Lesson 3: Heat of Solution
The process of dissolving is a process which involves the breaking and making of bonds, and that involves energy.
The breaking of bonds requires or absorbs energy. Using energy like that is called endothermic.
The formation of bonds releases energy. That is called exothermic.
Dissolution overall can be either endothermic or exothermic, depending on whether more energy was used to break the bonds, or more energy was released when new bonds were formed. If more energy is released in making bonds than is used in breaking bonds, the process is exothermic. If more energy is used than is released, the process is endothermic.
Example
As a very direct example of the heat involved in a dissolution reaction, I'd like you to try this experiment (which is also described in exercise 16). This can be done either at home or in the lab.
- Unless you have some kind of an allergy to detergent, I would like you to wet both of your hands and then put a little detergent on one hand.
- Close both hands.
- Then compare what happens to the hand that has the detergent in it to the other hand that just has the water.
- So please take a moment to do that now.
Well, you now know that at least one material has a very exothermic reaction when it dissolves in water.
Lab Work
Next I'd like you to measure the amount of energy involved in such a process. The way to do that is to do the experiment for which instructions are given in exercise 17 in your workbook.
When you do this experiment, use a precision thermometer (one marked in tenths of a degree) for measuring temperature. Check to see if we have temperature probes connected to a computer. If so, you can use that instead. Instructions for using it will be available. You need to write up that experiment as a formal lab report and turn it in. That means writing out the purpose, data, calculations, results, and answering the questions listed.
One additional note: In the phrase "heat of solution," the word "solution" refers to the process of dissolving rather than the physical mixture that is formed.