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Topic 5: Energetics/Thermochemistry

5.1 Measuring Energy Changes
Essential Idea:
  • The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.
  • Heat is a form of energy.
  • Temperature is a measure of the average kinetic energy of the particles.
  • Total energy is conserved in chemical reactions.
  • Chemical reactions that involve transfer of heat between the system and the surroundings are described as endothermic or exothermic.
  • The enthalpy change for chemical reactions is indicated in kJ/mol.
  • Enthalpy change values are usually expressed under standard conditions, given by H, including standard states.
Applications & Skills:
  • Calculation of the heat change when the temperature of a pure substance is changed.
  • A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated.
Nature of Science:
  • Fundamental principle—conservation of energy is a fundamental principle of science.
  • Making careful observations—measurable energy transfers between systems and surroundings.
PPT: 5.1 Intro to Enthalpy
PPT: 5.1 Enthalpy Calculations from Lab Data
PPT: 5.1 Enthalpy of Formation Equations & Reactions
Crash Course Enthalpy
Video Lesson: Thornley
Video Lab: Show Sample Lab Calculations
Enthalpy Practice Worksheet
5.2 Hess's Law
Essential Idea:
  • In chemical transformations energy can neither be created nor destroyed (the first law of thermodynamics).
  • The enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual steps.
Applications & Skills:
  • Application of Hess’s Law to calculate enthalpy changes.
  • Calculation of H reactions using Hf data.
  • Determination of the enthalpy change of a reaction that is the sum of multiple reactions with known enthalpy changes.
Nature of Science:
  • Hypotheses—based on the conservation of energy and atomic theory, scientists can test the hypothesis that if the same products are formed from the same initial reactants then the energy change should be the same regardless of the number of steps.
PPT: Hess's Law
Thornley Video: Hess's Law
Khan Academy: Hess's Law Example
Hess's Law Online Practice Problems
Additional Hess's Law Practice with Answer Key
5.3 Bond Enthalpies
Essential Idea:
  • Energy is absorbed when bonds are broken and is released when bonds are formed.
  • Bond-forming releases energy and bond-breaking requires energy.
  • Average bond enthalpy is the energy needed to break one mol of a bond in a gaseous molecule averaged over similar compounds.
Applications & Skills:
  • Calculation of the enthalpy changes from known bond enthalpy values and comparison of these to experimentally measured values.
  • Sketching and evaluation of potential energy profiles in determining whether reactants or products are more stable and if the reaction is exothermic or endothermic.
  • Discussion of the bond strength in ozone relative to oxygen in its importance to the atmosphere.
Nature of Science:
  • Models and theories—measured energy changes can be explained based on the model of bonds broken and bonds formed. Since these explanations are based on a model, agreement with empirical data depends on the sophistication of the model and data obtained can be used to modify theories where appropriate.
PPT: 5.3 Bond Enthalpy
Video Lesson: Thornley Bond Enthalpy
Bond Enthalpy Online Simulation