reactivity vs sensitiveness

reactivity and sensitiveness both are nouns.

Word	Noun	Adjective	Verb	Adverb
reactivity	Yes	No	No	No
sensitiveness	Yes	No	No	No

As nouns, sensitiveness is a hypernym of reactivity; that is, sensitiveness is a word with a broader meaning than reactivity:

reactivity: responsive to stimulation
sensitiveness: (physiology) responsiveness to external stimuli; the faculty of sensation

Other hypernyms of reactivity include sensibility, sensitivity.

reactivity (noun)	sensitiveness (noun)
responsive to stimulation	the ability to respond to physical stimuli or to register small physical amounts or differences
ready susceptibility to chemical change	the ability to respond to affective changes in your interpersonal environment
	(physiology) responsiveness to external stimuli; the faculty of sensation
	sensitivity to emotional feelings (of self and others)