1. Keeping Track of Energy During Chemical Change
A coherent way to treat energy in chemical reactions

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

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

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

5. Endothermic reaction
This is the standard energy diagram found in most texts.
But it doesn’t tell the whole story.

6. Energy Bar Charts
Show energy transfers between surroundings and system
Allow you to consider other energy accounts

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

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

9. Now reaction proceeds
During collisions, particles trade Eth for Ech as products are formed
After rearrangement, resulting particles move more slowly (lower Eth).

10. Consider all steps in process
1.Heating system increases Eth of reactant molecules
2.Energy is transferred from Eth to Ech now stored in new arrangement of atoms

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

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.

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

14. Exothermic reaction
System is now hotter than surroundings; energy flows out of system until thermal equilibrium is re-established

15. Consider all steps in process
1. Decrease in Ech results in increased Eth
2. System is now hotter than surroundings
3. Energy eventually moves from system to surroundings via heating

16. Contrast Conventional Diagram
This is the standard energy diagram found in most texts.
But, again,it doesn’t tell much of the story.

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 as the reaction proceeds.

18. What about a spontaneous endothermic process?
When NH4Cl dissolves in water, the resulting solution gets 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

19. Reaction useful for cold-packs
The system trades Eth for Ech
Eventually energy enters cooler system from warmer surroundings (you!)