2.3 Energy Changes During Chemical Reactions
Book for Endothermic and Exothermic Reactions.
Exothermic Reactions
These are reactions which release energy. Energy in chemical reactions is measured in kilojoules (kJ). Consider the following hypothetical reaction:
x2 + y2 → 2xy
50 kJ of energy are required to break the bonds that hold the x's and y's together in their respective molecules, but 100 kJ is released when xy forms.
50 kJ + x2 + y2 → 2xy + 100 kJ
simplified
x2 + y2 → 2xy + 50 kJ
The net result is that 50 kJ of energy are released.
Reactions of this sort which produce energy are called exothermic reactions. "Exo" means "out" and "therm" refers to heat. So, "exothermic" means that heat is coming out, or being produced.
Reactants → Products + energy
An example of an exothermic reaction occurs during cellular respiration in animals and plants. The reaction involves "burning" sugar (glucose C6H12O6 (s)) to produce energy. Oxygen from the air is required.
C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(l) + 2803 kJ