INTERMOLECULAR FORCES AND PROPERTIES (essential knowledge and power points)

*Interactions between intermolecular forces influence the solubility and separation of mixtures. (Explain the relationship between the solubility of ionic and molecular compounds in aqueous and nonaqueous solvents, and the intermolecular interactions between particles.) Describe the two methods that can be used to separate the components of a liquid since they cannot be separated with filtration.

1 The components of a liquid solution cannot be separated by filtration. They can, however, be separated using processes that take advantage of differences in the intermolecular interactions of the components. a. Chromatography (paper, thin-layer, and column) separates chemical species by taking advantage of the differential strength of intermolecular interactions between and among the components of the solution (the mobile phase) and with the surface components of the stationary phase. b. Distillation separates chemical species by taking advantage of the differential strength of intermolecular interactions between and among the components and the effects these interactions have on the vapor pressures of the components in the mixture.

Rules for Assigning Oxidation Numbers

1. The oxidation number of a monatomic ion is equal to its charge 2. For a polyatomic ion, the sum of the oxidation numbers must equal the ionic charge of the ion. 3. The oxidation number of a metal cation in a compund is the same as its ionic charge. 4.The oxidation number of hydrogen in a compound is +1 except in metal hydrides 5. The oxidation number of oxygen in a compound is -2 except in peroxides, for example, H2O2, where it is -1. 6. The oxidation number of an uncombined element is zero. 7.For any compound, the sum of the oxidation numbers of the atoms in the compound must equal zero.

How can we explain surface tension?

A molecule in the interior of a liquid is attracted by the molecules surrounding it, whereas a molecule at the surface of a liquid is attracted only by the molecules below it and on each side. This inward force pulls molecules from the surface to the interior, thereby reducing the surface area and causing the molecules at the surface to become more closely packed together. The liquid behaves almost as if it had a skin.Water has a high surface tension because of its strong hydrogen bonds. The surface tension of mercury is even stronger because of even stronger metallic bonds between the atoms of mercury.

*Gas properties are explained macroscopically—using the relationships among pressure, volume, temperature, moles, gas constant—and molecularly by the motion of the gas. (Explain the relationship between the motion of particles and the macroscopic properties of gases with: a. The kinetic molecular b. A particulate model. c. A graphical representation.) What is average kinetic energy and what is the equation used to calculate it?

All the particles in a sample of matter are in continuous, random motion. The average kinetic energy of a particle is related to its average velocity by the equation: EQN: KE = ½ mv2.

What is an Equilibrium Vapor Pressure Curve?

Any point along a vapor pressure versus temperature curve for a compound represents a pressure and temperature at which liquid and vapor are in equilibrium.

What is Capillary Action

Capillary action is the rise of liquids up very narrow tubes. Capillary action is the result of adhesive forces (forces of attraction between the liquid and another surface) being greater than cohesive intermolecular forces (forces holding the liquid together). The adhesive forces between the liquid and the walls of the tube tend to increase the surface area of the liquid. The surface tension of the liquid tends to reduce the area, thereby pulling the liquids up the tube. The liquid climbs until the adhesive and cohesive forces are balanced by the force of gravity on the liquid.

*Spectroscopy can determine the structure and concentration in a mixture of a chemical species. (Explain the properties of an absorbed or emitted photon in relationship to an electronic transition in an atom or molecule.)

Differences in absorption or emission of photons in different spectral regions are related to the different types of molecular motion or electronic transition: a. Microwave radiation is associated with transitions in molecular rotational levels. b. Infrared radiation is associated with transitions in molecular vibrational levels. c. Ultraviolet/visible radiation is associated with transitions in electronic energy levels.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship among the macroscopic properties of a substance, the particulate-level structure of the substance, and the interactions between these particles.) How do intermolecular forces affect the properties of ionic solids?

