( ( ( The energy ) ( needed ( to ( start ( a ( chemical reaction ) ) ) ) ) ) ( ( is ( called ( activation energy ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN energy)) (VP (VBN needed) (S (VP (TO to) (VP (VB start) (NP (DT a) (JJ chemical) (NN reaction))))))) (VP (VBZ is) (VP (VBN called) (NP (NN activation) (NN energy)))) (. .)))
The energy needed to start a chemical reaction is called activation energy.
(ROOT (S (NP (NP (DT The) (NN amount)) (PP (IN of) (NP (NP (NN energy)) (VP (VBN required) (S (VP (TO to) (VP (VB begin) (NP (DT a) (JJ chemical) (NN reaction))))))))) (VP (VBZ is) (VP (VBN known) (PP (IN as) (NP (DT the) (NN activation) (NN energy))))) (. .)))