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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. - PowerPoint PPT Presentation
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Keeping Track of Energy During Chemical ChangeKeeping Track of Energy During Chemical Change
A coherent way to treat energy in chemical reactions
A coherent way to treat energy in chemical reactions
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The Conventional Approach
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
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Modeling ApproachModeling 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
• 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
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Endothermic reactionEndothermic 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 energyenergy + 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
• 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 energyenergy + 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
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Endothermic reactionEndothermic 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.
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Energy Bar GraphsEnergy Bar Graphs• 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
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Consider role of Ek 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
• 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
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Heating system increases Ek
Heating system increases Ek• 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
Note intermediate stage
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Now reaction proceedsNow reaction proceeds• During collisions, particles trade Ek
for Ech as products are formed
• After rearrangement, resulting particles move more slowly (lower Ek).
• During collisions, particles trade Ek for Ech as products are formed
• After rearrangement, resulting particles move more slowly (lower Ek).
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Consider all steps in process
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
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
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Exothermic reactionExothermic 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
• 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
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Exothermic reactionExothermic 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.
• 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.
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Exothermic reactionExothermic 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
• 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
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Exothermic reactionExothermic reaction• System is now hotter than surroundings;
energy flows out of system until thermal equilibrium is re-established
• 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
Consider all steps in process
1. Decrease in Echresults in increased Ek
2. System is now hotter than surroundings3. Energy is transferred from system to
surroundings via heating
1. Decrease in Echresults in increased Ek
2. System is now hotter than surroundings3. Energy is transferred from system to
surroundings via heating
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Contrast Conventional Diagram
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.
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But what about energy used to start reaction?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
• 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
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What about a spontaneous endothermic process?
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
• 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
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Reaction useful for cold-packs
Reaction useful for cold-packs
• The system trades Ek for Ech
• Eventually energy enters cooler system from warmer surroundings (you!)
• The system trades Ek for Ech
• Eventually energy enters cooler system from warmer surroundings (you!)
