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Understanding Hess's Law and Enthalpy Changes in Reaction Processes

This tutorial explores Hess’s Law, which asserts that the enthalpy change of a reaction only depends on the initial and final states of the system, independent of the reaction pathway. Through a series of reactions involving K2CO3 and HCl, we calculate the heat evolved and absorbed, demonstrating the application of Hess's Law. The tutorial includes detailed calculations for enthalpy changes (ΔH) in various reactions, highlighting important principles in thermochemistry relevant to ongoing studies in chemistry.

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Understanding Hess's Law and Enthalpy Changes in Reaction Processes

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  1. Tutorial 5 Answer Presentation Hu Peiyu 12S7F

  2. Question 2 • a) Hess’s law states that the enthalpy change (ΔH) of a particular reaction is determined only by the initial and final state of the system regardless of the pathway taken.

  3. Question 2 • b) i) K2CO3+2HCl→2KCl+H2O+CO2 • ii) Heat evoved: q=m∙c∙ΔH=V/ρ∙c∙ΔT =30.0/1.0*4.2*5.2 =655J ΔHr=-q/n=625J/0.020mol=-32.8KJ/mol

  4. Question 2 • iii) because HCl is in excess in this reaction, the concentration of it need not to be exact.

  5. Question 2 • c) i) KHCO3+HCl→KCl+H2O+CO2 • ii) Heat absorbed: q=m∙c∙ΔH=m∙c∙ΔT =30.0*4.2*3.7 =466.2 ΔH=466.2/0.020=23.3KJ/mol

  6. Question 2 • d) Energy/ kJmol-1 K2CO3+2HCl ΔH=-32.8kJ Hd=? KCl+H2O+CO2 ΔH=23.3kJ KHCO3+HCl

  7. Question 2 • By Hess’s law • Hd(KHCO3)=23.3kJ-(-32.8kJ)=56.1kJ

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