Spontaneous Processes and Thermodynamic Equilibrium

The reaction between solid sodium and gaseous chlorine proceeds imperceptibly, if at all, until the addition of a drop of water sets it off. 

In both fundamental research and practical applications of chemistry, chemical reactions are performed by mixing the reactants and regulating external conditions such as temperature and pressure. Two questions arise immediately  

If the reaction is possible, what determines the ratio of products and reactants at equilibrium  

Predicting the equilibrium composition for a chemical reaction is the central goal of Unit 4, and in this chapter, we develop the conceptual basis for answering these two key questions. 

We can find out whether a proposed reaction is possible by determining whether it is a spontaneous thermodynamic process. In this context, spontaneous has a precise technical meaning (see later for clarification) that should not be confused with its conversational meaning, such as describing the spontaneous behavior of people in social situations. Thermodynamics can tell us whether a proposed reaction is possible under particular conditions even before we attempt the reaction. If the reaction is spontaneous, thermodynamics can also predict the ratio of products and reactants at equilibrium. But, we cannot use thermodynamics to predict the rate of a spontaneous reaction or how long it will take to reach equilibrium. These questions are the subject of chemical kinetics. To obtain a large amount of product from a spontaneous reaction in a short time, we need a reaction that is spontaneous and fast. 

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Chemical processes are central to the study of chemistry. The thermodynamic principles and relationships we have developed provide powerful tools for describing these processes, especially in predicting the spontaneity of the process and the equilibrium conditions that apply. 

An important use of the free energy function is to obtain a simple criterion for the occurrence of spontaneous processes and for thermodynamic equilibrium. According to the second law of thermodynamics,... 

The connection between entropy and the spontaneity of a reaction is expressed by the second law of thermodynamics The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process. Since the universe is made up of the system and the surroundings, the entropy change in the universe for any process is the sum of the entropy changes in the system (ASsys) and in the surroundings (ASsur,). Mathematically, we can express the second law of thermodynamics as follows ... 

The second law of thermodynamics states that the entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process. The mathematical statement of the second law of thermodynamics is given by... 

The second law of thermodynamics states that the entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process. We learn ways to calculate the entropy change of a system and of the surroundings, which together make up for the change in the entropy of the universe. We also discuss the third law of thermodynamics, which enables us to determine the absolute value of entropy of a substance. (18.4)... 

In thermodynamics, a metastable equilibrium state has at least three constraints. Two of these constraints apply to a stable equilibrium state, and the third prevents the system from achieving that state. On releasing the third constraint the system experiences a spontaneous process and achieves the stable equilibrium state. We have seen two examples so far, in Figures 4.1 and 4.6. These examples were chosen to follow from our definition of entropy, and show spontaneous processes having no overall energy change in the system. They show entropy acting as a thermodynamic potential. 

The standard entropy of a chemical reaction can be calculated from the absolute entropies of reactants and products. The third law of thermodynamics states that the entropy of a perfect crystalline substance is zero at 0 K. This law enables us to measure the absolute entropies of substances. Under conditions of constant temperature and pressure, the free-eneigy change AG is less than zero for a spontaneous process and greater than zero for a nonsponta-neous process. For an equilibrium process, AG = 0. 

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