Due to strong interactions between ions, ionic solids tend to have low vapor pressures, high melting points, and high boiling points. They tend to be brittle due to the repulsion of like charges caused when one layer slides across another layer. They conduct electricity only when the ions are mobile, as when the ionic solid is melted or dissolved in water or another solvent.

What is evaporation?

Evaporation is the process by which the liquid molecules escape the liquid (vaporize). For a liquid to evaporate, the particles must have enough energy to overcome the intermolecular forces that hold them together in the liquid phase. As the temperature of a liquid increases, more particles have sufficient energy to escape from the liquid. For this reason, the rate of evaporation is higher at higher temperatures.

*Gas properties are explained macroscopically—using the relationships among pressure, volume, temperature, moles, gas constant—and molecularly by the motion of the gas. (Explain the relationship between the macroscopic properties of a sample of gas or mixture of gases using the ideal gas law.) Describe how the relationship between the ideal gas law variables can be used to describe gas behavior?

Graphical representations of the relationships between P, V, T, and n are useful to describe gas behavior.

Hydrogen Bonding

Hydrogen Bonding is a special type of intermolecular force of attraction that exists between the hydrogen atom in a polar bond and an unshared electron pair on a nearby small electronegative atom (usually an F, O, or N) atom on another molecule. Hydrogen bonding is not really a chemical bond. Hydrogen bonding is a stronger form of dipole-dipole attraction.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when: a. The molecules are of the same chemical species b. The molecules are of two different chemical species.) Describe hydrogen bonding.

Hydrogen bonding is a strong type of intermolecular interaction that exists when hydrogen atoms covalently bonded to the highly electronegative atoms (N, O, and F) are attracted to the negative end of a dipole formed by the electronegative atom (N, O, and F) in a different molecule, or a different part of the same molecule.

*Gas properties are explained macroscopically—using the relationships among pressure, volume, temperature, moles, gas constant—and molecularly by the motion of the gas. (Explain the relationship between the macroscopic properties of a sample of gas or mixture of gases using the ideal gas law.) Describe daltons law of partial pressures.

In a sample containing a mixture of ideal gases, the pressure exerted by each component (the partial pressure) is independent of the other components. Therefore, the total pressure of the sample is the sum of the partial pressures. EQN: PA = Ptotal × XA, where XA = moles A/total moles; EQN: Ptotal = PA + PB + PC + ...

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship among the macroscopic properties of a substance, the particulate-level structure of the substance, and the interactions between these particles.) How do intermolecular forces affect the properties of covalent network solids?

In covalent network solids, the atoms are covalently bonded together into a three dimensional network (e.g., diamond) or layers of two-dimensional networks (e.g., graphite). These are only formed from nonmetals: elemental (e.g., diamond, graphite) or binary compounds of two nonmetals (e.g., silicon dioxide and silicon carbide). Due to the strong covalent interactions, covalent solids have high melting points. Three-dimensional network solids are also rigid and hard, because the covalent bond angles are fixed. However, graphite is soft because adjacent layers can slide past each other relatively easily.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship among the macroscopic properties of a substance, the particulate-level structure of the substance, and the interactions between these particles.) How do noncovalent interactions affect the properties in large biomolecules or polymers?

In large biomolecules or polymers, noncovalent interactions may occur between different molecules or between different regions of the same large biomolecule. The functionality and properties of such molecules depend strongly on the shape of the molecule, which is largely dictated by noncovalent interactions.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when: a. The molecules are of the same chemical species b. The molecules are of two different chemical species.) what type of reaction occurs in large biomolecules?

In large biomolecules, noncovalent interactions may occur between different molecules or between different regions of the same large biomolecule.

*Spectroscopy can determine the structure and concentration in a mixture of a chemical species. (Explain the amount of light absorbed by a solution of molecules or ions in relation to the concentration, path length, and molar absorptivity.)

In most experiments the path length and wavelength of light are held constant. In such cases, the absorbance is proportional only to the concentration of absorbing molecules or ions.

