( It ( ( is ( ( the ( second law ) ) ( of ( thermodynamics ( that ( explains ( why ( ( chemical reactions ) occur ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (NP (NP (DT the) (JJ second) (NN law)) (PP (IN of) (NP (NP (NNS thermodynamics)) (SBAR (WHNP (WDT that)) (S (VP (VBZ explains) (SBAR (WHADVP (WRB why)) (S (NP (NN chemical) (NNS reactions)) (VP (VBP occur))))))))))) (. .)))
It is the second law of thermodynamics that explains why chemical reactions occur.
(ROOT (S (NP (NNP Chemical) (NNS reactions)) (VP (VBP follow) (NP (NP (DT the) (NNS laws)) (PP (IN of) (NP (NNS thermodynamics))))) (. .)))