[Sonim194c]

My book introduces Thermochemistry with the concept of heat and how a calorimeter works. After that, it explains the story behind Hess Law and says that one of the reasons it was created is because a calorimeter can't calculate fast chemical reactions, but it doesn't really say the problem with it. Why can't we calculate fast chemical reactions? Won't the water in the calorimeter heat up or cool down after some time?

[Vidya]

There are many reactions which can not be carried out in water like combustion or single replacement reactions and many more reactions of different types.Hess's law helps in calculating  enthalpy change for any reaction.

[Sonim194c]

But what is the reason for fasst reactions?

[Vidya]

In fast reactions may heat lost to the surrounding is more which results in error.

[Sonim194c]

I thought the heat loss happened to every reaction. Why the lost is more with fast reactions?

[Vidya]

Due to more heat released in less time which results in more dissipation in surroundings.

[Sonim194c]

Ohh, many thanks!! So we would need time for all water be at the same temperature, but some heat is lost for the surroundings at the process.