*Matter exists in three states: solid, liquid, and gas, and their differences are influenced by variances in spacing and motion of the molecules. (Represent the differences between solid, liquid, and gas phases using a particulate level model.) Describe the characteristics of particles in the gas phase.

In the gas phase, the particles are in constant motion. Their frequencies of collision and the average spacing between them are dependent on temperature, pressure, and volume. Because of this constant motion, and minimal effects of forces between particles, a gas has neither a definite volume nor a definite shape.

Importance of Intermolecular Forces of Attraction

Intermolecular forces of attraction operate over short distances. If not for these forces, the condensed phases (liquids and solids) would not exist; gases would never change to liquids and solids. Properties of liquids, such as boiling point, vapor pressure, viscosity, and heat of vaporization, depend on the strengths of the intermolecular forces of attraction. Properties of solids, such as melting point and heat of fusion depend on the strengths of the intermolecular forces of attraction.

London (Dispersion) Forces and Induced dipoles

London (dispersion) forces are relatively weak attractive forces that occur in all covalent molecules but are the most noticeable in nonpolar molecules and the nonbonded atoms of the noble gases (Ar, C8H18). LDF occur because the electron cloud becomes distorted as the electrons shift to an unequal distribution. It is during this instant that a molecule develops a temporary dipole. This temporary dipole introduces a similar response in neighboring molecules, thus producing a short-lived attraction between molecules. The ease with which the electron "cloud" of an atom can be distorted is called polarizability. The larger the molecule, the easier it becomes to induce dipoles since the electron cloud is larger and more easily distorted (more polarizable). Weak dipoles can be induced in nonpolar material by the presence of ions (Ion-induced dipoles) and strongly polar materials (Dipole-Induced dipoles).

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when: a. The molecules are of the same chemical species b. The molecules are of two different chemical species.) How can the relative strength of London dispersion forces be determined?

London dispersion forces are a result of the Coulombic interactions between temporary, fluctuating dipoles. London dispersion forces are often the strongest net intermolecular force between large molecules. a. Dispersion forces increase with increasing contact area between molecules and with increasing polarizability of the molecules. b. The polarizability of a molecule increases c. The term "London dispersion forces" should not be used synonymously with the term "van der Waals forces."

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship among the macroscopic properties of a substance, the particulate-level structure of the substance, and the interactions between these particles.) How do the properties of liquids and solids relate to their strengths and types of intermolecular forces?

Many properties of liquids and solids are determined by the strengths and types of intermolecular forces present. Because intermolecular interactions are broken when a substance vaporizes, the vapor pressure and boiling point are directly related to the strength of those interactions. Melting points also tend to correlate with interaction strength, but because the interactions are only rearranged, in melting, the relations can be more subtle.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship among the macroscopic properties of a substance, the particulate-level structure of the substance, and the interactions between these particles.) How do intermolecular forces affect the properties of metallic solids?

Metallic solids are good conductors of electricity and heat, due to the presence of free valence electrons. They also tend to be malleable and ductile, due to the ease with which the metal cores can rearrange their structure. In an interstitial alloy, interstitial atoms tend to make the lattice more rigid, decreasing malleability and ductility. Alloys typically retain a sea of mobile electrons and so remain conducting.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship among the macroscopic properties of a substance, the particulate-level structure of the substance, and the interactions between these particles.) How do intermolecular forces affect the properties of molecular solids?

Molecular solids are composed of distinct, individual units of covalently-bonded molecules attracted to each other through relatively weak intermolecular forces. Molecular solids generally have a low melting point because of the relatively weak intermolecular forces present between the molecules. They do not conduct electricity because their valence electrons are tightly held within the covalent bonds and the lone pairs of each constituent molecule. Molecular solids are sometimes composed of very large molecules or polymers.

*Interactions between intermolecular forces influence the solubility and separation of mixtures. (Using particulate models for mixtures: a. Represent interactions between components. b. Represent concentrations of components.)

