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Keeping Track of Energy During Chemical Change

A coherent way to treat energy in chemical reactions

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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

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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

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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 energyenergy + CaCO3 CaO + CO2 (g)

• If uncertain, use analogy from algebraIf 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 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

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Heating system increases Eth• 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 Eth

for Ech as products are formed

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

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Consider all steps in process1.Heating system increases Eth of reactant

molecules2.Energy is transferred from Eth to Ech 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 Ech than the reactants

• CH4 + 2O2 CO2 + 2H2O + energy

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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.

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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

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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 Ech results in increased Eth

2. System is now hotter than surroundings3. 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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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.

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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

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Reaction useful for cold-packs

• The system trades Eth for Ech

• Eventually energy enters cooler system from warmer surroundings (you!)