As my previous post was a bit unclear and unspecific, I am reformulating it. First of all this is a question that I had while reading some concepts my book introduces, so this isn't a homework question in anyway.
The context is: I have started reading the thermochemistry chapter of my book, it starts explaining the concept of heat and how a calorimeter works. After that, it shows the story behind Hess' Law and says that one of the reasons it was created is because a calorimeter can't be used to measure the enthalpy variation of too slow reactions, explosive reactions and too fast reactions. As a result, there was a need to create an indirect way of calculating the enthalpy variation, and this problem could be solved with Hess Law.
Now, I know why we can't use the calorimeter for too slow reactions and explosive reactions, but I would like to understand what is the problem with too fast reactions. Too fast reactions releases/absorbs heat on a small time period, but this shouldn't be a problem, because the calorimeter will get at equilibrium as other reactions.
As suggested, I am adding the paragraph of the textbook:
As we saw, the heat involved in a transformation can be determined using a calorimeter. However, for many reasons, the calorimeter can't be used to determine the heat for every chemical reaction. There are situations where the reaction can be too slow or fast, can be explosive, it can even occur simultaneously with others undesirable reaction, what makes it impossible to measure the enthalpy variation for a specific reaction. How can we determine the heat involved in these type of transformations?
( I am brazilizan and the book is called Química: Cotidiano e Transformações)
So, what is the problem with too fast reactions?