Thermochemistry — Which of the following processes is exothermic under standard conditions?

Introduction / Context:
Determining whether a reaction is exothermic or endothermic is central to reactor design, safety, and energy balances. Exothermic reactions release heat (negative enthalpy change), while endothermic reactions absorb heat. This question asks you to evaluate several well-known transformations and identify if any are exothermic as stated.

Given Data / Assumptions:

• Standard thermochemical data trends apply.
• No catalysts or alternate pathways that could change overall enthalpy sign.
• Physical states are standard (graphite is the stable allotrope of carbon at STP).

Concept / Approach:

Graphite to diamond raises enthalpy (diamond is metastable and higher in energy) → endothermic. Splitting water (electrolysis) requires energy input to break strong O–H bonds → endothermic. Dehydrogenation removes H2 from alkanes to form alkenes; this generally absorbs heat (breaking C–H and forming a C=C and H–H results in positive enthalpy) → endothermic. Hence, none of the listed processes are exothermic as written.

Step-by-Step Solution:

Verification / Alternative check:

Standard enthalpies of formation confirm positive ΔH for these transformations. Industrial dehydrogenations are run in fired heaters to supply heat.

Why Other Options Are Wrong:

A/B/C are individually endothermic. Option E (not among the original choices) can be exothermic but is not offered as the principal selection; the correct answer among the listed is “none of these”.

Common Pitfalls:

Assuming all phase or structural changes are exothermic; overlooking that bond-breaking steps generally require input energy.

Final Answer:

none of these