Chem 16 Principles of Chemistry PDF

Chem 16 | Principles of Chemistry 1  UNIT 9 Liquids Chem 16 | Principles of Chemistry 2  Comparison between States of Matter ▪ The fundamental difference between states of matter is the distance between particles....

Chem 16 | Principles of Chemistry 1  UNIT 9 Liquids Chem 16 | Principles of Chemistry 2  Comparison between States of Matter ▪ The fundamental difference between states of matter is the distance between particles. 3 Chem 16 | Principles of Chemistry  Comparison between States of Matter ▪ The fundamental difference between states of matter is the distance between particles. State Shape and Volume Compressibility Ability to Flow Gas Conforms to shape and High High volume of container Liquid Conforms to shape of Very low Moderate container; volume limited by surface Solid Maintains its own shape and Almost none Almost none volume 4 Chem 16 | Principles of Chemistry  Attractive Forces ▪ Intramolecular or bonding forces are found within a molecule. ▪ Intermolecular or nonbonding forces are found between molecules. 5 Chem 16 | Principles of Chemistry  Attractive Forces – Intermolecular forces arise from the attraction between molecules with partial charges, or between ions and molecules. – Intermolecular forces are relatively weak compared to bonding forces because they involve smaller charges that are farther apart. – properties of liquids such as boiling point and melting point reflect the strength of IMFs 6 Chem 16 | Principles of Chemistry  Attractive Forces Table 12.2 Comparison of Bonding and Nonbonding (Intermolecular) Forces 7 Chem 16 | Principles of Chemistry  Attractive Forces Table 12.2 Comparison of Bonding and Nonbonding (Intermolecular) Forces (continued) 8 Chem 16 | Principles of Chemistry  Intermolecular Forces Types of Intermolecular forces: (1) Dispersion (London) forces van der Waals forces (2) Dipole – Dipole forces (3) Hydrogen Bonding (4) Ion-dipole forces 9 Chem 16 | Principles of Chemistry  Dispersion Forces – a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles – also called London Dispersion Forces – weakest IMF and present in all molecules – the only force found in nonpolar molecules 10 Chem 16 | Principles of Chemistry  Dispersion Forces A. When atoms are far apart they do not influence one other. B. When atoms are close together, the instantaneous C. The process occurs dipole in one atom induces a throughout the sample. dipole in the other. 12-11  Dipole-dipole Forces – attractive forces between polar molecules – similar in origin to H-bonding but weaker – increases with increasing polarity 12 Chem 16 | Principles of Chemistry  Dipole-dipole Forces – attraction between dipole molecules. – for molecules of approximately equal mass and size, the strength of intermolecular attractions increases with increasing polarity. 13 Chem 16 | Principles of Chemistry  Hydrogen Bonding – An intermolecular hydrogen bond is the attraction between the H atom of one molecule and a lone pair of the N, O, or F atom of another molecule. – strongest IMF – a special type of dipole-dipole attraction 14 Chem 16 | Principles of Chemistry  Ion-Dipole Forces – occurs when ionic compound is mixed with a polar compound 15 Chem 16 | Principles of Chemistry  Summarizing Intermolecular Forces Dispersion Weak Present in all molecules Increase with molar mass Dipole – Dipole Present in polar molecules Hydrogen Bonds Strongest IMF in a pure substance Ion – dipole Present in mixture of ionic and polar compounds 16 Chem 16 | Principles of Chemistry Intermolecular Forces 17 Chem 16 | Principles of Chemistry Exercise 1 Indicate what type of bonding is holding the atoms together in one molecule of the following. (a) NH3 (b) HCl (c) NaCl in water (d) F2 18 Chem 16 | Principles of Chemistry  Properties of Liquids - Viscosity - Surface Tension - Vapor Pressure - Volatility - Boiling Point 19 Chem 16 | Principles of Chemistry  Viscosity Resistance of a liquid to flow. Increasing molecular weight, increasing intermolecular forces, increases viscosity. Decreasing temperature, increases viscosity. SI unit for viscosity: kg m-1 s-1 20 Chem 16 | Principles of Chemistry  Viscosity Relationship to Intermolecular Forces: Liquids with strong intermolecular forces (like hydrogen bonding or van der Waals forces) tend to have higher viscosity. For example, glycerol (with extensive hydrogen bonding) is more viscous than water. Weak intermolecular forces result in low viscosity, as seen in substances like ethanol or hexane. 21 Chem 16 | Principles of Chemistry Surface Tension Surface tension is the energy required to increase the surface area of a liquid. – The stronger the intermolecular attractive forces, the higher the surface tension will be. – Increasing the temperature of a liquid decreases its surface tension. 22 Chem 16 | Principles of Chemistry  Surface Tension Relationship to Intermolecular Forces: Strong intermolecular forces, such as hydrogen bonding in water, create higher surface tension. Water molecules at the surface experience an inward pull due to these forces, minimizing surface area. Liquids with weaker forces, such as benzene, have lower surface tension. 23 Chem 16 | Principles of Chemistry  Vapor Pressure Vapor Pressure is the pressure exerted by its vapor when the liquid and vapor are in dynamic equilibrium. 24 Chem 16 | Principles of Chemistry  Vapor Pressure Relationship to Intermolecular Forces: Strong intermolecular forces reduce the tendency of molecules to escape into the gas phase, leading to low vapor pressure (e.g., water). Weak intermolecular forces increase vapor pressure because molecules escape more easily (e.g., diethyl ether). 25 Chem 16 | Principles of Chemistry  Volatility The rate at which a chemical will evaporate. The weaker the intermolecular forces in the liquid, the more easily molecules can escape, the higher the vapor pressure. Example: At 25 °C, CCl4 is more volatile than CBr4, because dispersion forces between its molecules are smaller than in CBr4. 26 Chem 16 | Principles of Chemistry  Volatility Relationship to Intermolecular Forces: Strong intermolecular forces reduce volatility (e.g., water is less volatile due to hydrogen bonds). Weak forces increase volatility (e.g., alcohols or hydrocarbons with weaker dipole-dipole or London dispersion forces). 27 Chem 16 | Principles of Chemistry  Boiling Point The temperature at which the vapor pressure of a liquid equals the external pressure. Increasing temperature, increases the vapor pressure. 28 Chem 16 | Principles of Chemistry  Boiling Point Relationship to Intermolecular Forces: Liquids with strong intermolecular forces require more energy to overcome these forces, resulting in higher boiling points (e.g., water: 100°C). Weak forces correspond to lower boiling points (e.g., acetone: 56°C). 29 Chem 16 | Principles of Chemistry Exercise 2 1. Based on their composition and structure, list CH2Cl2, CH3CH2CH3, and CH3CH2OH in order of: (a) increasing intermolecular forces, (b) increasing viscosity, (c) decreasing surface tension. 30 Chem 16 | Principles of Chemistry Chem 16 | Principles of Chemistry 31