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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?

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    $\begingroup$ Possible duplicate of Question About Calorimeter and Fast Reactions $\endgroup$ – Buck Thorn Sep 3 at 6:25
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    $\begingroup$ Too fast reactions will start (and probably end) before you close the lid, which is bad for the precision. $\endgroup$ – Ivan Neretin Sep 3 at 9:41
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    $\begingroup$ @IvanNeretin Exactly! That is why I said this in my comment to the OP’s other same question: “So, provided a fast reaction does not start unless and until you trigger it in the calorimeter, I do not see a problem. The calorimeter simply makes a heat measurement. Your intuition on this seems correct: the water temperature will change accordingly.” $\endgroup$ – Ed V Sep 3 at 12:23
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    $\begingroup$ Suppose the “fast” reaction was that of vinegar and baking soda. It would be quite comical and futile to mix them in the calorimeter, get the top properly screwed on, etc. But, change the procedure to add a starting time delay: put the measured quantity of vinegar in the calorimeter, drop in the baking soda in gelatin capsules, properly assemble the calorimeter, etc., and then the reaction will start when enough gelatin dissolves. So, reaction speed is not necessarily a problem and the book’s advice was not definitive. $\endgroup$ – Ed V Sep 3 at 13:33
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    $\begingroup$ So my book is wrong? I have seen other books that don't say anything about fast reactions, they just point out for too slow and explosive reactions. $\endgroup$ – Sonim Blenim Sep 3 at 15:56

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