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Chapter 11

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Jamie Donaldson

Chapter 11 – Liquids, Solids, and Intermolecular Forces 11.2 solids, liquids, and gases: A Molecular Comparison The densities of liquid (0.998) and solid (0.917) are greater than the density of gas (5.90*10 ). This is due to the fact that the molecules of solid and liquid are closely packed together however on the other hand gas molecules have a lot of distance between one another. Water is in gaseous state at 100°C, liquid at 20°C, and solid at 0°C. State Density Shape Volume Strength of IMF Gas Low Indefinite Indefinite Weak Liquid High Indefinite Definite Moderate Solids High Definite Definite Strong Thermal energy overcomes the intermolecular forces in liquids and this allows them to more around one another. This is not possible in solids; they are locked in position and are only able to vibrate. Solids and liquids cannot be easily compressed since there is not much space between the particles but in gas since there is a large distance between the molecules it can be compressed. The molecules and atoms within a solid are fixed so they have a definite shape. Solids can be crystalline (ordered structure) or amorphous (no long range order). Changing the temperature or pressure can change the state of a matter. Solid  liquid requires heat and liquid  solid requires cold temperatures Liquid  gas requires heat or decreasing pressure and gas  liquid requires cold temperatures or an increase in pressure. 11.3 Intermolecular Forces: the forces that hold condensed states together The strength of the intermolecular force between atoms and molecules determines the state. Intermolecular forces are due to interactions between charges, partial charges and temporary charges. Protons and electrons are attracted to one another. Their potential energy decreases, as they get closer together. Intermolecular forces are weaker than bonding forces. In bonding forces (greater charges) the molecules and atoms are closer together than compared to intermolecular forces (smaller charges). Dispersion forces (London forces) are present in all molecules and atoms. Dispersion forces are due to fluctuation in electron distribution (unevenly distributed). One side of the molecule or atom is going to slightly more negative than the other side. The side without the electrons will be positive due to the nucleus’ charge. The magnitude of the dispersion force depends on how easily the electrons in the atom or molecule can move or polarize in response to an instantaneous dipole. A larger electron cloud = greater dispersion force. So dispersion force increases as molar mass increases. The shape of a molecule also plays a key role in the magnitude of a dispersion force. The larger the area for interaction between the molecules or atoms the greater the boiling point. Dipole-Dipole forces exist in all polar molecules. Polar molecules have permanent dipole. The positive end of one polar molecule attracts the negative end of another polar molecule. Polar molecules have a greater boiling point than non-polar molecules. The polarity is important in determining miscibility (mix without separating states). Polar molecules are miscible with other polar molecules but not with non-polar molecules (oil and water cannot mix). Hydrogen Bonding takes place when a hydrogen atom is bonded to fluorine, oxygen or nitrogen. Hydrogen has a large partial positive charge due to the large electro difference. Hydrogen can approach these atoms very closely. Hydrogen bonds can occur between molecules. Hydrogen is the stronger than dispersion forces and dipole-dipole forces. Ion-dipole force occurs in mixtures of ionic compounds and polar compounds. This is the strongest intermolecular force. 11.4 intermolecular forces in action: surface tension, viscosity, and capillary action Surface tension molecules at the surface have fewer molecules to interact with compared to molecules in the interior. The molecules at the surface are less stable and therefore have higher potential energy. The surface tension is of a liquid is the energy required to increase the surface area by a unit amount. The tendency of a liquid to make its surface area smaller creates a skin that resists penetration. Surface tension decreases as intermolecular force decreases. Viscosity is the resistance to liquid flow. Viscosity is measured in poise (p), which is 1g/cm*s. viscosity is greater in molecules with stronger intermolecular forces because if molecules are strongly attracted to each other they don’t flow around easily. Viscosity also depends on the shape of a molecule. Longer molecules have greater viscosity. Thermal energy overcomes intermolecular forces; this causes molecules to flow more easily. Capillary action is the ability of a liquid to flow against gravity up a narrow tube. Capillary action occurs because of two forces; cohesive forces (attraction between the molecules in the liquid) and adhesive forces (attraction between the molecules and the surface of the tube). Cohesive forces help keep the liquid together while adhesive forces causes the liquid to spread out to the surface of the tube. If adhesive forces are less than cohesive forces the liquid will not rise. When adhesive forces are greater than cohesive forces the meniscus will be concave and vise versa. 11.5 Vaporization and Vapor Pressure Vaporization is the process by which thermal energy can overcome intermolecular forces and produce a state change from liquid to gas. The higher the temperature the more the molecules move around (possess more energy). The transition from liquid to gas is known as vaporization. The transition from gas to liquid is called condensation. Increasing the temperature will increase the rate of vaporization. The rate of vaporization increases with increasing surface area and the rate of vaporization increases with decreasing strength in intermolecular forces. Liquids that vaporize easily are called volatile and those that do not vaporize easily are known as nonvolatile. Vaporization is an endothermic process because it takes energy to vaporize molecules. Condensation is exothermic because is heat is released. Heat of vaporization (ΔH vaps the amount of heat required to vaporize one mole of a liquid to gas. Heat of vaporization will always be positive because energy is absorbed. When condensation takes place the same amount of heat will be involved however it is emitted rather than absorbed (negative value). If water is in a closed container than there will be liquid water turning into gas but there will also be gas condensing into water because they have no where else to go. If water remains a constant temperature than the rate of evaporation remains constant. The rate of condensation and the rate of vaporization will become equal (dynamic equilibrium will be reached). The pressure of a gas in dynamic equilibrium with its liquid is called vapor pressure. When a system in dynamic equilibrium is disturbed, the system responds so as to minimize the disturban
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