Delta g and delta a in ideal gas expansion
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The First Law of thermodynamics deals with the consequences of energy conservation. Quantities considered are heat, work, energy and enthalpy. The First Law is exact, and leads to equations describing energy flow. In this part of the lecture we will focus on analyzing the nature of spontaneous processes in nature. This leads to the formulation of the Second Law of thermodynamics, and to the introduction of new quantities: entropy and free energy. As all scientific laws, the Second Law is based on observation of nature. Interestingly, the Second Law is not exact, it is a statistical law which works "practically always", and leads to inequalities describing the direction of spontaneous change. The combination of the First and Second Law places additional constraints on future events compared to application of one of the laws alone.
We will start with a discussion of spontaneous processes and introduction of entropy.
Spontaneous processes
We all have an everyday experience of spontaneous processes, processes which happen "on their own", without external intervention. A gas will expand if the external pressure is lower than the gas pressure p, heat will flow from a hot body to a cold body, salt dissolves in water, a mixture of reactive substances will react to form products, we ourselves become older as time goes by. Spontaneous processes have a very important characteristic: directionality or irreversibility. They only proceed in one direction, and the reverse processes are not observed to occur spontaneously. Thus, we do not find a gas being compressed when the external pressure is lower than the gas pressure, heat flowing from a colder to a hotter body, solutions separating into components or stable substances suddenly decomposing into reactive species. Such processes may be brought about, but they need external intervention. For example, we can compress a gas by doing work on the system or distill out salt from sea water by supplying the energy needed to evaporate the water.
We will start with a discussion of spontaneous processes and introduction of entropy.
Spontaneous processes
We all have an everyday experience of spontaneous processes, processes which happen "on their own", without external intervention. A gas will expand if the external pressure is lower than the gas pressure p, heat will flow from a hot body to a cold body, salt dissolves in water, a mixture of reactive substances will react to form products, we ourselves become older as time goes by. Spontaneous processes have a very important characteristic: directionality or irreversibility. They only proceed in one direction, and the reverse processes are not observed to occur spontaneously. Thus, we do not find a gas being compressed when the external pressure is lower than the gas pressure, heat flowing from a colder to a hotter body, solutions separating into components or stable substances suddenly decomposing into reactive species. Such processes may be brought about, but they need external intervention. For example, we can compress a gas by doing work on the system or distill out salt from sea water by supplying the energy needed to evaporate the water.
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