The material world is constantly changing. The changes give rise to all sorts of phenomena, including light, thundder, flood, storms, and heat observed in the weather. Did you ever ask yourself what make the material change? Well, former scientists have analyzed the question, and they have provided a simple answer to this. Energy causes all the changes. However, the concept of energy and how it behalve are very difficult to master. On the other hand, once you have mastered it, you will appreciate the simplicity and realize the beauty of this concept.
Thermodynamics discusses changes based on amounts of energy. Since energy is conserved, energy transferred into a system is called internal energy. The amount of energy in a reaction remains the same regardless whether the reaction (or change) takes place in one or several steps. This principle is illustrated by the Hess's law, the application of which gives the estimate of energy in a process. Measurements of energy is called calorimetry, and they can be measured under constant volume or constant pressure. A system tends to minimize its Gibb's free energy, G, and such a tendency leads to the concept of chemical equilibria. These topics require lengthy discussion, and each is a sub-unit of study.
Chemical Kinetics which deals with the rate of reactions. A good example to illustrate the two factors is the existence of diamond and graphite. At room temperature and pressure, thermaldynamics indicates that the stable form of carbon is graphite. From a thermaldynamic point of view, diamond should convert to graphite. But the reaction rate (kinetics) is so slow that there is no detectable change.
The temperature dependence associated with chemical kinetics is discussed in terms of activation energy, which is often perceived as the energy barrier that has to be overcome in order for a reaction to proceed.
However, kinetics of chemical reaction also deals with rate laws, elementary steps, and mechanicsms of reactions.
The links given above have been prepared for other courses, and they have a slightly different style.
Example 1
Solution
Glass is a meta stable state compared to crystalline state.
Arrangements of molecules or groups of atoms in glass does not have a
long range order, and the state is often referred to as frozen solid.
Polymers are in glassy states when cooled, because the long molecules
do not have time to properly align themselves into crystalline states.
Over a long period of time, the polymers will become more crystalline.
As a result, they become brittle. For example, a pair of old glass frame
belonging to your grandfather are very brittle.
Discussion
Realizing that there two factors governing the changes in material
let you understand some engineering problems.