Why don't endothermic reactions violate the 2nd Law of Thermodynamics?



Answers:
well, it is possible to decrease entropy in a reaction. if i cool some water down and it freezes, then my highly disordered liquid forms a highly ordered crystal, and the entropy of my water is lower.

but, you have to consider the statement of the second law, which is that the entropy of the *universe* is always increasing. it costs you some energy to cool your water down - you have to run a freezer or something - and the production of this energy at some other point in the process comes with an increase in entropy that is greater than the decrease you get from freezing.

as for an endothermic reaction, i am not sure which reaction you are speaking of. so i will give a simple example of an endothermic reaction:

H2O (l) -> H2O (g)

when water evaporates, it cools its container and draws heat in. however, this is accompanied by an increase in the entropy, since you go from a more-ordered (liquid) state to a less-ordered (gaseous) state.

typically, an endothermic reaction will involve some kind of phase change, where for example you mix two solids together and you end up with a liquid and a solid. here is an example of that:
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/ENDO2/PAGE1.HTM

so, the random thermal energy is converted into useful ordered bond energy - this is a decrease in entropy. but it comes at the expense of breaking *other* bonds and using their energy, and an overall increase in entropy of the system.
Because you have to consider the entire system that the reaction is taking place in...and the fact that energy comes in many forms