Which of the following statements is not correct about Hess's Law of constant heat summation:
Correct answer: D. It tells whether the reaction is exothermic or endothermic.
- A. It helps in calculating the heat of reaction.
- B. It helps in calculating the heat of combustion.
- C. It helps in calculating the heat of formation.
- D. It tells whether the reaction is exothermic or endothermic.
Explanation
Hess's Law of constant heat summation is a fundamental principle that allows for the calculation of the total enthalpy change of a chemical reaction, regardless of the pathway taken, by summing the enthalpies of intermediate steps. It is widely used to calculate the heat of reaction, combustion, and formation. However, it does not inherently indicate whether a reaction is exothermic or endothermic. This requires knowledge of the sign and magnitude of the calculated enthalpy change. Therefore, the correct answer is Option D: It tells whether the reaction is exothermic or endothermic, as this is not a direct application of Hess's Law.
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About Thermochemistry and Energetics of Chemical Reactions
Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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