Keeping Track of Energy During      Chemical Change      A coherent way to treat energy in             chemical reactions ...
The Conventional Approach• Treatment of energy in reactions is  vague  • Where/how is energy stored is left    unanswered ...
Modeling Approach• Use energy bar diagrams to represent energy  accounts at various stages of reaction• Provide mechanism ...
Endothermic reaction• How do you know on which side to write the  energy term?   • If you had to supply energy to the reac...
Endothermic reaction• This is the standard energy diagram  found in most texts.• But it doesn’t tell the whole story.     ...
Energy Bar Charts• Show energy transfers between  surroundings and system• Allow you to consider other energy  accounts   ...
Consider role of Eth• How does heating the reactants  result in an increase in Ech?• Energy to rearrange atoms in  molecul...
Heating system increases Eth• Hotter, faster molecules (surroundings) transfer  energy to colder, slower molecules (system...
Now reaction proceeds• During collisions, particles trade Eth  for Ech as products are formed• After rearrangement, result...
Consider all steps in process1.Heating system increases Eth of reactant  molecules2.Energy is transferred from Eth to Ech ...
Exothermic reaction• How do you know on which side to write the  energy term?   • If energy flows from system to surroundi...
Exothermic reaction• CH4 + 2O2 → CO2 + 2H2O + energy   • Place energy bars for Ech   • Postpone (for now) examination of e...
Exothermic reaction• Now take into account changes in Eth   • When reactant molecules collide to produce     products that...
Exothermic reaction• System is now hotter than surroundings;  energy flows out of system until thermal  equilibrium is re-...
Consider all steps in process1. Decrease in Ech results in increased Eth2. System is now hotter than surroundings3. Energy...
Contrast Conventional Diagram• This is the standard energy diagram found in  most texts.• But, again,it doesn’t tell much ...
But what about energy used      to start reaction?• Save activation energy for later - in the  study of reaction kinetics ...
What about a spontaneous endothermic process?• When NH4Cl dissolves in water, the  resulting solution gets much colder• Wh...
Reaction useful for cold-packs• The system trades Eth for Ech• Eventually energy enters cooler system  from warmer surroun...
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Keeping Track of Energy in Chemical Reactions

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Presentation by Larry Dukerich on how to represent energy transfer during chemical reactions. This presentation is part of the training provided by the Modeling Instruction in Chemistry Program.

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Keeping Track of Energy in Chemical Reactions

  1. 1. Keeping Track of Energy During Chemical Change A coherent way to treat energy in chemical reactions 1
  2. 2. The Conventional Approach• Treatment of energy in reactions is vague • Where/how is energy stored is left unanswered • How energy is transferred between system and surroundings is ignored 2
  3. 3. Modeling Approach• Use energy bar diagrams to represent energy accounts at various stages of reaction• Provide mechanism for change• Connect thermal and chemical potential energy• Focus on what is happening during the course of the reaction 3
  4. 4. Endothermic reaction• How do you know on which side to write the energy term? • If you had to supply energy to the reactants, the products store more energy energy + CaCO3 → CaO + CO2 (g) • If uncertain, use analogy from algebra If 3 + y = x, which is greater, y or x? • Consistent with generalization that separated particles have more energy 4
  5. 5. Endothermic reaction• This is the standard energy diagram found in most texts.• But it doesn’t tell the whole story. 5
  6. 6. Energy Bar Charts• Show energy transfers between surroundings and system• Allow you to consider other energy accounts 6
  7. 7. Consider role of Eth• How does heating the reactants result in an increase in Ech?• Energy to rearrange atoms in molecules must come from collisions of molecules• Low energy collisions are unlikely to produce molecular rearrangement 7
  8. 8. Heating system increases Eth• Hotter, faster molecules (surroundings) transfer energy to colder, slower molecules (system)• Now reactant molecules are sufficiently energetic to produce reaction 8
  9. 9. Now reaction proceeds• During collisions, particles trade Eth for Ech as products are formed• After rearrangement, resulting particles move more slowly (lower Eth). 9
  10. 10. Consider all steps in process1.Heating system increases Eth of reactant molecules2.Energy is transferred from Eth to Ech now stored in new arrangement of atoms3. Resulting system is cooler - requires continued heating to bring Eth back up to level required to sustain reaction 10
  11. 11. Exothermic reaction• How do you know on which side to write the energy term? • If energy flows from system to surroundings, then the products must store less Ech than the reactants • CH4 + 2O2 → CO2 + 2H2O + energy 11
  12. 12. Exothermic reaction• CH4 + 2O2 → CO2 + 2H2O + energy • Place energy bars for Ech • Postpone (for now) examination of energy required to initiate reaction. • Like consideration of the motion of a ball the moment it begins to roll downhill - don’t worry about initial push. 12
  13. 13. Exothermic reaction• Now take into account changes in Eth • When reactant molecules collide to produce products that store less energy, new molecules move away more rapidly 13
  14. 14. Exothermic reaction• System is now hotter than surroundings; energy flows out of system until thermal equilibrium is re-established 14
  15. 15. Consider all steps in process1. Decrease in Ech results in increased Eth2. System is now hotter than surroundings3. Energy eventually moves from system to surroundings via heating 15
  16. 16. Contrast Conventional Diagram• This is the standard energy diagram found in most texts.• But, again,it doesn’t tell much of the story. 16
  17. 17. But what about energy used to start reaction?• Save activation energy for later - in the study of reaction kinetics • If this really bothers you, ask yourself how the energy used to start the reaction compares to energy released during course of the reaction 17
  18. 18. What about a spontaneous endothermic process?• When NH4Cl dissolves in water, the resulting solution gets much colder• What caused the Eth to decrease? • Some Eth of water required to separate ions in crystal lattice. • Resulting solution has greater Ech than before 18
  19. 19. Reaction useful for cold-packs• The system trades Eth for Ech• Eventually energy enters cooler system from warmer surroundings (you!) 19

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