I’m curious about the principles behind heat release and absorption in chemical reactions. What determines whether a reaction releases heat (exothermic) or absorbs heat (endothermic)? Is it related to specific types of bonds breaking or forming, or are there other factors involved?
In: Chemistry
It mostly depends on which bonds form and which bonds break.
Burning hydrogen and oxygen to water can seen as a three-step process:
* Split two hydrogen molecules apart into four hydrogen atoms. Hydrogen attracts other hydrogen, so splitting them needs a bit of energy.
* Split one oxygen molecule into two oxygen atoms. Oxygen attracts other oxygen, so splitting it needs a bit of energy.
* Let each oxygen atom react with two hydrogen atoms. Hydrogen and oxygen strongly attract each other, so this step releases a lot of energy. Some of that energy ends up splitting more hydrogen and oxygen (which continues the reaction), the rest ends up as heat.
It’s a bit like rearranging magnets – some arrangements have the magnets all aligned nicely, going from a more chaotic arrangement to this ordered layout will make the magnets snap together quickly, releasing some energy as heat (and sound).
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