Reaction pathway diagrams indicate energy changes for specific reactions

Steps:

• Examine the diagram's energy profile: rising energy from reactants to products signals an endothermic reaction.
• Identify reactants and products: water vapor breaking into hydrogen and oxygen atoms matches atomic dissociation.
• Check stoichiometry: 1 H₂O yields 2 H and 1 O, aligning with the pathway's labeled intermediates.
• Confirm direction: forward path shows bond breaking, not formation.

Why B is correct:

• It represents the endothermic dissociation of H₂O(g) into atoms, where energy input breaks O-H bonds, per bond energy definitions.

Why the others are wrong:

• A: Exothermic formation of water, with energy release, opposite of an uphill diagram.
• C: Exothermic recombination of O atoms, showing energy decrease.
• D: Endothermic but produces molecular H₂ and O₂, not atomic products.

Final answer: B