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Published byHugo Butler Modified over 9 years ago
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Keeping Track of Energy During Chemical Change
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– 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
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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 + Na 2 HCO 3 NaOH + CO 2 (g) Think of an algebraic equation If 3 + y = x, which is greater, y or x? Consistent with generalization that separated particles have more energy
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Endothermic reaction This is the standard energy diagram found in most texts. But it doesn’t tell the whole story.
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Energy Bar Charts Show energy transfers between surroundings and system Allow you to consider other energy accounts
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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
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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
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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
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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 ).
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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
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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 CaO + H 2 O Ca(OH) 2 + energy
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Exothermic reaction CaO + H 2 O Ca(OH) 2 + energy – Place energy bars for E ch
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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
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Exothermic reaction – System is now hotter than surroundings; energy flows out of system until thermal equilibrium is re- established
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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
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Contrast Conventional Diagram This is the standard energy diagram found in most texts. But, again,it doesn’t tell much of the story
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What about a spontaneous endothermic process? When Baking Soda (NaHCO 3 ) is added to vinegar (HC 2 H 3 O 2 ), the resulting reaction gets colder What caused the E th to decrease? – Some E th of vinegar was needed to separate the bonds in the reactants – Resulting products have greater E ch than before
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Reaction useful for cold-packs The system trades E th for E ch Eventually energy enters cooler system from warmer surroundings (you!)
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