The misconception that I’ll be discussing today is that burning must be an endothermic reaction because it takes energy to get it started. If you think about a candle or burning a log in the fireplace, you have to use a heat source to get the fire started so a lot of students think that means that it’s an endothermic reaction. But if you think about the whole process, the burning log gives off much more energy than it takes to light it.
It’s easier to explain this to the students if you think about the model of skiing. To get up a hill, a skier has to put in a lot of energy. They either have to get on a ski lift, which uses electricity, or they have to walk all the way up to the top of the hill…that’s like an endothermic reaction, one the requires a lot of energy. To get down the hill, however, the skier just has to push off and they glide down to the bottom of the hill. Energy is given off in that case so it’s like an exothermic reaction. The reactions that we are talking about today…the lighting of the match, the lighting of a candle, the burning of a piece of paper…they’re like skiing downhill, but it’s like there’s a bump at the top of the hill, so you have to exert some energy to get over that bump, but once you get over the bump, then you can glide all the way down the hill. The energy that it takes to get over that bump is called activation energy. Some reactions have higher activation energies than others.
To light a match all you have to do is scrape it on the ground or on the side of the box, and that little bit of energy is enough to kick it over the bump, if you will, at the top of the hill, and allow that reaction, that exothermic reaction to give off quite a bit of heat. To light a piece of paper on fire, you have to hold the heat source up to it. Once it gets enough activation energy, the paper will light on fire and continue to burn.
So an endothermic reaction is one that requires more energy than it produces, and an exothermic reaction is one that produces more energy than it requires. Because lighting a candle takes a very small amount of energy and in turn it gives off a lot of energy, it’s still considered an exothermic reaction. There’s just an activation energy that it has to overcome to get that reaction started.
There’s another related concept that can be difficult for students to understand, catalysts or enzymes. There are a lot of models out there to explain how catalysts work. I’ve seen lock and key or that they grab two molecules and pull them together. But what many catalysts really do is lower the activation energy by giving the reaction an alternate mechanism. They make the reaction more efficient at that temperature or they allow it to occur at a lower temperature. That happens in our body … there are reactions that don’t usually happen at normal body temperature. If you add catalysts or enzymes, then they can. Catalysts can allow a reaction to occur at room temperature or body temperature by lowering the activation energy.
Hydrogen peroxide will naturally break up into water and oxygen on it’s own very slowly but if you add a catalyst you can make it happen instantaneously. It lowers the activation energy so it can occur faster at that temperature. So the lock and key models… they’re ok at an elementary level but once you start getting up into a middle school physical science class or a high school chemistry class, the students can understand activation energy and reaction mechanisms, and that’s about the time that you should start introducing the correct concept for what a catalyst or an enzyme does which is to reduce the activation energy of a reaction so that it proceeds faster or can react at a lower temperature.
Explaining exactly how a catalyst accomplishes this feat is more complicated and listeners would be directed to Wikipedia’s entry on catalysis for more information.
So thank you for joining me in this discussion on the science misconception that combustion reactions are endothermic because it takes energy to get them started.