The strength of dispersion forces between molecules can affect physical properties of the substances they make up. In other words… the amount of energy needed to break an intermolecular bond in a compound, in a substance can determine such things as melting and boiling points and solubility! Let’s learn more!!... Melting and Boiling Points Solids are able to melt, and liquid substances are able to boil, when there is enough energy present for particles to overcome their forces of attraction with surrounding particles to break free and move around more. For substances to melt or boil they have to break the intermolecular bonds between their particles. If the particles can break free and move around more, then their state will change. i.e.: solids liquids and liquids gases Bond energies tell the amount of energy that is required to break the intermolecular bonds between particles. As you increase the temperature of a substance, you increase the energy in its ions or molecules. So depending on how strong the forces of attraction are between particles, a substance may require more energy (higher temperatures) in order to break the bonds. Factors That Cause High Melting and Boiling Points Larger molecules that have more electrons have more vibrations within their molecules. This means that they can easily cause an uneven distribution of charge within their molecules. This increases the strength of the dispersion forces between larger molecules, making their bonds stronger, and a lot more difficult to break. Their bonds are stronger, so they require a lot more energy to break them which means higher melting and boiling points. Factors That Cause Low Melting and Boiling Points The shape of molecules can also affect their boiling and melting point. Molecules with a spherical shape have a smaller surface area than a straight chain molecule even if they have the same number of electrons. A smaller surface area means there is less chance of causing an uneven distribution of charge within the molecule. This also means less opportunities to induce a charge on a nearby molecule. Therefore, substances with more spherical shaped molecules will have weaker dispersion forces. Weaker dispersion forces mean that there is less energy (lower temperatures) required to break the bonds holding molecules together. Ionic and Molecular Compounds Ionic compounds generally have extremely high melting points because their bonds are much stronger and more difficult to break. It takes a lot of energy. For example, calcium fluoride, CaF2. Molecular Compounds generally have low boiling points because they have much weaker dispersion forces between their particles. So it takes a lot less energy (lower temperatures) to break the intermolecular bonds. For example, bromide, Br2. Mechanical Properties of Solids Ionic compounds are brittle; they are not malleable and ductile like most metals. This is because ionic compounds consist of positive and negative charges which attract each other. If you tried to alter a solid ionic compound by bending it or hitting it with a hammer, it would cause the charges to go out of line and suddenly instead of opposite charges being aligned, like charges become aligned and repel each other. This creates a line or plane along which substances break because there are no intermolecular forces of attraction. An example is magnesium oxide, MgO. Mechanical Properties of Solids With molecular compounds, if they are polar molecules they are quite brittle because there are positive and negative charges within the dipoles that can become aligned with their like charge and repel, causing a plane along which they could be broken. They also tend not to be as hard as ionic compounds because polar molecules are only “slightly” charged and so that allows for weaker intermolecular forces and more movement and space between particles. Ex. Ice cubes If the molecular compounds are non-polar, however, they are often quite soft even though they are solids at room temperature. This is because the forces of attraction that may occur temporarily between their molecules are random and constantly changing. This allows molecules to be able to slide all over each other causing a substance to be soft but still have enough attraction between particles to be solid. A good example is wax. Electrical Conductivity In order to conduct electrical currents, negative and/ or positive charges have to be able to move freely and independently of one another. As we know metallic elements are the best conductors of electricity, but what about ionic and molecular compounds? Electrical Conductivity Ionic Compounds are only good conductors of electricity when they are in a liquid state or dissolved in water. In a solid state the electrons in ionic compounds are held too tightly within the individual ions to be able to move about and generate an electric current or flow of charge. However, in the liquid state, or when the compound is dissolved in water, the electrons are not held as tightly and are able to move around each other, or in the case of the dissolved compound, through the water. Electrical Conductivity Generally, molecular compounds can’t conduct electricity at all because the electrons are all localized within the molecules, and even though polar molecules are slightly charged, the charges can’t leave the molecules and move around. Non-polar molecular compounds can’t conduct electricity at all. Solubility 1. 2. 3. Solubility depends on the forces between particles. In order for a solid to dissolve there are three energy changes that must happen. The forces between the particles in the solid must be broken. (Between ions and ions or molecules and molecules). The forces between the particles in the liquid must be broken. 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. Conclusion We just learned how dispersion forces between molecules or ions in a compound can have an affect on the physical properties of the related substance. We learned that the strength of the dispersion forces in compounds affects: the melting and boiling points, malleability, hardness, solubility, and electrical conductivity of that substance! Now let’s do a Lab!!