Keeping Track of Energy During Chemical Change

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 • How energy is transferred between system and surroundings is ignored

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

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

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 molecules must come from collisions of molecules • Low energy collisions are unlikely to produce molecular rearrangement

Heating system increases Eth • Hotter, faster molecules (surroundings) transfer energy to colder, slower molecules (system) • Now reactant molecules are sufficiently energetic to produce reaction

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

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

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

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.

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

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

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

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

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.

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

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

Keeping Track of Energy During Chemical Change