A chemist can convert substance A to substance D by one of four two-step pathways

CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics

Problem

A chemist can convert substance A to substance D by one of four two-step pathways. Enthalpy changes for each elementary step are given in the table. Path I: A →\rightarrow B +40 kJ+40\,\text{kJ} B →\rightarrow D −120 kJ-120\,\text{kJ} Path II: A →\rightarrow C −60 kJ-60\,\text{kJ} C →\rightarrow D −10 kJ-10\,\text{kJ} Path III: A →\rightarrow E +30 kJ+30\,\text{kJ} E →\rightarrow D +10 kJ+10\,\text{kJ} Path IV: A →\rightarrow F −70 kJ-70\,\text{kJ} F →\rightarrow D +25 kJ+25\,\text{kJ} Which pathway is most exothermic overall? (a) Path IV (b) Path I (c) Path III (d) Path II

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