1. 7.2 Thermochemistry Objective 2

2. Essential Questions What is heat?
What is an exothermic process? Endothermic? How can you use heat to tell which type it is?
What is enthalpy?
What is ΔH? How can you use ΔH to determine if a reaction is endothermic or exothermic?

3. Thermochemistry
Thermochemistry is the study of changes in heat in chemical reactions

4. Energy Thermal energy is heat
Heat is transfer of energy from a hotter object to a colder one

5. Heat
Heat and temperature are NOT the same because temperature measures kinetic energy (speed) of the particles in a substance
No such thing as COLD in chemistry there is only heat flowing out
Heat and Energy both measured in Joules (J)

6. Energy (cont) What does the law of conservation of energy say?
Energy cannot be created nor destroyed
If energy cannot be created nor destroyed then why is there heat?
Energy is being transferred or changed

7. Exothermic and Endothermic
Depending on how the heat/energy is being transferred a process is either exothermic or endothermic
Exothermic process is when energy is being released as heat to the surroundings
Endothermic process is when energy is being absorbed as heat from its surroundings

8. Exo and Endo (cont)
If in an experiment if the temperature of the surroundings increases the reaction would be exothermic
If in an experiment if the temperature of the surroundings decreases the reaction would be endothermic

9. Exo and Endo (cont)
When Al is added to CuSO4 the mixture heats up so it would be exothermic.

10. State Changes
If the state change absorbs energy (takes in energy) to occur it is endothermic
If the state change releases energy (energy has to leave) than it is exothermic
Example: melting would be endo, but freezing would be exo

11. Enthalpy
Enthalpy (H) is used to measure the heat of a system (what you are studying)
ΔH is the change in enthalpy which means the change in heat for the system
ΔH = Hproducts – Hreactants

12. Enthalpy Diagrams (don’t need to write)
Exothermic Reaction: CaO (s) + H2O (l) → Ca(OH)2 (s) ΔH = kJ
These enthalpy diagrams show exothermic and endothermic processes: a) the reaction of calcium oxide and water and b) the decomposition of sodium bicarbonate. Identifying In which case is the enthalpy of the reactant(s) higher than that of the product(s)?

13. Enthalpy Diagrams (don’t need to write)
Endothermic Reaction: 2 NaHCO3 (s) → Na2CO3 (s) + H2O (g) + CO2 (g) ΔH = 129 kJ
These enthalpy diagrams show exothermic and endothermic processes: a) the reaction of calcium oxide and water and b) the decomposition of sodium bicarbonate. Identifying In which case is the enthalpy of the reactant(s) higher than that of the product(s)?

14. Enthalpy (cont)
Exothermic reactions have a negative ΔH value because the reactants are higher in enthalpy than the products.
Endothermic reactions have a positive ΔH value because the products have a higher enthalpy.
ΔH can be written next to the reaction like in this exo reaction:
2 H2 (g) + O2 (g) → 2 H2O (l) ΔH = -572 kJ

15. Enthalpy (cont) ΔH can be written as part of the reaction
For exo reactions the ΔH is written as a product because it is being released by the reaction
2 H2 (g) + O2 (g) → 2 H2O (l) kJ
For endo the ΔH is written as a reactant because energy must be absorbed for it to react
2 H2O (l) kJ → 2 H2 (g) + O2 (g)

16. Enthalpy (cont)
Identify the following processes as exothermic or endothermic:
B2H6 (g) + 3 O2 (g) kJ → B2O3 (s) + 3 H2O (l)
Na (s) + ½ Cl2 (g) → NaCl (s) ΔH = -411 kJ
CO2 (g) → C (s) + O2 (g) ΔH = kJ
3C (s) + 4 H2 (g) → C3H8 (g) kJ
Sweat evaporating on your skin
Water boiling on the stove
CO2 freezing to form dry ice
endo
exo
endo
exo
endo
endo
exo

17. Essential Questions What is heat?
What is an exothermic process? Endothermic? How can you use heat to tell which type it is?
What is enthalpy?
What is ΔH? How can you use ΔH to determine if a reaction is endothermic or exothermic?

18. 7.2 Tracked Assignment
Worksheet