1. Energy & Entropy Changes
Chapter 4 Notes
Energy & Entropy Changes

4. Changes In Energy
When matter undergoes a change, there is often a change in energy.
Energy can change from one form to another, but the total amount of energy always remains the same.

5. Endothermic vs. Exothermic
Reactions can be classified in two ways depending on the energy involved.
Reactions can be endothermic or exothermic.

6. Endothermic Reactions
An endothermic reaction absorbs energy and feels cold.
Endothermic reactions occur when energy is absorbed as bonds are broken.

7. Cold Packs are Endothermic

8. Exothermic Reactions
An exothermic reaction releases energy and feels hot.
Exothermic reactions occur when energy is released as bonds are formed.

9. Hot Packs are Exothermic

10. Organization of Particles
Chemists can also measure the amount of disorder of the particles in a reaction.
This “disorder” is called entropy and is abbreviated with the symbol “S”.

11. Entropy
The particles in a solid are highly organized and therefore have a low entropy.

12. Entropy
The particles in a liquid are less organized than a solid and therefore have a higher entropy.

13. Entropy
The particles in a gas are highly disorganized and therefore have a very high entropy.

14. Entropy in Chemical Reactions
4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)
What are the reactants in the above reaction?

15. Entropy in Chemical Reactions
4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)
What are the reactants in the above reaction?
Fe and O2

16. Entropy in Chemical Reactions
4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)
2. What are the products in the above reaction?

17. Entropy in Chemical Reactions
4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)
2. What are the products in the above reaction?
Fe2O3

18. Entropy in Chemical Reactions
4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)
Which side has a higher entropy and why?

19. Entropy in Chemical Reactions
4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)
Which side has a higher entropy and why?
The reactants have a higher entropy because a gas has more disorder than a solid.

20. Chapter 4 Activity 1 Summary
Write 5 Fill-in-the-blank questions about today’s notes (1 level 1, 2 level 2, and 2 level 3). Make the blanks large enough to write the answers in (do not put the answers in the blanks yet!).