The strength of dispersion forces between The strength of dispersion forces between molecules can affect physical properties of the molecules can affect physical properties of the substances they make up.substances they make up.

In other words… the amount of energy needed In other words… the amount of energy needed to break an intermolecular bond in a compound, to break an intermolecular bond in a compound, in a substance can determine such things as in a substance can determine such things as melting and boiling points and solubility!melting and boiling points and solubility!

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Melting and Boiling PointsMelting and Boiling Points Solids are able to melt, and liquid substances are able to Solids are able to melt, and liquid substances are able to

boil, when there is enough energy present for particles to boil, when there is enough energy present for particles to overcome their forces of attraction with surrounding overcome their forces of attraction with surrounding particles to break free and move around more.particles to break free and move around more.

For substances to melt or boil they have to break the For substances to melt or boil they have to break the intermolecular bondsintermolecular bonds between their particles. If the between their particles. If the particles can break free and move around more, then their particles can break free and move around more, then their state will change. state will change.

i.e.: solids i.e.: solids liquids and liquids liquids and liquids gases gases Bond energies tell the amount of energy that is required to Bond energies tell the amount of energy that is required to

break the intermolecular bonds between particles.break the intermolecular bonds between particles. As you increase the temperature of a substance, you As you increase the temperature of a substance, you

increase the energy in its ions or molecules. So depending increase the energy in its ions or molecules. So depending on how strong the forces of attraction are between on how strong the forces of attraction are between particles, a substance may require more energy (higher particles, a substance may require more energy (higher temperatures) in order to break the bonds.temperatures) in order to break the bonds.

Factors That Cause High Melting Factors That Cause High Melting and Boiling Pointsand Boiling Points

Larger molecules that have more electrons have Larger molecules that have more electrons have more vibrations within their molecules. This more vibrations within their molecules. This means that they can easily cause an uneven means that they can easily cause an uneven distribution of charge within their molecules. distribution of charge within their molecules.

This This increases increases the strength of the dispersion the strength of the dispersion forces between larger molecules, making their forces between larger molecules, making their bonds stronger, and a lot more difficult to break. bonds stronger, and a lot more difficult to break.

Their bonds are stronger, so they require a lot Their bonds are stronger, so they require a lot more energy to break them which means more energy to break them which means higher higher melting and boiling points. melting and boiling points.

Factors That Cause Low Melting Factors That Cause Low Melting and Boiling Pointsand Boiling Points

The shape of molecules can also affect their boiling and The shape of molecules can also affect their boiling and melting point.melting point.

Molecules with a spherical shape have a smaller surface Molecules with a spherical shape have a smaller surface area than a straight chain molecule even if they have the area than a straight chain molecule even if they have the same number of electrons.same number of electrons.

A smaller surface area means there is less chance of A smaller surface area means there is less chance of causing an uneven distribution of charge within the causing an uneven distribution of charge within the molecule. This also means less opportunities to induce a molecule. This also means less opportunities to induce a charge on a nearby molecule. charge on a nearby molecule.

Therefore, substances with more spherical shaped Therefore, substances with more spherical shaped molecules will have molecules will have weakerweaker dispersion forces. dispersion forces.

Weaker dispersion forces mean that there is less energy Weaker dispersion forces mean that there is less energy ((lower temperatureslower temperatures) required to break the bonds holding ) required to break the bonds holding molecules together.molecules together.

Ionic and Molecular CompoundsIonic and Molecular Compounds

Ionic compounds generally have extremely high Ionic compounds generally have extremely high melting points because their bonds are much melting points because their bonds are much stronger and more difficult to break. It takes a stronger and more difficult to break. It takes a lot of energy. For example, calcium fluoride, lot of energy. For example, calcium fluoride, CaFCaF22..

Molecular Compounds generally have low Molecular Compounds generally have low boiling points because they have much weaker boiling points because they have much weaker dispersion forces between their particles. So it dispersion forces between their particles. So it takes a lot less energy (lower temperatures) to takes a lot less energy (lower temperatures) to break the intermolecular bonds. For example, break the intermolecular bonds. For example, bromide, Brbromide, Br22..

Mechanical Properties of SolidsMechanical Properties of Solids

Ionic compounds are brittle; they are not malleable Ionic compounds are brittle; they are not malleable and ductile like most metals. This is because ionic and ductile like most metals. This is because ionic compounds consist of positive and negative charges compounds consist of positive and negative charges which attract each other. which attract each other. If you tried to alter a solid ionic compound by bending If you tried to alter a solid ionic compound by bending it or hitting it with a hammer, it would cause the it or hitting it with a hammer, it would cause the charges to go out of line and suddenly instead of charges to go out of line and suddenly instead of opposite charges being aligned, like charges become opposite charges being aligned, like charges become aligned and repel each other.aligned and repel each other.This creates a line or plane along which substances This creates a line or plane along which substances break because there are no intermolecular forces of break because there are no intermolecular forces of attraction. An example is magnesium oxide, MgO.attraction. An example is magnesium oxide, MgO.

