In the previous question, you should have concluded that cold packs undergo an ENDOTHERMIC reaction. This is a reaction that absorbs heat from its surroundings and has an "uphill" energy diagram, characterized by a positive (t) value for H. Here's where things get really interesting: The chemical reaction inside a cold pack is SPONTANEOUS despite the fact that it has a positive H value. This is strange! Why does the reaction keep going on its own, even though it must pull energy from its surroundings to do that? Let's look at why: Recall from the lecture that to be spontaneous a reaction must have a negative value for G : G=HTS Now you can answer this question: How is possible for G to be negative if H is positive? (Remember, T is in Kelvin, so T is always positive.) Choose the correct ending to this statement: For G to be negative when H is positive... PS - There is a short TED-Ed talk on cold packs that doesn't completely answer this question, but you might enjoy it. S must be a big positive value. (Large increase in entropy) S must be a big negative value. (Large decrease in entropy)