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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• 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

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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 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

• 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

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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 ChartsEnergy 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

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Consider role of Eth 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

• 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 Eth

Heating system increases Eth• 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

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Now reaction proceedsNow reaction proceeds• During collisions, particles trade Eth for Ech as products are formed

• After rearrangement, resulting particles move more slowly (lower Eth).

• During collisions, particles trade Eth for Ech as products are formed

• After rearrangement, resulting particles move more slowly (lower Eth).

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Consider all steps in process

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

1.Heating system increases Eth of reactant molecules

2.Energy is transferred from Eth to Ech now stored in new arrangement of atoms

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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 Eth • When reactant molecules collide to produce products that store less energy, new molecules move away more rapidly

• Now take into account changes in Eth • When reactant molecules collide to produce products that store less energy, new molecules move away more rapidly

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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 Ech results in increased Eth

2. System is now hotter than surroundings

3. Energy eventually moves from system to surroundings via heating

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

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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.