Thermochemistry check: classify the reaction C (solid) + O2 (gas) → CO2 (gas) in terms of heat effect.

Introduction:
Combustion reactions are central to energy engineering. Identifying whether a reaction releases or absorbs heat is a basic thermochemistry skill with direct safety and process control implications.

Given Data / Assumptions:

• Reaction: carbon burning completely to carbon dioxide.
• Standard conditions for enthalpy of formation.
• No catalysts or intermediates specified.

Concept / Approach:
The standard enthalpy of formation of CO2 is negative, reflecting heat release when forming stable CO2 from elemental carbon and oxygen. Hence, the reaction is exothermic. Autocatalysis refers to reactions accelerated by one of their own products, which is not implied here.

Step-by-Step Solution:
Use enthalpy of formation: ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants).ΔH°f(CO2) ≈ −393.5 kJ/mol; ΔH°f(C) = 0; ΔH°f(O2) = 0.Compute: ΔH°rxn ≈ −393.5 kJ/mol ⇒ heat released ⇒ exothermic.

Verification / Alternative check:
Practical observation: burning carbon (e.g., coal, charcoal) releases heat readily used for power and heating applications.

Why Other Options Are Wrong:

• Endothermic: would require heat input, contrary to data.
• Autocatalytic: no product-catalyzed pathway indicated.
• None of these: incorrect because exothermic is correct.

Common Pitfalls:
Confusing formation of CO with CO2; incomplete combustion to CO still releases heat but less than complete combustion to CO2.

Final Answer:
Exothermic