  Bond Polarity Examples; H - Cl DEN = 3.0 - 2.1 = 0.9 (polar covalent) O-H DEN = 3.5- 2.1 = 1.4 (polar covalent) C-N DEN = 3.0 - 2.5 = 0.5 (polar covalent) C- H DEN = 2.5 - 2.1 = 0.4 (nonpolar covalent) 26  Bond Polarity In addition to classifying a bond as being polar covalent, we can represent the polarity with partial charges (d+, or d-) and a dipole moment. The partial negative charge (d-) goes on the atom in the bond with the higher EN value and the partial positive charge (d+) goes on the atom in the bond with the lower EN value. d- d+ d+ d- d+ d- O-H C-N H - Cl 3.0 2.1 2.5 3.0 2.1 3.0 27  Bond Polarity The polarity of the bond can also be represented with a dipole moment. The dipole moment points in the direction of the atom with the higher EN value. O-H C-N H - Cl 3.0 2.1 2.5 3.0 2.1 3.0 28  Molecular Polarity In order to determine the polarity of a O H H molecule, we need to take into account each bond in the molecule and its geometry or shape. First, determine if each bond in polar covalent, nonpolar covalent or ionic. If N H H the bond is polar covalent, draw a H dipole moment for that bond. H O-H DEN = 3.5- 2.1 = 1.4 (polar covalent) C N-H DEN = 3.0- 2.1 = 0.9 (polar covalent) Cl Cl C-H DEN = 2.5- 2.1 = 0.4 (nonpolar covalent) Cl C - Cl DEN = 3.0- 2.5 = 0.5 (polar covalent) 29  Molecular Geometry (Shape) # of Number of Molecular Geometry Molecular Geometry Approximate Electron Lone Pairs (3D structure) Bond Angles Groups 180o 2 0 linear 180° 0 trigonal planar 120° 120o 3 1 bent