Attractions

Particle-Particle Attractions and Boiling Point Temperatures

The temperature at which a liquid boils differs for different substances because it depends on the force of attractions between a substances particles. The stronger the interparticle attractions, the higher the temperature at which the substance will boil. Therefore, if you could predict the relative strengths of different attractions, you could predict which of two substances should have a higher boiling point temperature. Predicting the relative strengths of attractions between particles requires combining some of your skills from previous chapters with new skills that you will develop while studying this chapter. For example, given a name of a substance, you need to be able to write its formula, and you need to be able to tell whether it represents a binary ionic compound, an ionic compound with polyatomic ion(s), a binary covalent compound, a binary acid, or an oxyacid. Because you need to be able to draw Lewis structures and predict molecular shapes, you should, if necessary, review these topics.

The figure below shows the connections between the skills mentioned above and the first of the predictions we will be learning to make in this chapter. The skills it lists are all interrelated: to do any of them successfully, you need to be able to do the ones that lie above it.

Connections Between Skills This figure shows the connections between the skills covered in earlier chapters and the skills presented in this chapter. To master the new skills, you need to have mastered the old ones. Start at the top of the sequence and work your way down, convincing yourself as you go along that you can do each task.

If you can predict the types of attractions between particles in two different substances, you can also predict the relative strengths of those attractions, and then the relative boiling point temperatures for the substances. Increased strength of attractions leads to decreased rate of evaporation, decreased rate of condensation at equilibrium, decreased concentration of vapor, and decreased vapor pressure at a given temperature. As a result, higher temperatures are necessary to reach the vapor pressure required for boiling. In summary,

Stronger attractions between particles lead to lower equilibrium vapor pressures and higher boiling point temperatures.

Click here to see a Sample Study Sheet that summarizes this procedure.

Click here to see an example of predicting relative strengths of attractions.

Click here to see an example of predicting relative vapor pressures and boiling point temperatures.

Click here to see an exercise for predicting relative strengths of attractions.

Click here to see an exercise for predicting relative vapor pressures and boiling point temperatures.