Matter energy changes

We now consider briefly the matter of electrode potentials. The familiar Nemst equation was at one time treated in terms of the solution pressure of the metal in the electrode, but it is better to consider directly the net chemical change accompanying the flow of 1 faraday (7 ), and to equate the electrical work to the free energy change. Thus, for the cell... 

Batch calorimeters are instmments where there is no flow of matter in or out of the calorimeter during the time the energy change is being measured. Batch calorimeters differ in the way the reactants are mixed and in the method used to detennine the enthalpy change. Enthalpy changes can be measured by the various methods... 

The second part of the first law of thermodynamics arises when the requirement that the process be adiabatic is dropped recall that this means the system is not insulated, and processes can be caused by heating and cooling. In a general process (the only assumption is that matter is not added or removed from the system), if an amount of work W is done on the system and the energy changes by DE then the heat supplied to the system Q is defined by... 

Spontaneous processes result in the dispersal of matter and energy, hi many cases, however, the spontaneous direction of a process may not be obvious. Can we use energy changes to predict spontaneity To answer that question, consider two everyday events, the melting of ice at room temperature and the formation of ice in a freezer. 

Although there is no change in the total of the mass numbers, the quantity of matter does change significantly. Some matter is changed to energy. 

Without any more information, we can t say which of the two possibilities actually happens. The nice thing about thermodynamics is that it doesn t matter. Now let s see if you can be convinced of this. We know we can add free energies of individual reactions to get the free-energy change of another reaction. 

The thermodynamic standard state of a substance is its most stable state under standard pressure (1 atm) and at some specific temperature (usually 25°C). Thermodynamic refers to the observation, measurement and prediction of energy changes that accompany physical changes or chemical reaction. Standard refers to the set conditions of 1 atm pressure and 25°C. The state of a substance is its phase gas, liquid or solid. Substance is any kind of matter all specimens of which have the same chemical composition and physical properties. 

Strictly, it should be a relation between AG" and AG, but not AH9 or AG9, because what matters is the enthalpy or free energy change under the reaction conditions, not the standard conditions. 

Figure 4.3 Free energy changes in redox reactions mediated by microbes, (a) Oxidation of reduced inorganic compounds linked to reduction of O2. (b) Oxidation of organic matter CH2O linked to reduction of various organic and inorganic oxidants. pH = 7 and unit oxidant and reductant activities except (Mn +) = 0.2mM and (Fe +) = ImM...

In broad terms the decomposition of organic matter under anaerobic conditions is expected to be slower than nnder aerobic conditions becanse the free energy changes for the reactions involved are mnch smaller (Table 4.1 and Fignre 4.3). For example, for the aerobic decomposition of CH2O ,... 

One can consider two facets of the solvation process, the energetics and the kinetics. Clearly, the kinetics will not matter if reactions take place on a time scale that is much faster or much slower than the solvation process. However, if reaction and solvation occur on the same time scale, the considerable energy changes that the solvation process can engender will affect the reaction. In fact, exactly what solvent motions take place during the solvation process may well be important. Thus, it is of interest to understand the kinetics of the solvation process. 

This is a short but critically important section. When a system is at equilibrium, it has no tendency to change in either direction (forward or reverse) and will remain in its state until it is disturbed from outside the system. For example, when a block of metal is at the same temperature as its surroundings, it is in thermal equilibrium with them, and energy has no tendency to flow into or out of the block as heat. When a gas confined to a cylinder by a piston has the same pressure as the surroundings, the system is in mechanical equilibrium with the surroundings, and the gas has no tendency to expand or contract (Fig. 7.21). When a solid, such as ice, is in contact with its liquid form, such as water, at certain conditions of temperature and pressure (at 0°C and 1 atm for water), the two states of matter are in physical equilibrium with each other, and there is no tendency for one form of matter to change into the other form. Physical equilibria, which include vaporization as well as melting, are dealt with in detail in Chapter 8. When a chemical reaction mixture reaches a certain composition, it seems to come to a halt. A mixture of substances at chemical equilibrium has no tendency either to produce... 

Even if AG is a large negative quantity the reaction is, of course, not necessarily fast. The rate depends on the activation barrier that the reactants must overcome to reach the transition state. If the barrier is too high, then no matter how exothermic the reaction is, it cannot take place. However, in the absence of special effects there is usually a qualitative correlation between a reaction s net energy change and its energy of activation. This point is discussed further in Section 2.6. 

In 1900 Max Planck proposed a solution to the problem of black-body radiation described above. He suggested that when electromagnetic radiation interacts with matter, energy can only be absorbed or emitted in certain discrete amounts, called quanta. Planck s theory will not be described here, as it is highly technical. In any case, Planck s proposal was timid compared with the theory that followed. He supposed that quanta were only important in absorption and emission of radiation, but that otherwise the wave theory did not need to be modified. It was Einstein who took a more radical step in 1905 (the year in which he published his first paper on the theory of relativity and on several other unrelated topics). Einstein s analysis of the photoelectric effect is crucial, and has led to a complete change in the way we think of light and other radiation. 

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