Calculate ΔHo for the chemical reaction:

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SUMMARY

The calculation of ΔH° for the chemical reaction is determined using the formula ΔH° = ∑ΔHBroken - ∑ΔHFormed. The correct calculation yields -108 kJ, confirming that the answer key is accurate. The confusion arises from the distinction between bond enthalpy and ΔH°, where a positive bond enthalpy indicates a negative ΔH°. This highlights the importance of understanding the relationship between bond enthalpies and reaction enthalpies in thermodynamics.

PREREQUISITES
  • Understanding of thermodynamic principles, specifically enthalpy changes.
  • Familiarity with bond enthalpy concepts and calculations.
  • Knowledge of chemical reaction equations and stoichiometry.
  • Ability to perform basic arithmetic operations with thermodynamic values.
NEXT STEPS
  • Study the concept of bond enthalpy in detail, focusing on its implications in chemical reactions.
  • Learn about Hess's Law and its application in calculating reaction enthalpies.
  • Explore advanced thermodynamic concepts, including Gibbs free energy and its relation to ΔH°.
  • Practice calculating ΔH° for various chemical reactions using different methods.
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Chemistry students, educators, and professionals seeking to deepen their understanding of thermodynamics and reaction enthalpies.

AMan24
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Homework Statement


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Homework Equations


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The Attempt at a Solution


2(255) - [(1)(159) + (1)(243)]
= +108KJ

The answer key says its b) -108KJ. Am i doing something wrong or is the book wrong?
 
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When using bond enthalpies, the way you calculate the heat of reaction is
\Delta H° = \sum \Delta H_{\textrm{Broken}} - \sum \Delta H_{\textrm{Formed}}
As in, "the sum of enthalpy changes by broken bonds minus the sum of enthalpy changes by formed bonds." So, the correct answer is -108 kJ.
 
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ΔH° is not the same thing as a bond enthalpy. Quite the opposite - positive bond enthalpy means negative ΔH° (think why).
 
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