1. Energy of Reactions
Chemistry 11

2. ENERGY CHANGES IN CHEMICAL REACTIONS
Molecules are held together by CHEMICAL BONDS.
In order to break a bond, energy must be added to the bond.
Energy changes in the reaction can be shown graphically.

3. 2 HCl(g) + energy → H2(g) + Cl2(g)
The separated atoms (H2(g) and Cl2(g)) contain more energy than the original molecule (HCl(g)), so energy is needed to cause the reaction.

4. Classifying Reactions:
Can classify reactions based on their energy changes.
Whether they take in or release energy (usually as heat)
If a reaction takes in MORE energy to break bonds than it GIVES OFF to form bonds, the reaction will require a NET INPUT OF ENERGY, and it is called…
ENDOTHERMIC

5. Endothermic Reactions
An ENDOTHERMIC reaction ABSORBS heat from its surroundings (heat Enters the system).
 feels cold
Ex. Decomposition of Potassium chlorate
KClO3(s) kJ → K+(aq) + ClO3-(aq)
Graphically represented as:
Energy

6. If a reaction takes in LESS energy to break bonds than it GIVES OFF to form bonds, the reaction will GIVE OFF ENERGY, and it is classified as…
EXOTHERMIC

7. Exothermic Reactions:
An Exothermic reaction GIVES OFF heat to the surroundings (heat exits the reaction)
 it feels warm
Ex. Your furnace burning natural gas.
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) + 891kJ
Graphically represented as:
Energy

8. Enthalpy (heat energy):
Is the heat contained in the system
Symbol = H
The change in heat (enthalpy) during a reaction is H
ΔH = H PRODUCTS – H REACTANTS
Where:
HPRODUCTS = the heat contained in the PRODUCTS
HREACTANTS = the heat contained in the REACTANTS

9. Writing the Energy Term (Enthalpy)
Exothermic and endothermic reactions can each be shown in two ways:
ENDOTHERMIC:
A + 50 KJ → B
A → B; ΔH = +50 kJ
EXOTHERMIC:
C → D + 80 KJ
C → D; ΔH = -80 kJ
Note:
H is +ve for an endothermic reaction
And
H is –ve for an exothermic reaction