1. Unit 8 – Notes Page 7
Endothermic and Exothermic Reactions

2. Energy: The ability to do work Chemical bonds store energy
Breaking bonds takes energy
Forming bonds releases energy

3. Activation Energy
The minimum amount of energy required for a reaction to take place

4. Heat of Reaction, ΔH A change in the internal energy of the reaction
+ ΔH: Reactants lower energy than products
- ΔH: Reactants higher energy than products

5. Enthalpy Amount of energy a system or substance contains
Another way to say ΔH

6. Surroundings
Where the reaction is taking place: the beaker, the test tube, the classroom, the universe, etc.

7. System
Substance(s) / chemicals involved in a reaction or phase change.

8. Open System

9. Closed System

10. Isolated System

11. Exothermic Reactions Exo = out Thermic = heat
Energy (heat, light) is given off from a system when a reaction takes place

12. Endothermic Reactions
Endo = into, inside
Thermic = heat
Energy (heat, light) is taken into a system when a reaction takes place

13. Exothermic Reactions General equation: Reactants  Products + Energy
Takes less energy to break bonds in reactants than is released when new bonds form in products
Negative ΔH: Exothermic

14. Example: Combustion Always exothermic Burns and combines with oxygen
Give off heat as energy and light

15. Example: Hot Hands How does an instant hot pack work?
Exothermic reaction of iron in oxygen:
4Fe(s) + 3O2(g)  2Fe2O3(s)

16. Endothermic Reactions
General equation:
Reactants + Energy  Products
Needs a constant input of energy to continue.
Takes more energy to break bonds in reactants than is released when new bonds form.
Positive ΔH: Endothermic

17. Example: Photosynthesis
6 CO2 + 6 H2O → C6H12O6 + 6 O2
Plants need to aborb energy from the sun (or other source of light)
Absorbs energy  endothermic

18. (NO3)NH4 (s) + H2O(l)  NH4(OH) (aq) + HNO3 (aq)
Example: Cold Pack 
How does an instant cold pack work?
(NO3)NH4 (s) + H2O(l)  NH4(OH) (aq) + HNO3 (aq)
Squeezing the cold pack breaks an inner bag of water, and the water mixes with a chemical inside the pack. The chemical and water combine in an endothermic reaction. The energy needed for the reaction to take place comes from the water, which gets colder as the reaction proceeds.

19. Phase Change Ice  Water Solid gold  molten gold
Heat moves from the surroundings to the system

20. Solid  Liquid  Gas (Melt, evaporate)
Requires heat (energy). It takes energy from the environment. This is ENDOTHERMIC.
Gas  Liquid  Solid (Condense, freeze)
Requires absence of heat (energy). It releases energy into its surroundings.
Making ice is EXOTHERMIC!