According to my experience in this area , the dry Neem leaves were soked in distilled Water ford at least 7 hours (optimum time) at room temperature, filtered, and added to an aqueous  HCl solution with a final concentration of 1.0 M. You can use other acids like HNO3 or H2SO4.  

You may want to search for the terms "enthalpy of solution" or "enthalpy of mixing".  Any good physical chemistry textbook, as well as wikipedia, ought to help.

Try this for starters: http://www.ausetute.com.au/heatsolution.html

Thank you Greg