When the temperature reaches the boiling point, the average kinetic energy of the liquid particles is sufficient to overcome the forces of attraction that hold molecules in the liquid state. Vapor pressure is caused by an equilibrium between molecules in the gaseous state and molecules in the liquid state. When molecules in the liquid state have sufficient kinetic energy they may escape from the surface and turn into a gas. Molecules with the most independence in individual motions achieve sufficient kinetic energy velocities to escape as gases at lower temperatures.
The vapor pressure will be higher more gas molecules are present and therefore the compound will boil at a lower temperature. Molecules which strongly interact or bond with each other through a variety of intermolecular forces can not move easily or rapidly and therefore, do not achieve the kinetic energy necessary to escape the liquid state.
Therefore, molecules with strong intermolecular forces will have higher boiling points. This is a consequence of the increased kinetic energy needed to break the intermolecular bonds so that individual molecules may escape the liquid as gases. What is the general trend in the melting and boiling data? As the chain length numbers of carbons increases the melting and boiling points of the alkanes gradually increase for these compounds.
The two processes more or less cancel each other out energetically; thus, there is no barrier to solubility. Show Answer. Note: The actual boiling points arenonane, Skip to main content. Chapter 3: An Introduction to Organic Compounds.
Search for:. Physical Properties of Alkanes Objectives After completing this section, you should be able to arrange a number of given straight-chain alkanes in order of increasing or decreasing boiling point or melting point. Key Term Make certain that you can define, and use in context, the key term below.
Exercises For each of the following pairs of compounds, select the substance which you expect to have the higher boiling point: octane and nonane. Solubility in Water When a molecular substance dissolves in water, the following must occur: break the intermolecular forces within the substance.
In the case of the alkanes, these are the Van der Waals dispersion forces. In water, the primary intermolecular attractions are hydrogen bonds. Need help with Chemistry? One to one online tuition can be a great way to brush up on your Chemistry knowledge. Write an equation to show how ethanoic acid behaves as a weak acid in its reaction with water.
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