呵呵呵~~这是我之前问教授问题的答覆,相信对大家一定会有帮助, 祝大家化学欧趴 哇哈哈哈! Enthalpy refers to the internal energy of a system, or a reaction. Normally we are just concerned with the change in enthalpy in a reaction. The change in enthalpy can be positive or negative, and the value tells us how much heat is released or absorbed in a reaction. In an exothermic reaction, heat is released, and enthalpy change is negative. In an endothermic reaction, heat is absorbed, and the enthalpy change is positive. As for entropy, it refers to the randomness (or state of disorder) of a process or reaction. In nature, a process which has a positive entropy change is thermodynamically favorably and can proceed spontaneously. The reaction will proceed in the direction that produces a positive change in entropy. However, entropy change needs to be considered along with change in enthalpy at the temperature the process or reaction occurs. Ultimately, for a reaction or process to be spontaneous in the direction the free energy must decrease. Free energy is just another thermodynamic function that is also related to spontaneity. Free energy is defined as G = H - TS. One just needs to be concerned with change in free energy in a process or reaction. If the change in free energy of a reaction is negative, the reaction is spontaneous. H means enthalpy and S is entropy. G is defined as H - TS because one needs to consider both enthalpy change and entropy change to determine if a process is spontanous at a particular temperature. Becuase enthalpy change does not tell us whether a reaction is spontanous or not, we need the entropy term to help us determine if the reaction under consideration is spontanous. For example, if a reaction is endothermic, it would not be spontaneous at low temperatures. But at high temperatures, it may become spontaneous becasue of the heat input and the entropy term dominates. For a reaction to occur, delta S of the universe must be positive, meaning that from a whole picture there is an increase in the rondomness of the universe. For example, consider the formation of a solid or crystal. In forming a solid or crystal, the system become more ordered, but it has to release heat to the surroundings, which is a disordered energy. Because of this increase in entropy, the formation of solids or crystals is thermodynamically favorably. on page 424 in the textbook table 10.6 In case 2, delta S is positive and delta H is also positive. The result is that the reaction is spontaneous at high temperatures. If you examine the equation G = H - TS, when both H and TS terms are positive, we are not sure if G will be negative. At high temperatures, the TS term dominates, H - TS will become negative and the reaction will be spontaneous. In this case, the sign of H is less important. That is why exothermicity is relatively unimportant. Even though delta H is positive, which dose not favor spontaneity, but the -TS term is more negative so H - TS will still be negative. In case 3, delta S is negative and delta H is also negative. Again we do not know if G will be negative from the signs, as -T multiplied by a negative delta S will make this term positive. A negative delta H plus a positive TS term may or may not give a negative delta G. However, at low temperatures, -TS will be a smaller number and delta H becomes more important. If delta H is large and negative, delta G will be negative. So under this condition, exothermicity of the reaction is dominant, or quite important. A state function means that the value of the term, such as enthalpy and entropy, only depends on the state (i.e., temperature and pressure) the chemical or system exists. So for enthalpy and entropy, which are state functions, we only need to be concerned with their values at the initial state and final state. We do not need to know by what process one the system goes from the initial state to the final state. It is like point A and point B. Point A has a certain position in location, as well as point B. You can go from point A to point B anyway you like, but the two points are not changed. Reversible process means that if the process can return to its initial state. Since some energy is used or wasted in carrying out the process, all real processes are not reversible. However, if friction is significantly reduced, the process may approach a reversible process. Irreversible process means the process does not go back to its initial state, since some energy has lost as heat, in general, in doing this process. --

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