1. Plot a graph Tb (ºC) v Mr of alkanes using graph paper.
2. State & explain the trends in terms of the type & size of the intermolecular forces, which exist in the substances.
Trend: As the number of Carbon and Hydrogen atoms in the chain increases, so does the boiling point.
Explanation: Alkanes only contain Carbon-to-Carbon covalent bonds or Carbon to hydrogen covalent bonds and the electronegativity difference between the Carbon and Hydrogen atoms is very small (0.4) so the bond is only slightly polar. Therefore, again it will only be Van der Waals forces that are present and the temporary dipoles that have been induced will cause a weak attraction between the atoms. As the size of the alkanes increases, so does the molecular mass and surface area meaning the alkanes will have many more locations where dipoles could be induced. Due to this, the Van der Waals forces of attraction will be much more abundant within the larger molecules and so it will require a lot more energy to break these attractions. Thus, creating a higher boiling point.
Complete the table for the relative solubility in water of the homologous series of alcohols (C1-6) access data from the power point available on Moodle.
Alcohols Mr Relative solubility in water
Methanol, CH3OH 12.0 + (4 x 1.0) + 16.0 = 32.0 ∞
Ethanol, C2H5OH (2 x 12.0) + (6 x 1.0) + 16.0 = 46.0 ∞
Propanol, C3H7 OH (3 x 12.0) + (8 x 1.0) + 16.0 = 60.0 ∞
Butanol, C4H9 OH (4 x 12.0) + (10 x 1.0) + 16.0 = 74.0 0.11
Pentanol, C5H11 OH (5 x 12.0) + (12 x 1.0) + 16.0 = 88.0 0.030
Hexanol, C6H13 OH (6 x 12.0) + (14 x 1.0) + 16.0 = 102.0 0.0058
1. Comment on the general trend.
2. Explain the trends in terms of the type & size of the intermolecular forces, which exist in the substances. Use labelled diagrams to help.
Trend: As the number of Carbon and Hydrogen atoms in the chain increases, the solubility of the alcohols in water decreases.
Explanation: Alcohols consist of a hydrocarbon chain with a hydroxyl group attached (-OH). The hydroxyl group contains a strong intermolecular force known a hydrogen bond. This type of intermolecular force can only be formed between a Hydrogen atom and either a Nitrogen, Oxygen or Fluorine atom (The most electronegative elements in the periodic table.). This means as well as the hydroxyl hydrogen ‘bond’, the alcohol can also form the same intermolecular force of attraction with the water molecules. Hydrogen Bonds are special forms of dipoles that occur when the hydrogen atom attaches itself to a very electronegative atom with a lone pair of electrons. The electron pair is pulled away from the hydrogen atom exposing the nucleus. This nucleus now behaves like a bare proton generating a strong electrostatic attraction. As hydrogen is so small, it has a very high charge density and the bond is extremely polarised. When the hydrocarbon chain is short in an alcohol, it is soluble in water. This is because the hydroxyl group is known as hydrophilic, as it can form hydrogen bonds to the neighbouring water molecules. Whereas, the hydrocarbon part of the alcohol is known as hydrophobic, this is because it cannot form the same hydrogen bonds to the water molecules. Therefore, when more carbons are added (making the chain longer) the ratio of the hydrophobic part of the molecule to the hydrophilic part becomes uneven. (The hydrophobic increases) Consequentially, the solubility of the alcohol decreases until it becomes virtually insoluble in water.
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• Solutions, Solubility and alcohol PowerPoint
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