( ( ( ( ( Sometimes ( ( that reaction ) ( is positive ) ) ) , ) and ) ( sometimes ( it ( is negative ) ) ) ) . )
(ROOT (S (S (ADVP (RB Sometimes)) (NP (DT that) (NN reaction)) (VP (VBZ is) (ADJP (JJ positive)))) (, ,) (CC and) (S (ADVP (RB sometimes)) (NP (PRP it)) (VP (VBZ is) (ADJP (JJ negative)))) (. .)))
Sometimes that reaction is positive, and sometimes it is negative.
(ROOT (S (S (VP (VBZ Is) (NP (NP (DT the) (NN heat)) (PP (IN of) (NP (NP (NN reaction)) (PP (IN for) (NP (DT an) (JJ endothermic) (NN reaction)))))))) (VP (VBZ is) (ADJP (JJ positive))) (. .)))