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 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 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 + CaCO 3  CaO + CO 2 (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 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 + CaCO 3  CaO + CO 2 (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. 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 Show energy transfers between surroundings and system Allow you to consider other energy accounts

7 Consider role of E th How does heating the reactants result in an increase in E ch ? Energy to rearrange atoms in molecules must come from collisions of molecules Low energy collisions are unlikely to produce molecular rearrangement How does heating the reactants result in an increase in E ch ? 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 E th Hotter, faster molecules (surroundings) transfer energy to colder, slower molecules (system) Now reactant molecules are sufficiently energetic to produce reaction 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 E th for E ch as products are formed After rearrangement, resulting particles move more slowly (lower E th ). During collisions, particles trade E th for E ch as products are formed After rearrangement, resulting particles move more slowly (lower E th ).

10 Consider all steps in process 1.Heating system increases E th of reactant molecules 2.Energy is transferred from E th to E ch now stored in new arrangement of atoms 1.Heating system increases E th of reactant molecules 2.Energy is transferred from E th to E ch 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 E ch than the reactants CH 4 + 2O 2  CO 2 + 2H 2 O + energy 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 E ch than the reactants CH 4 + 2O 2  CO 2 + 2H 2 O + energy

12 Exothermic reaction CH 4 + 2O 2  CO 2 + 2H 2 O + energy Place energy bars for E ch 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. CH 4 + 2O 2  CO 2 + 2H 2 O + energy Place energy bars for E ch 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 E th When reactant molecules collide to produce products that store less energy, new molecules move away more rapidly Now take into account changes in E th 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 E ch results in increased E th 2. System is now hotter than surroundings 3. Energy eventually moves from system to surroundings via heating 1. Decrease in E ch results in increased E th 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. 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. 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 NH 4 Cl dissolves in water, the resulting solution gets colder What caused the E th to decrease? Some E th of water required to separate ions in crystal lattice. Resulting solution has greater E ch than before When NH 4 Cl dissolves in water, the resulting solution gets colder What caused the E th to decrease? Some E th of water required to separate ions in crystal lattice. Resulting solution has greater E ch than before

19 Reaction useful for cold-packs The system trades E th for E ch Eventually energy enters cooler system from warmer surroundings (you!) The system trades E th for E ch Eventually energy enters cooler system from warmer surroundings (you!)