Keeping Track of Energy During Chemical Change

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 • Reconnect kinetic 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 Graphs • Show energy transfers between surroundings and system • Allow you to consider other energy accounts

7. Consider role of Ek • 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 Ek • Hotter, faster molecules (surroundings) transfer energy to colder, slower molecules (system) • Now reactant molecules are sufficiently energetic to produce reaction Note intermediate stage

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

10. Consider all steps in process 1.Heating system increases Ek of reactant molecules 2.Energy is transferred from Ek to Ech now stored in new arrangement of atoms 3. Resulting system is cooler - requires continued heating to bring Ek back up to level required to sustain reaction

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 Ek • When reactant molecules collide to produce products that store less energy, new molecules move away more rapidly Note intermediate stage

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 Echresults in increased Ek 2. System is now hotter than surroundings 3. Energy is transferred 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 during course of the reaction

18. What about a spontaneous endothermic process? • When NH4Cl dissolves in water, the resulting solution gets much colder • What caused the Ek to decrease? • Some Ek 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 Ek for Ech • Eventually energy enters cooler system from warmer surroundings (you!)