Mechanical Properties of SolidsMechanical Properties of Solids

With molecular compounds, if they are polar molecules they are With molecular compounds, if they are polar molecules they are quite brittle because there are positive and negative charges quite brittle because there are positive and negative charges within the dipoles that can become aligned with their like charge within the dipoles that can become aligned with their like charge and repel, causing a plane along which they could be broken. and repel, causing a plane along which they could be broken. They also tend not to be as hard as ionic compounds because They also tend not to be as hard as ionic compounds because polar molecules are only “slightly” charged and so that allows for polar molecules are only “slightly” charged and so that allows for weaker intermolecular forces and more movement and space weaker intermolecular forces and more movement and space between particles. Ex. Ice cubesbetween particles. Ex. Ice cubesIf the molecular compounds are non-polar, however, they are If the molecular compounds are non-polar, however, they are often quite soft even though they are solids at room often quite soft even though they are solids at room temperature. This is because the forces of attraction that may temperature. This is because the forces of attraction that may occur temporarily between their molecules are random and occur temporarily between their molecules are random and constantly changing. This allows molecules to be able to slide constantly changing. This allows molecules to be able to slide all over each other causing a substance to be soft but still have all over each other causing a substance to be soft but still have enough attraction between particles to be solid. A good enough attraction between particles to be solid. A good example is wax.example is wax.

Electrical ConductivityElectrical Conductivity

In order to conduct electrical currents, In order to conduct electrical currents, negative and/ or positive charges have negative and/ or positive charges have to be able to move freely and to be able to move freely and independently of one another.independently of one another.

As we know metallic elements are the As we know metallic elements are the best conductors of electricity, but what best conductors of electricity, but what about ionic and molecular compounds?about ionic and molecular compounds?

Electrical ConductivityElectrical Conductivity

Ionic Compounds are only good conductors of Ionic Compounds are only good conductors of electricity when they are in a liquid state or electricity when they are in a liquid state or dissolved in water. In a solid state the electrons dissolved in water. In a solid state the electrons in ionic compounds are held too tightly within in ionic compounds are held too tightly within the individual ions to be able to move about and the individual ions to be able to move about and generate an electric current or flow of charge.generate an electric current or flow of charge.

However, in the liquid state, or when the However, in the liquid state, or when the compound is dissolved in water, the electrons compound is dissolved in water, the electrons are not held as tightly and are able to move are not held as tightly and are able to move around each other, or in the case of the around each other, or in the case of the dissolved compound, through the water.dissolved compound, through the water.

Electrical ConductivityElectrical Conductivity

Generally, molecular compounds can’t Generally, molecular compounds can’t conduct electricity at all because the conduct electricity at all because the electrons are all localized within the electrons are all localized within the molecules, and even though polar molecules, and even though polar molecules are slightly charged, the molecules are slightly charged, the charges can’t leave the molecules and charges can’t leave the molecules and move around.move around.

Non-polar molecular compounds can’t Non-polar molecular compounds can’t conduct electricity at all.conduct electricity at all.

Solubility Solubility depends on the forces between

particles. In order for a solid to dissolve there are three energy changes that must happen.

1. The forces between the particles in the solid must be broken. (Between ions and ions or molecules and molecules).

2. The forces between the particles in the liquid must be broken.

3. There must be an attraction between the particles of the solid and the liquid.

If the force of attraction between the particles of the liquid and solid is much greater than the attraction between the like particles, then the solid has a much greater chance of dissolving.

Solubility

For ionic compounds, they are usually soluble in a polar substance like water because there is a force of attraction between the charged particles of the ionic compound, and the water, causing the compound to break down and dissolve in the water. Ex. KCl

Solubility

The same goes for polar molecular compounds. They generally dissolve well in other polar substances like water.

This is because there is a force of attraction between the charged particles in the compound and in the water. Ex. NH3

Polar compounds do not however, dissolve in non-polar solvents because the forces of attraction are much greater between the charges of the individual compound.

The force of attraction between the polar compound and the non-polar solvent is pretty much non-existent.

Solubility

Non-polar molecules do not dissolve in polar solvents. This is because the forces of attraction between the water molecules are very strong, so the water molecules remain attached to each other rather than attracting the non-polar molecules. An example is iodine in water.

Non-polar molecules will, however, dissolve in non-polar solvents. There is a greater attraction between the molecules in the solid and liquid, causing the solid to break down and dissolve. An example is iodine in kerosene.

ConclusionConclusion

We just learned how dispersion forces between We just learned how dispersion forces between molecules or ions in a compound can have an molecules or ions in a compound can have an affect on the physical properties of the related affect on the physical properties of the related substance. substance.

We learned that the strength of the dispersion We learned that the strength of the dispersion forces in compounds affects: the melting and forces in compounds affects: the melting and boiling points, malleability, hardness, solubility, boiling points, malleability, hardness, solubility, and electrical conductivity of that substance!and electrical conductivity of that substance!

Now let’s do a Lab!! Now let’s do a Lab!!