1. Energy Changes

2. 2.4 - Energy Changes bond breaking: bond making:
Enthalpy (H): heat contained in a system
In a chemical reaction, there is BOTH bond breaking AND bond making
bond breaking:
bond making:
energy needed (absorbed)
energy released

3. Net total energy change is for a reaction is:
DH = Hproducts – Hreactants

4. Endothermic Exothermic
surroundings
feel cold
surroundings
feel hot

5. Endothermic Exothermic
energy is absorbed from the surroundings into the system
energy for bond breaking ____ energy for bond making
energy is a _____________
energy of products _____ energy of reactants
DH = Hproducts – Hreactants = _________
energy is released from the system into the surroundings
energy for bond breaking ____ energy for bond making
energy is a ______________
Energy of products ______ energy of reactants
DH = Hproducts – Hreactants = __________
>
<
reactant
product
>
<
positive
negative

6. Endothermic Exothermic
HCl + 50kJ → H+ + Cl-
H+ + Cl- → HCl + 50kJ
H =
H =