Intermolecular Forces There are two types of forces of interest to chemists: 1. Intramolecular forces 2. Intermolecular forces

Intermolecular Forces There are two types of forces of interest to chemists: 1. Intramolecular forces 2. Intermolecular forces Intramolecular forces: Forces that act between atoms in the same molecule.

Intermolecular Forces There are two types of forces of interest to chemists: 1. Intramolecular forces 2. Intermolecular forces Intramolecular forces: Forces that act between atoms in the same molecule. Intermolecular forces: Forces that act between molecules.

The forces that hold together a liquid or solid are intermolecular forces. Intramolecular forces are responsible for the stability of individual molecules.

Consider the cooling of a gas Consider the cooling of a gas. By lowering the temperature we decrease the kinetic energy (recall from kinetic theory of gases that the KE is proportional to the temperature) of the gas molecules.

Consider the cooling of a gas Consider the cooling of a gas. By lowering the temperature we decrease the kinetic energy (recall from kinetic theory of gases that the KE is proportional to the temperature) of the gas molecules. At some point during the cooling process, molecules are moving so slowly that they can no longer overcome the attractive forces between the molecules – a liquid forms.

Liquids possess definite volume, but assume the shapes of their containers. The forces in liquids are not strong enough to hold the molecules in rigid positions.

Generally, intermolecular forces are much weaker than the covalent bonds holding atoms together in molecules.

Generally, intermolecular forces are much weaker than the covalent bonds holding atoms together in molecules. For example, it usually requires much less energy to evaporate a liquid than to decompose the molecules of the liquid.

Generally, intermolecular forces are much weaker than the covalent bonds holding atoms together in molecules. For example, it usually requires much less energy to evaporate a liquid than to decompose the molecules of the liquid. For example: it takes about 41 kJ to vaporize 1 mole of water. It takes 930 kJ to break all the O H bonds in 1 mole of water.

Types of Intermolecular Forces

Types of Intermolecular Forces Dipole – dipole forces: Forces between molecules that have permanent (non-zero) dipole moments.

Types of Intermolecular Forces Dipole – dipole forces: Forces between molecules that have permanent (non-zero) dipole moments. Dipole – dipole forces act between polar molecules (recall that polar molecules have permanent non-zero dipole moments).

These forces are of electrostatic origin and they can be understood in terms of Coulomb’s law.

These forces are of electrostatic origin and they can be understood in terms of Coulomb’s law. The larger the dipole moment, the greater the attractive force.

The larger the dipole moment, the greater the attractive force. These forces are of electrostatic origin and they can be understood in terms of Coulomb’s law. The larger the dipole moment, the greater the attractive force. Possible orientation of polar molecules in a solid. + - + - + - + - + - + - + - + - + -

The larger the dipole moment, the greater the attractive force. These forces are of electrostatic origin and they can be understood in terms of Coulomb’s law. The larger the dipole moment, the greater the attractive force. Possible orientation of polar molecules in a solid. The dipoles are aligned for maximum attractive interaction. + - + - + - + - + - + - + - + - + -

In a liquid, the molecules are not held so rigidly and they align themselves in such a way that on average, the attractive interaction is at a maximum.

It would be expected that the stronger the dipole – dipole attraction among molecules, the higher the melting point of the substance.

It would be expected that the stronger the dipole – dipole attraction among molecules, the higher the melting point of the substance. molecule dipole moment (D) melting point (oC) CH4 0 -183 SiH4 0 -185 PH3 0.55 -133 H2S 0.95 -83 CHCl3 0.99 -64

Note that melting point is not just determined by polarity Note that melting point is not just determined by polarity. The following molecules are nonpolar. Compound Melting point (oC) CH4 -182.5 CF4 -150.0 CCl4 -23.0 CBr4 90.0 CI4 171.0

Ion – dipole forces: Forces between ions and molecules that have a permanent (non-zero) dipole moment.

Ion – dipole forces: Forces between ions and molecules that have a permanent (non-zero) dipole moment. Ion – dipole forces can be understood in terms of Coulomb’s law. The ion involved can be either a cation or an anion.

Ion – dipole forces: Forces between ions and molecules that have a permanent (non-zero) dipole moment. Ion – dipole forces can be understood in terms of Coulomb’s law. The ion involved can be either a cation or an anion. Na+ + - I- + -

Ion – dipole forces: Forces between ions and molecules that have a permanent (non-zero) dipole moment. Ion – dipole forces can be understood in terms of Coulomb’s law. The ion involved can be either a cation or an anion. The strength of ion – dipole interactions depends on the charge and size of the ion and the magnitude of the dipole moment. Na+ + - I- + -

The charges on cations are generally more concentrated, because cations are usually smaller than anions.

The charges on cations are generally more concentrated, because cations are usually smaller than anions. Therefore, for equal charges a cation is able to interact with more dipoles than an anion.

The charges on cations are generally more concentrated, because cations are usually smaller than anions. Therefore, for equal charges a cation is able to interact with more dipoles than an anion. Ion – dipole interactions play an important role in determining the degree of solubility of ionic compounds in water.

The three-dimensional structure of sodium chloride is largely the result of strong electrostatic forces between the Na+ and Cl- ions.

The three-dimensional structure of sodium chloride is largely the result of strong electrostatic forces between the Na+ and Cl- ions. When a crystal of NaCl dissolves in water, the three-dimensional network of ions breaks up into individual units, which are then stabilized through interaction between ions and the polar water molecules.

These ions are said to be hydrated, because they are surrounded by water molecules arranged in a specific manner.

These ions are said to be hydrated, because they are surrounded by water molecules arranged in a specific manner. Hydration: The process in which an ion or a molecule is held strongly to water molecules in an aqueous solution.

