From The Chemistry Book
7. Energy is exchanged or transformed in all chemical reactions and physical changes of matter.
7. a. Students know how to describe temperature and heat flow in terms of the motion of molecules (or atoms).
Temperature is a measure of the average kinetic energy of molecular motion in a sample. Heat is energy transferred from a sample at higher temperature to one at lower temperature. Often, heat is described as flowing from the system to the surroundings or from the surroundings to the system. The system is defined by its boundaries, and the surroundings are outside the boundaries, with the universe frequently considered as the surroundings.
7. b. Students know chemical processes can either release (exothermic) or absorb (endothermic) thermal energy.
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 covered in Standard Set 8, Reaction Rates, in this section.
7. c. Students know energy is released when a material condenses or freezes and is absorbed when a material evaporates or melts.
Physical changes are accompanied by changes in internal energy. Changes of physical state either absorb or release heat. Evaporation and melting require energy to overcome the bonds of attractions in the corresponding liquid or solid state. Condensation and freezing release heat to the surroundings as internal energy is reduced and bonds of attraction are formed.
7. d. Students know how to solve problems involving heat flow and temperature changes, using known values of specific heat and latent heat of phase change.
Qualitative knowledge that students gained by mastering the previous standards will help them to solve problems related to the heating or cooling of a sub-stance over a given temperature range. Specific heat is the energy needed to change the temperature of one gram of substance by one degree Celsius. The unit of specific heat is joule/gram-degree.
During phase changes, energy is added or removed without a corresponding temperature change. This phenomenon is called latent (or hidden) heat. There is a latent heat of fusion and a latent heat of vaporization. The unit of latent heat is joule/gram or kilojoule/mole. Students should be able to diagram the temperature changes that occur when ice at a temperature below zero is heated to superheated steam, which has temperatures above 100C.
