When speaking of change, chemists refer to the system and the surroundings. Together, the system and the surroundings make up the universe. You choose the system that you want to investigate and you can imagine drawing a dashed surface around the system, to separate it from the surroundings. When considering change you think about what the system is doing. When we stretched the rubber band we asked is it producing heat energy or releasing heat energy. If the system is an open system, energy can pass between the system and the surroundings. For example a teakettle heating on the stove is an open system. A closed system is a system that is isolated form the surroundings and heat can neither enter nor leave. This is very, very difficult to achieve. An attempt at a closed or isolated system is a thermos bottle. As we know a thermos bottle slow the movement of heat, but eventually the hot soup or cold beverage in the thermos bottle will exchange heat with the surroundings. When heat energy is released from the system, the surroundings gain that energy, while the system loses energy. If your hand is resting on the system, you feel heat energy coming from the system. When a system loses heat energy, there is a loss in the amount of enthalpy that the system contains. The change in enthalpy is negative. When heat energy is absorbed by a system, he surroundings must give that heat energy to the system. Energy must come from somewhere. The law of conservation of energy stats that energy cannot be created or destroyed. It can only be transferred from one location to another. If you hand is resting on the system, you feel heat energy leaving your hand and going to the system. When a system gains energy, there is an increase in the amount of energy the system posses. The change of enthalpy is positive. Whenever a change occurs, particles inside the system are rearranged. The new arrangement is either more or less disorganized. When the new arrangement is more disorganized the entropy has increased. The change in entropy is positive and the final entropy is a larger value than the initial entropy. Conversely, when the new arrangement is less spread out or more organized, the entropy has decreased. The entropy change is negative and the finial entropy is a lower value than the initial entropy. When materials change from gas to liquid, or liquid to solid the entropy decreases. Changes can occur either spontaneously or not. There are two factors that affect spontaneity are changes in enthalpy and changes in entropy. Exothermic changes drive a process toward spontaneity. This is because substances are produced that have lower energy than the reactants from which thy formed. Lower-energy states are favorable. Changes that result in an increase in entropy of the system also drive a process toward spontaneity. This is because nature tends to become more disorganized over time. If both factors change in the direction that favors spontaneity, the reaction will definitely be spontaneous. If neither factor changes in the direction that favors spontaneity, the reaction will definitely not be spontaneous. If only one change occurs in a favorable direction, the more dominant change will determine if the reaction is spontaneous. A third factor determines if a process will be spontaneous. Consider the freezing of water to form ice. If the water is left on the counter, freezing is not spontaneous. However, if the water is placed in the freezer. the process becomes spontaneous. The temperature at which a process occurs can affect whether or not a process will be spontaneous. The Gibbs free energy is a measurement that can be used to tell whether a change will occur spontaneously. If the change in Gibbs free energy is negative, the change will occur spontaneously at the given temp.
The change in Gibbs free energy can be determined using this equation:
ΔG= ΔH – TΔS (where the temp is measured in kelvins)
If ΔH and ΔS both have the same sign, ΔG could be positive or negative depending on whether ΔS or ΔH is more influential in determining the sign of ΔG. If ΔG is positive the reaction is not spontaneous at room temp. Polymers are molecules made of long strings of monomers that are attached to each other. This structure causes materials that are made of polymers to exhibit some unusual behaviors. When we heated the rubber band with the hair dyer we encountered this by the rubber band getting shorter or contracting. We need to compare the enthalpy and entropy of the two states of the rubber band. When the rubber band is in a stretched state, the entropy (disorder) is low because the molecules are pulled relatively straight and lined up. The enthalpy is also low when the rubber band is in a stretched state because the aligned molecules are able to experience attractions to each other. Molecules exhibit a smilier relationship between distance and energy. Molecules experience relatively weak attractions for each other when they are close together. This means when a molecule us attracted to another it is at a lower energy (enthalpy) state. When the rubber band is in a contracted state, the entropy (disorder) is high because the molecules are tangled around each other. The enthalpy is also high because the molecules are father apart from other molecules. When a molecule does not have other nearby molecules to be attracted to, the molecule is at a higher energy (enthalpy). The enthalpy factor favors the rubber band becoming stretched. Nature favors results that end with high enthalpy. At room temp the change in entropy dominates the change in enthalpy because a rubber band does not stretch at room temp by itself. The rubber band stretched when you decreased its temperature by cooling it with ice. This means that at a lower temp the stretched state is favored. The change in enthalpy drives the rubber band toward a stretched state. Therefore, at a lower temp the enthalpy factor becomes dominant over the entropy factor. Because the rubber band is stretched when cooled with ice and is contracted at room temp, increasing temp causes the rubber band to contract. As the temp increases the change in entropy makes the contraction of the rubber band more and more spontaneous. Therefore, the rubber band contracts more than it does at room temp. Polymers are probably the most versatile substances that technology has produced. They can change in what they feel like and scientist can change their properties. One of the most exciting fields in which polymers are used is medicine. A polymer called a thermoplastic polymer is used to stick the working parts of the mechanical heart to the heart itself. Thermoplastics are usually polyesters or polymers in which the repeating monomer forms an ester linkage between an organic acid and an alcohol. Polyethylene terephthalate is a copolymer of terephthalic acid and ethylene glycol connected by the ester functional group. Properties of polymers are such that they can be so sturdy that they can be used to construct cars and buildings, or so elastic that they can be used to make trampolines and playground balls. The fields of polymer chemistry and materials science are exciting and employ many chemists.