These ions are said to be hydrated, because they are surrounded by water molecules arranged in a specific manner. Hydration: The process in which an ion or a molecule is held strongly to water molecules in an aqueous solution. The water molecules play the role of an “electrical insulator”. Water molecules “shield” the charged ions Na+ and Cl- from each other.

Solvation: Describes the interaction of the solute molecules or ions with the solvent molecules.

Water has a relatively large dipole moment (1 Water has a relatively large dipole moment (1.8 D) and is therefore a good solvent for many ionic compounds.

Water has a relatively large dipole moment (1 Water has a relatively large dipole moment (1.8 D) and is therefore a good solvent for many ionic compounds. NaCl will not dissolve in nonpolar liquids such as benzene and carbon tetrachloride – these liquids have a zero dipole moment.

Dispersion Forces

Dispersion Forces Dispersion Forces (Also called London Forces): Attractive forces between molecules which arise as a result of temporary dipoles induced in the molecules.

Dispersion Forces Dispersion Forces (Also called London Forces): Attractive forces between molecules which arise as a result of temporary dipoles induced in the molecules. These interactions will be important for nonpolar molecules.

When a sodium ion is placed near a neutral atom like helium, the electron density of the neutral atom becomes distorted, as a result of the electrostatic force exerted by the sodium ion.

When a sodium ion is placed near a neutral atom like helium, the electron density of the neutral atom becomes distorted, as a result of the electrostatic force exerted by the sodium ion. Na+

When a sodium ion is placed near a neutral atom like helium, the electron density of the neutral atom becomes distorted, as a result of the electrostatic force exerted by the sodium ion. He atom before charge distortion. Na+

When a sodium ion is placed near a neutral atom like helium, the electron density of the neutral atom becomes distorted, as a result of the electrostatic force exerted by the sodium ion. He atom before charge distortion. Na+ Na+ + -

When a sodium ion is placed near a neutral atom like helium, the electron density of the neutral atom becomes distorted, as a result of the electrostatic force exerted by the sodium ion. He atom before charge distortion. Charge cloud of He gets distorted by the presence of the Na+ ion. Na+ Na+ + -

When a sodium ion is placed near a neutral atom like helium, the electron density of the neutral atom becomes distorted, as a result of the electrostatic force exerted by the sodium ion. He atom before charge distortion. Charge cloud of He gets distorted by the presence of the Na+ ion. Na+ Na+ + - Na+ + -

When a sodium ion is placed near a neutral atom like helium, the electron density of the neutral atom becomes distorted, as a result of the electrostatic force exerted by the sodium ion. He atom before charge distortion. Charge cloud of He gets distorted by the presence of the Na+ ion. Ion – induced dipole interaction. Na+ Na+ + - Na+ + -

A dipole moment is said to be induced, that is, we have an induced dipole moment in the atom, because there is now a separation of positive and negative charges in the atom.

A dipole moment is said to be induced, that is, we have an induced dipole moment in the atom, because there is now a separation of positive and negative charges in the atom. A similar situation exists when a polar molecule approaches a helium atom. Consider a HCl molecule close to a helium atom.

A dipole moment is said to be induced, that is, we have an induced dipole moment in the atom, because there is now a separation of positive and negative charges in the atom. A similar situation exists when a polar molecule approaches a helium atom. Consider a HCl molecule close to a helium atom. + -

A dipole moment is said to be induced, that is, we have an induced dipole moment in the atom, because there is now a separation of positive and negative charges in the atom. A similar situation exists when a polar molecule approaches a helium atom. Consider a HCl molecule close to a helium atom. induced dipole + - + - + -

A dipole moment is said to be induced, that is, we have an induced dipole moment in the atom, because there is now a separation of positive and negative charges in the atom. A similar situation exists when a polar molecule approaches a helium atom. Consider a HCl molecule close to a helium atom. induced dipole + - + - + - + - + -

A dipole moment is said to be induced, that is, we have an induced dipole moment in the atom, because there is now a separation of positive and negative charges in the atom. A similar situation exists when a polar molecule approaches a helium atom. Consider a HCl molecule close to a helium atom. induced dipole dipole – induced dipole interaction + - + - + - + - + -

The extent to which a dipole moment can be induced depends on the charge of the ion, or the strength of the permanent dipole, and the polarizability of the neutral atom or molecule.

The extent to which a dipole moment can be induced depends on the charge of the ion, or the strength of the permanent dipole, and the polarizability of the neutral atom or molecule. Polarizability: A measure of how easily the electron density in an atom or molecule can be distorted.

Consider the cooling of a rare (inert) gas Consider the cooling of a rare (inert) gas. Each atom collides with other atoms.

Consider the cooling of a rare (inert) gas Consider the cooling of a rare (inert) gas. Each atom collides with other atoms. A collision almost always distorts the spherical symmetry of the charge cloud – hence a temporary dipole is created within the atom.

Consider the cooling of a rare (inert) gas Consider the cooling of a rare (inert) gas. Each atom collides with other atoms. A collision almost always distorts the spherical symmetry of the charge cloud – hence a temporary dipole is created within the atom. This temporary dipole can instantaneously induce a dipole in its nearest neighbor.

Consider the cooling of a rare (inert) gas Consider the cooling of a rare (inert) gas. Each atom collides with other atoms. A collision almost always distorts the spherical symmetry of the charge cloud – hence a temporary dipole is created within the atom. This temporary dipole can instantaneously induce a dipole in its nearest neighbor. If the temperature is low, these interactions – between the induced dipoles – may be sufficiently strong to hold the atoms together. Hence, a liquid starts to form.