Standard 7b Preknowledge
7b) Students know chemical processes can either release (exothermic) or absorb (endothermic) thermal energy.
CALIFORNIA FRAMEWORKS SUMMARY: Endothermic processes absorb heat, and their equations can be written with heat as a reactant. Exothermic processes release heat, and their equations can be written with heat as a product. The net heat released to or absorbed from the surroundings comes from the making and breaking of chemical bonds during a reaction. Students understand and relate heat to the internal motion of the atoms and molecules. They also understand that breaking a bond always requires energy and that making a bond almost always releases energy. The amount of energy per bond depends on the strength of the bond.
The potential energy of the reaction system may be plotted for the different reaction stages: reactants, transition states, and products. This plot will show reactants at lower potential energy than products for an endothermic reaction and reactants at higher potential energy than products for an exothermic reaction. A higher energy transition state usually exists between the reactant and product energy states that affect the reaction rate.