explain how water's ability to absorb heat is used to measure energy changes in chemical reactions?

Answer 1

When an exothermic reaction takes place some amount of energy is released. If you measure the elevation of a quantity water you should know the energy released by the formula
E = mcdt m mass of water, c = 4.18J/g, elevation of temperature: dt

Answer 2

Q=mcdT

basicly by using this relation u can estimate the heat transfer but the system ull have during investigations would be more complex. probably u ll have a jacket around ur reactor so heat transffered will be expressed by heat transfer coefficients for different reactor hydrodynamics. if u give details about ur reactor a more specific modelling can be made.