Particulate representations of solutions communicate the structure and properties of solutions, by illustration of the relative concentrations of the components in the solution and drawings that show interactions among the components.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship among the macroscopic properties of a substance, the particulate-level structure of the substance, and the interactions between these particles.) How do we understand how intermolecular interactions help to establish macroscopic properties?

Particulate-level representations, showing multiple interacting chemical species, are a useful means to communicate or understand how intermolecular interactions help to establish macroscopic properties.

*Matter exists in three states: solid, liquid, and gas, and their differences are influenced by variances in spacing and motion of the molecules. (Represent the differences between solid, liquid, and gas phases using a particulate level model.) How can the arrangement of solids be described and what is true regarding the structure of both types?

Solids can be crystalline, where the particles are arranged in a regular three-dimensional structure, or they can be amorphous, where the particles do not have a regular, orderly arrangement. In both cases, the motion of the individual particles is limited, and the particles do not undergo overall translation with respect to each other. The structure of the solid is influenced by interparticle interactions and the ability of the particles to pack together.

*Interactions between intermolecular forces influence the solubility and separation of mixtures. (Calculate the number of solute particles, volume, or molarity of solutions.) How can solution composition be expressed?

Solution composition can be expressed in a variety of ways; molarity is the most common method used in the laboratory. EQN: M = nsolute /Lsolution

*Interactions between intermolecular forces influence the solubility and separation of mixtures. (Calculate the number of solute particles, volume, or molarity of solutions.) What is the difference in macroscopic particles of homogeneous mixtures and heterogeneous mixtures?

Solutions, also sometimes called homogeneous mixtures, can be solids, liquids, or gases. In a solution, the macroscopic properties do not vary throughout the sample. In a heterogeneous mixture, the macroscopic properties depend on location in the mixture.

What are the factors affecting vapor pressure?

Strength of Intermolecular Forces Liquids whose components have high intermolecular forces have relatively low vapor pressures because the molecules need high energies to escape to the vapor phase. Size of the Molecule Larger molecules have relatively low vapor pressures, mainly because of the large dispersion forces. (The more electrons a substance has, the more polarizable it is, and the greater the dispersion forces are.) Temperature At a given temperature, vapor pressure is constant. As the temperature increases, vapor pressure increases because the rate of evaporation is greater and vapor particles have more energy.

*Interactions between intermolecular forces influence the solubility and separation of mixtures. (Explain the relationship between the solubility of ionic and molecular compounds in aqueous and nonaqueous solvents, and the intermolecular interactions between particles.) What makes substances tend to be soluble in one another?

Substances with similar intermolecular interactions tend to be miscible or soluble in one another.

What is surface tension?

Surface tension is a measure of the inward forces that must be overcome to expand the surface of a liquid. Surface tension is what allows bugs to "walk" on water and water drops to be spheres.

*Spectroscopy can determine the structure and concentration in a mixture of a chemical species. (Explain the amount of light absorbed by a solution of molecules or ions in relation to the concentration, path length, and molar absorptivity.) What is the beer-lambert law?

The Beer-Lambert law relates the absorption of light by a solution to three variables according to the equation: EQN: A = εbc. The molar absorptivity ε describes how intensely a sample of molecules or ions absorbs light of a specific wavelength. The path length b and concentration c are proportional to the number of absorbing species.

*Gas properties are explained macroscopically—using the relationships among pressure, volume, temperature, moles, gas constant—and molecularly by the motion of the gas. (Explain the relationship between the motion of particles and the macroscopic properties of gases with: a. The kinetic molecular b. A particulate model. c. A graphical representation.) Describe the relationship between temperature and average kinetic energy.

The Kelvin temperature of a sample of matter is proportional to the average kinetic energy of the particles in the sample.

*Gas properties are explained macroscopically—using the relationships among pressure, volume, temperature, moles, gas constant—and molecularly by the motion of the gas. (Explain the relationship between the motion of particles and the macroscopic properties of gases with: a. The kinetic molecular b. A particulate model. c. A graphical representation.) What does the Maxwell-Boltzmann distribution provide?

