The document discusses energy from chemicals and chemical reactions. It defines exothermic and endothermic reactions, where exothermic reactions release energy and have a negative enthalpy change, and endothermic reactions absorb energy and have a positive enthalpy change. Examples of each type of reaction are given. The document also provides an example calculation of determining the enthalpy change of a reaction using bond energies. Finally, it summarizes hydrogen fuel cells, noting they convert chemical energy to electrical energy through redox processes, have advantages of being renewable and non-polluting but disadvantages of being expensive and dangerous.

1. LESSON 10
29/02/20
ENERGY FROM CHEMICALS

2. • ∆H = bond breakage (energy absorb)-bond formation(energy released)
• ∆H is negative(<0)
Example of reactions:
• physical: freezing, condensation
• Chemical: combustion
EXOTHERMIC REACTION
Ea

3. EXOTHERMIC REACTION
Energy level of reactants higher than products

4. • ∆H = bond breakage (energy absorb)-bond formation (energy released)
• ∆H is positive(<0)
Example of reactions:
• physical: melting, boiling
• Chemical: thermal decomposition
ENDOTHERMIC REACTION
Ea

5. ENDOTHERMIC REACTION
Energy level of reactants lower than products

6. ENTHALPHY CHANGE
∆H= Energy absorbed in bond breaking - Energy released in bond forming
Bond Energy (kJ/mol)
H-H 436
O=O 496
O-H 460
2 H-H + O=O—> 2H-O-H
2H-H bonds and 1 O=O bond broken
4 O-H bonds formed
∆H = [2(436) + 496] -[4(460)]
= -472 kJ/mol
The reaction is exothermic

7. Hydrogen Fuel cell
Positive electrode:
O2 (g) + 2H2O (l) + 4e- —> 4OH- (aq)
Negative electrode:
2H2 (g) + 4OH- (aq) —> 4H2O (l) + 4e-
• Fuel cell converts chemical energy into electrical energy
• Generates electricity via redox processes
Overall reaction
Advantages
- hydrogen is renewable
- No pollution
Disadvantages
-expensive
-dangerous
2H2 (g) + O2 (g) —> 2H2O (l)
∆H= -484 kJ/mol