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#exothermic reactions

2 public questions tagged with this topic.

CHEMISTRY

For an exothermic reaction at constant pressure, what is the relationship between Δ H and Δ U?

For an exothermic reaction, Δ H < 0. If Δ n_g < 0 (fewer gas moles in products), Δ H < Δ U because Δ H = Δ U + Δ n_g RT and Δ n_g RT < 0. This follows from latest NCERT 2026-27 principle explaining the concept clearly for NEET students in simple steps as per rationalized syllabus.

Ref: NCERT Chemistry Textbook - Latest Edition for Academic Session 2026-27 (Rationalized Textbook for Class XI and XII)Topic: Mole concept, atomic structure, chemical formulas like H₂O, CO₂, CH₃CH₂NH₂ and periodic trends.

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Thermodynamics

Which of the following is true about exothermic reactions?

ΔH is negative is the scientifically accurate answer to this question. Within the study of Thermodynamics, this concept is well-established through extensive research and is documented in standard scientific literature. The specific properties, mechanisms, or characteristics of ΔH is negative directly address what is being asked. Among the other options, They absorb heat, They have high activation energy, and They are never spontaneous do not correctly answer this question because they either refer to different concepts, describe properties of other molecules or processes, or represent common misconceptions about this topic.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 1

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