Recently, my sixth grade daughter studied endothermic and exothermic reactions. She was confused about this, based upon the results of an experiment conducted in the class. It took a lot of people to successfully answer a difficult question she had. This page was created to possibly help others who don't understand endothermic and exothermic reactions as well as they want or need to.
The basic definitions are easy to understand:
The problem comes in trying to state which specific examples of reactions are which, because usually a lot of things are going on at the same time.
Here was the experiment which confused her: Mix baking soda and vinegar, stick a thermometer in the solution and watch it go down. This concludes our demonstration of an endothermic reaction.
Here was the question my daughter asked: "How can you be adding heat to something and its temperature be going down? If the temperature is going down, it must be giving off heat, and that is exothermic!"
Now this is a VERY good question, and no one she talked to could give her a good explanation as to just how you could be adding heat to the solution, and yet its temperature be dropping. Here is how we (me, along with the help of four Purdue engineering graduates!) were finally able to resolve her confusion.
First- we must define the system. The vinegar and baking soda mixed together to form a solution are the system. EVERYTHING else - the container, the air, the people doing the experiment- are the surroundings. Focus on just our little system, and don't let everything else going on around the system add confusion.
Second- we must understand an important concept about the conservation of energy. Everything is trying to get to the same temperature. Pour a cold drink into a glass and what happens after about an hour? Assuming you didn't drink it, the temperature of the drink is now the same as its surroundings.
Third- to change from one state to another (from liquid to solid, from solid to gas, etc.) takes energy. To change from one molecular structure to another (from separate substances mixed to form a solution) takes energy.
Put some ice cubes in a container, and apply heat.
So, in this endothermic example, the temperature of the ice and water stayed the same at the exact instant of the reaction. As long as heat continues to be applied after the change from ice to water, either from our burner flame, or even just from sitting in a warm room, the temperature of the water goes up.
Continue to apply heat to the melting water, until the water begins to boil.
So, in this endothermic example, the temperature of the water and steam stayed the same at the exact instant of the reaction. As the hot gas is realeased into its surroundings, the water vapor will cool and possibly condense on some surface (like on the outside of that cold drink glass we talked about before). Then it will sit around for a while longer until it all becomes the same temperature as the room. None of this has anything to do with determining whether the boiling water was an endothermic reaction. The water absorbed heat - that is all we need to know.
We mix vinegar and baking soda, and observe the temperature of the solution. This is a much more complex example than the water we used before, because we are mixing two substances which react with each other to form a solution.
The molecular structure of the vinegar and baking soda combined and reacting form kinetic energy. Heat energy from the surroundings is required because in the process, they are also combining to form a new molecular structure which has potential energy (the goop). The change from kinetic energy to potential energy requires heat. So much heat is needed that the temperature drops because it can't get the heat fast enough. This explanation did not help my daughter as well as the previous examples, but is a little more exact explanation as to how the temperature can be dropping, even though you are adding heat.
Understanding exothermic reactions is pretty easy. Most reactions are exothermic. A log burns, it gives of heat. An atom splits, it gives off heat. The key to understanding endothermic reactions is to stay focused on the one question - is the system absorbing heat from its surroundings? Stay focused on the little system, and don't let everything else going on around the system add confustion.