Boiling Point and Melting Point.Boiling and melting points result from the weakening of inter molecular forces between covalent molecules.
The melting and and boiling points range from very low (dispersion forces) to high (hydrogen bonds). The stronger the force is, the more energy is required to melt the solid or boil the liquid.
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1.Dispersion Forces:Effect of Mass:
If the molecule has more mass, it means that the number of electrons present is greater, and thus gives more strength to the dispersion forces that require more energy to break them, and these molecules will have higher melting and boiling points than others. Effect of Surface: In hydrocarbons, the forces increase over surface area where the strength of dispersion forces depends on the contact of as many regions of the molecules as possible. Hydrocarbons with lower number of carbons and shorter chains have lower boiling and melting points than others. |
2.Molecules with Net Dipoles:The force of attraction between the molecules is greater so the energy required to break the attraction is greater compared to non-polar molecules. The temperatures for the melting and boiling are also higher than any non-polar molecule. Similarly, as the mass increases, the dipoles will carry a greater charge so the temperature of melting and boiling increases. However, there are exceptions, as illustrated in the diagram on the right.
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SOLUBILITY
"Like dissolves Like"-Non-polar solvents dissolve non-polar solutes.
In dissolution process, forces are broken but they must be compensated by molecule interaction between the two substances of the same nature. *Keep in mind that water, or H2O, is a polar solvent that forms hydrogen bonds. For example, Hexane (C6H14), and Pentane (C5H12) are both non-polar held together by London Dispersion forces and dissolution occurs between them.
-Non-polar substances do not dissolve in water since: 1. Water molecules are polar 2. Water molecules are held together by hydrogen bonds Why not dissolve?Not enough energy is present to break the hydrogen bonds since hydrogen bonds are stronger than dispersion forces, and there is no compensating force between a non-polar molecule and a water molecule.
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Briefly, C30H62, or paraffin wax (non-polar) dissolves in oil (non-polar), hexane C6H14 (non-polar) or CCl4 (non-polar).
Paraffin wax will NOT dissolve in neither water nor ethanol nor any other polar substance, while C6H12O6, or glucose (polar) dissolves in both water and ethanol, but not in oil or any other non-polar substance. |
Water dissolves polar molecules like alcohols: CH3OH, CH3CH2OH.
-Moreover with alcohols, which are hydrocarbon derivatives, as the chain gets longer it becomes more difficult for dissolution to take place because more hydrogen bonds need to be broken to give space for the alcohol and not enough hydrogen bonds can be formed after that to compensate the previous broken bonds.
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Solute SolidsIn the case where solids are solutes, solubility increases as the temperature increases because as the solid is closer to its melting point, the better its inter molecular forces' compatibility with that of the liquid.
Why?Keep in mind that the inter molecular forces present within a solid are really strong in order to keep the molecule altogether It is far stronger compared to the forces present within a liquid. By applying this principle, we come to observe that solids that show greater solubility tend to have low melting points.
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