The Maxwell-Boltzmann distribution provides a graphical representation of the energies/ velocities of particles at a given temperature.

What are Intermolecular Forces of Attraction?

The attractive forces between individual particles (atoms, molecules, ions) of a substance are known as intermolecular forces of attraction. These forces are quite weak relative to intramolecular forces, that is, covalent and ionic bonds within compounds.

*Matter exists in three states: solid, liquid, and gas, and their differences are influenced by variances in spacing and motion of the molecules. (Represent the differences between solid, liquid, and gas phases using a particulate level model.) How can the arrangement of the particles in liquids be described?

The constituent particles in liquids are in close contact with each other, and they are continually moving and colliding. The arrangement and movement of particles are influenced by the nature and strength of the forces (e.g., polarity, hydrogen bonding, and temperature) between the particles.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when: a. The molecules are of the same chemical species b. The molecules are of two different chemical species.) Describe ion-dipole forces.

The dipole moment of a polar molecule leads to additional interactions with other chemical species. Ion-dipole forces of attraction are present between ions and polar molecules. These tend to be stronger than dipole-dipole forces.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when: a. The molecules are of the same chemical species b. The molecules are of two different chemical species.) describe dipole iduced dipole interactions.

The dipole moment of a polar molecule leads to additional interactions with other chemical species. . Dipole-induced dipole interactions are present between a polar and nonpolar molecule. These forces are always attractive. The strength of these forces increases with the magnitude of the dipole of the polar molecule and with the polarizability of the nonpolar molecule.

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when: a. The molecules are of the same chemical species b. The molecules are of two different chemical species.) Describe dipole-dipole interactions.

The dipole moment of a polar molecule leads to additional interactions with other chemical species. Dipole-dipole interactions are present between polar molecules. The interaction strength depends on the magnitudes of the dipoles and their relative orientation. Interactions between polar molecules are typically greater than those between nonpolar molecules of comparable size because these interactions act in addition to London dispersion forces.

*Gas properties are explained macroscopically—using the relationships among pressure, volume, temperature, moles, gas constant—and molecularly by the motion of the gas. (Explain the relationship among non-ideal behaviors of gases, interparticle forces, and/or volumes.) Explain the differences between ideal and real gases and how these differences can be used to explain deviations from the ideal gas law.

The ideal gas law does not explain the actual behavior of real gases. Deviations from the ideal gas law may result from interparticle attractions among gas molecules, particularly at conditions that are close to those resulting in condensation. Deviations may also arise from particle volumes, particularly at extremely high pressures.

*Gas properties are explained macroscopically—using the relationships among pressure, volume, temperature, moles, gas constant—and molecularly by the motion of the gas. (Explain the relationship between the motion of particles and the macroscopic properties of gases with: a. The kinetic molecular b. A particulate model. c. A graphical representation.) Describe the kinetic molecular theory and the max-well Boltzmann distribution.

The kinetic molecular theory (KMT) relates the macroscopic properties of gases to motions of the particles in the gas. The Maxwell-Boltzmann distribution describes the distribution of the kinetic energies of particles at a given temperature.

*Gas properties are explained macroscopically—using the relationships among pressure, volume, temperature, moles, gas constant—and molecularly by the motion of the gas. (Explain the relationship between the macroscopic properties of a sample of gas or mixture of gases using the ideal gas law. What is the ideal gas law and what does it relate?

The macroscopic properties of ideal gases are related through the ideal gas law: EQN: PV = nRT.

How can you determine the boiling point of a substance?

The normal boiling point of a substance is the temperature at which the vapor pressure is equal to 1 atm (760 mm Hg, 101.3 kPa).

*Intermolecular forces can explain the physical properties of a material. (Explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when: a. The molecules are of the same chemical species b. The molecules are of two different chemical species.) How can the relative strength and orientation dependence of dipole-dipole and ion-dipole forces be understood?

