1. Unit 7- Chemical Reactions: Particles and Energy
Chemical Energy, Ech

2. Energy Storage and Transfer
Energy, like money can be stored and moved between different accounts
It’s still money regardless of the account
Let’s consider the nrg accounts we will use:
Thermal or kinetic energy, Eth or Ek
Phase energy, Eph
Chemical potential energy, Ech

3. Chemical potential energy, Ech
Energy due to attractions of atoms within particles or molecules.
These attractions are described as chemical bonds because they are directed between specific atoms in the particle/molecule.

4. Constructing an Energy Bar Chart
Consider this example
A cup of hot coffee cools as it sits on the table.
1. Determine what is in the system
cup coffee
Everything else is in the surroundings

5. Conventional Energy Diagram
This is the standard energy diagram for exothermic rxns found in most texts.
But, it doesn’t tell much of the story.

6. Energy Bar Charts
Show energy transfers between surroundings and system
Allow you to consider other energy accounts

7. Heating system increases Eth
Hotter, faster molecules (surroundings) transfer energy to colder, slower molecules (system)
Now reactant molecules are sufficiently energetic to produce reaction

8. Now reaction proceeds
During collisions, particles trade Eth for Ech as products are formed
After rearrangement, resulting particles move more slowly (lower Eth).

9. 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
3. Resulting system is cooler - requires continued heating to bring Eth 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 Now take into account changes in Eth
When reactant molecules collide to produce products that store less energy, new molecules move away more rapidly with a higher temp

12. Exothermic reaction
System is now hotter than surroundings; energy flows out of system until thermal equilibrium is re-established

13. 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

14. What about a spontaneous endothermic process?
What does spontaneous mean?
When NH4Cl dissolves in water, the resulting solution gets much colder
What caused the Eth to decrease?
Where did the Eth go?
Ech
Some Eth of water required to separate ions (break bonds) in the crystal lattice.
Resulting solution has greater Ech than before
At the expense of Eth

15. Reaction useful for cold-packs
The system trades Eth for Ech
Eventually energy enters cooler system from warmer surroundings (you!)