Why does Hess's law allow you to determine the enthalpy change of a reaction?

The most important agent of erosion is? What goes in dry and comes out wet the longer it is the stronger it gets? What is The lining of the body cavities? What is the paint i.e chemical composition used in color shifting toys? Is a black hole living or non living? What is the total acidity of sapota wine? How many yards are in 3 4 of a mile? Does wine ferment in your stomach? Which one has pH value greater than 10.5 A blood B lemon juice C milk D lime water? What happens to diaphragm and the chest cavity when you inhale? Does the number of protons equal the periodic number? What can you infer about autotrophic organisms? What is the anatomy of an organ? What are some example of density? Who put forward a theory about black holes? What direction do you have to face a bird house? What is the smallest thing ever measured? Does the paramecium have a respiratory system? What river that flows by Vienna Budapest and Belgrade and then along the border between Bulgaria and Romania into the Black Sea? What stimuli do your eyes respond to?

Why does Hess's law allow you to determine the enthalpy change of a reaction?