When chemists are speaking of a change, they refer to the system and the surroundings. The system and the surroundings make up the universe. When considering change, you think about what the system is doing. For example, you ask, “Is the enthalpy of the system increasing or decreasing?” etc. If the system is an open system, energy is able to pass between the system and the surroundings. However, if the system is a closed system is it isolated from the surroundings and heat can neither enter nor leave.
When heat energy is release from the system, the surroundings gain energy and the system loses energy. When a system loses energy there is a loss in the amount of enthalpy the system contains. The change in enthalpy is negative. On the other hand, when heat energy is absorbed by the system, the surroundings must give that heat energy to the system because energy must come from somewhere. The law of conservation of energy states that energy cannot be created or destroyed. It is merely transferred from one place to another. When a system gains energy the change in enthalpy is positive.
When a change takes place, the particles inside the system are rearranged. The rearrangement of these particles are either more or less disorganized. If the particles become more disorganized the entropy has increased but if the particles become more organized the entropy has decreased.
The changes in enthalpy and entropy are the two factors that affect the spontaneity of a change. Exothermic changes drive a process toward spontaneity because the products have lower energy than the reactants from which they are formed. Lower energy states are more favorable. Also an increase in entropy drives a process toward spontaneity because nature tends to become more disorganized over time. There is a third factor that affects spontaneity, that is the temperature at which the process occurs. There is something called Gibbs free energy which conveniently combines the three factors that affect spontaneity. Gibbs free energy can be used to tell whether a change will happen spontaneously. If the change in Gibbs free energy is negative the change will be spontaneous. If it is positive the change will not happen spontaneously. The equation for Gibbs free energy is ΔG=ΔH−TΔS.
Polymers are molecules made up of a long chain of repeating monomers joined together. Materials made of polymers exhibit unusual behaviors. When the rubber band is stretched the entropy is low because the molecules are lined up and the enthalpy is also low because the aligned molecules are able to experience attractions to each other. When the rubber band is contracted the entropy is high because the molecules are tangled and the enthalpy is high because the molecules are farther apart from other molecules. The rubber band contracted more when we heated it up and stretched when it cooled.
Polymers are probably the most versatile substance that technology has produced. Scientists have learned how to control the properties of polymers like varying the composition of the monomers, the chemical types of monomers and the degree of cross-linking between polymer strands. A field in which polymers are used is medicine. There is a polymer called a thermoplastic polymer which is used to stitch the working parts of the mechanical heart to the heart itself. Thermoplastics are usually polyesters or polymers where the repeating monomer forms an ester linkage between an organic acid and an alcohol. Polymers are used a lot in the biomedical field. For example, they are used to provide skin grafts for the treatment of serious burns and they are used in prosthetic limb technology. Polymers can be so sturdy they are used to construct cars and buildings. Also they can be so elastic that they are used to make trampolines.