Combustion thermochemistry: For a typical exothermic combustion process carried out at comparable conditions, the internal energy of the combustion products is __________ relative to that of the reactants.

Introduction:
Understanding how energy is stored and released in combustion is central to thermodynamics and reactor design. When fuels burn, chemical bonds rearrange and energy is liberated as heat and (often) work. This question focuses on the internal energy comparison between products and reactants for exothermic combustion under comparable reference conditions.

Given Data / Assumptions:

• Combustion is exothermic under standard reference states.
• Comparison is made at the same reference temperature and pressure unless otherwise stated.
• No shaft work extraction during the comparison; we are comparing state functions.

Concept / Approach:
For an exothermic reaction, the change in internal energy is negative at a given temperature: ΔU = U_products − U_reactants < 0. Similarly, the enthalpy change ΔH is negative. Physically, this means the products, at the same reference temperature, contain less chemical energy (lower internal energy) than the reactants, with the difference released as heat to the surroundings (or converted to work in engines/turbines).

Step-by-Step Solution:
Write the first law for a reacting system at constant T for state comparison: ΔU = U_p − U_r.Exothermic combustion → ΔU < 0.Therefore, U_p < U_r ⇒ products have less internal energy than reactants (at the same T, reference state).

Verification / Alternative check:
Standard heats of combustion (negative values) tabulated at 25°C imply products are at a lower internal energy level. Any temperature rise during actual combustion is a process effect; when both sides are brought back to the same T, the energy difference remains negative.

Why Other Options Are Wrong:

• More/equal: Contradict exothermicity at equal reference conditions.
• “Depends on state”: While temperature affects U, at equal reference T the sign is fixed (less).
• “Indeterminate”: With the exothermic assumption and equal reference T, the determination is clear.

Common Pitfalls:
Confusing process temperature rise (hot products) with state comparison at the same T; internal energy is a state function, so compare at equal T and P.

Final Answer:
less