Energy changes during reactions

Fig. 22.7. Thermodynamic driving forces for various anaerobic (top) and aerobic (bottom) microbial metabolisms during mixing of a subsea hydrothermal fluid with seawater, as a function of temperature. Since the driving force is the negative free energy change of reaction, metabolisms with positive drives are favored thermodynamically those with negative drives cannot proceed. The drive for sulfide oxidation is the mirror image of that for hydrogentrophic sulfate reduction, since in the calculation 02(aq) and H2(aq) are in equilibrium.

Since the possible violence of reactions lies in the heat liberated and the temperature which may be reached, the energy change during the reaction has been selected to present the reaction safety in the ISI. This is a feasible approach since the formation enthalpies are known for most substances. 

For approximate estimation of enthalpy changes during reactions, use can be made of empirical bond energies (Table 6-7) which represent the approximate enthalpy changes (-AH°) for formation of compounds in a gaseous state from atoms in the gas phase. Other more comprehensive methods of approximation have been developed 49/50... 

The actual reaction of sodium with chlorine occurs all at once rather than in a stepwise manner, but energy calculations can be made more easily if we imagine a series of hypothetical steps for which experimentally measured energy values can be obtained. There are five contributions that must be taken into account to calculate the overall energy change during the formation of solid NaCI from solid sodium metal and gaseous chlorine molecules ... 

Figure 5.1 Energy changes during a chemical reaction.

Figure 5.3 Free energy changes during a multistep reaction.

The Gibbs free energy change during a reaction is a measure of the reversible work (other than pressure-volume work) that can be obtained from the process at constant T and p. Since cellular processes are isothermal and isobaric, free energies are the quantities of choice in studying metabolic processes with respect to their ability to carry out the work of cells. 

Figure 10.36 Free energy changes during the course of a chemical reaction.

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