The relative strength and orientation dependence of dipole-dipole and ion-dipole forces can be understood qualitatively by considering the sign of the partial charges responsible for the molecular dipole moment, and how these partial charges interact with an ion or with an adjacent dipole.

*Matter exists in three states: solid, liquid, and gas, and their differences are influenced by variances in spacing and motion of the molecules. (Represent the differences between solid, liquid, and gas phases using a particulate level model.) How can the volume of substances in the solid and liquid phase be described?

The solid and liquid phases for a particular substance typically have similar molar volume because, in both phases, the constituent particles are in close contact at all times.

Why are intermolecular forces of attraction important?

The strength of the intermolecular forces can be used to determine whether a covalent compound exists as a solid, liquid, or gas under standard conditions. Solids have the strongest intermolecular forces of attraction between their particles. The intermolecular forces of attraction between the molecules of liquids are not as strong as those found between the particles of a solid. Gases have the weakest intermolecular forces of attraction between their particles

Ion-Dipole Forces

The type of attractive forces that exists between an ion and the partial charge on the end of a polar molecule is known as an ion-dipole Force. The magnitude of the attraction increases as either the charge on the ion or the magnitude of the dipole moment increases.Ion-dipole forces are especially important for solutions of ionic substances in polar liquids.

Dipole-Dipole Forces

The type of intermolecular force of attraction that occurs between all polar molecules is known as dipole-dipole. Dipole-dipole forces are generally weaker than ion-dipole forces. For molecules of approximately equal mass and size, the strengths of intermolecular attractions increase with increasing polarity.

*Spectroscopy can determine the structure and concentration in a mixture of a chemical species. (Explain the properties of an absorbed or emitted photon in relationship to an electronic transition in an atom or molecule.)

The wavelength of the electromagnetic wave is related to its frequency and the speed of light by the equation: EQN: c = λν The energy of a photon is related to the frequency of the electromagnetic wave through Planck's equation (E = ℎν).

What is vapor pressure?

Vapor pressure is the pressure that develops in the gas phase above a liquid when the liquid is placed in a closed container. The pressure of the gas depends on the number of gas particles present. The greater the number of gas particles, the greater the vapor pressure. Liquids with a high vapor pressure are said to be volatile. This means they evaporate rapidly from an open dish.

Equilibrium Vapor Pressure

When a liquid is placed in a closed container, it starts evaporating just as it would in an open beaker. In the closed container, however, gas particles cannot escape. As the gas particles move, they collide with the walls of the container and the surface of the liquid. When the gas particles collide with the liquid, very few bounce off; almost all condense to the liquid state again. Initially the rate at which the particles evaporate is much greater than the rate at which they condense. As the gas particles increase in number, they collide with the liquid surface more frequently. Eventually the rate at which the liquid particles evaporate is equal to the rate at which the gas particles condense. Under these conditions the liquid and gas are said to be in equilibrium, and vapor pressure is constant, representing the equilibrium vapor pressure.

*Spectroscopy can determine the structure and concentration in a mixture of a chemical species. (Explain the properties of an absorbed or emitted photon in relationship to an electronic transition in an atom or molecule.) What occurs when a photon is absorbed/emitted by an atom or molecule?

When a photon is absorbed (or emitted) by an atom or molecule, the energy of the species is increased (or decreased) by an amount equal to the energy of the photon.

convex meniscus (relationship between adhesion and cohesion?)

adhesion < cohesion

concave meniscus (relationship between adhesion and cohesion?)

adhesion > cohesion

What is the difference between intramolecular and intermolecular forces?

intramolecular forces - forces within a molecule that hold atoms together, that is, covalent bonds. Intermolecular forces - forces between molecules that hold molecules to each other. These intermolecular forces are collectively referred to as Van der Waals Forces. They are much weaker than covalent bonds.