# How is the energy of reactants in a chemical reaction found ?

A pretty rudimentary question, but I'm reviewing the concept of energy diagrams and activation energy and I'm curious to know how chemists find the energy of reactants and of a transition state in a laboratory.

What kinds of instruments are used ? As well, aren't transition states only something we can imagine ? So how is it that chemistry are able to find their energy in a lab ?

Energy itself is usually not measured, but rather change in energy ($\Delta E$) is the quantity of practical importance. One may be interested in change in free energy as well, $\Delta G$, changes in enthaly $\Delta H$, and even change in entropy $\Delta S$.
A common way to evaluate the increased energy of transition state ($\Delta E^{\ddagger}$) above the reactant is via the Arhennius Plot. Here, one evaluates the rates of reaction ($k$) at a set of different temperatures ($T$ in Kelvin). It is fairly easy to show that a plot of the $\ln(k)$ vs. $1/T$ gives a line whose slope is related to $\Delta H^{\ddagger}$, and where the intercept is related to $\Delta S^{\ddagger}$. (The ideal gas constant is in here too, but it's easy to accomodate.)