The mechanism for the conversion of compound P to compound S consists of the following elementary steps:
P → Q ΔH = +40 kJ
Q → R ΔH = −70 kJ
R → S ΔH = −15 kJ
Based on these values, what is the overall enthalpy change, ΔHrxn∘, for the reaction P → S ?
a) −30kJ
b) −45kJ
c) +40kJ
d) −15kJ