Keeping Track of Energy During
Chemical Change
A coherent way to treat energy in
chemical reactions
1
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
2
Modeling 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
3
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
4
Endothermic reaction
• This is the standard energy diagram
found in most texts.
• But it doesn’t tell the whole story.
5
Energy Bar Graphs
• Show energy transfers between
surroundings and system
• Allow you to consider other energy
accounts
6
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
7
Heating system increases Ek
• Hotter, faster molecules (surroundings) transfer
energy to colder, slower molecules (system)
• Now reactant molecules are sufficiently
energetic to produce reaction
Note intermediate
stage
8
Now reaction proceeds
• During collisions, particles trade Ek
for Ech as products are formed
• After rearrangement, resulting particles move
more slowly (lower Ek).
9
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
10
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
11
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.
12
Exothermic 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
Note intermediate
stage
13
Exothermic reaction
• System is now hotter than surroundings;
energy flows out of system until thermal
equilibrium is re-established
14
Consider all steps in process
1. Decrease in Echresults in increased Ek
2. System is now hotter than surroundings
3. Energy is transferred from system to
surroundings via heating
15
Contrast Conventional Diagram
• This is the standard energy diagram found in
most texts.
• But, again,it doesn’t tell much of the story.
16
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
17
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
18
Reaction useful for cold-packs
• The system trades Ek for Ech
• Eventually energy enters cooler system
from warmer surroundings (you!)
19