Thermodynamics spontaneity

The oxidation of hydrogen to water (Hj -t- i Oj -> HjO) is thermodynamically spontaneous and the energy released as a result of the chemical reaction appears as heat energy, but the decomposition of water into its elements is a non-spontaneous process and can be achieved only by supplying energy from an external source, e.g. a source of e.m.f. that decomposes the water electrolytically. Furthermore, although the heat produced by the spontaneous reaction could be converted into electrical energy, the electrical... 

FIGURE 12.17 Iron nails stored in oxygen-free water (left) do not rust, because the oxidizing power of water itself is weak. When oxygen is present (as a result of air dissolving in the water, right), oxidation is thermodynamically spontaneous and rust soon forms. 

Equation (3) defines the equilibrium condition under the constraint that temperature and pressure are constant. A related consequence of the Second Law is that if AG < 0 the reaction of the reactant to product is thermodynamically spontaneous. Thermodynamic spontaneity means that... 

In mammalian cells, the two most common forms of covalent modification are partial proteolysis and ph osphorylation. Because cells lack the ability to reunite the two portions of a protein produced by hydrolysis of a peptide bond, proteolysis constitutes an irreversible modification. By contrast, phosphorylation is a reversible modification process. The phosphorylation of proteins on seryl, threonyl, or tyrosyl residues, catalyzed by protein kinases, is thermodynamically spontaneous. Equally spontaneous is the hydrolytic removal of these phosphoryl groups by enzymes called protein phosphatases. 

Notice that the word spontaneous has a different meaning in thermodynamics than it does in everyday speech. Ordinarily, spontaneous refers to an event that takes place without any effort or premeditation. For example, a crowd cheers spontaneously for an outstanding performance. In thermodynamics, spontaneous refers only to the natural direction of a process, without regard to whether it occurs rapidly and easily. Chemical kinetics, which we introduce in Chapter 15, describes the factors that determine the speeds of chemical reactions. Thermodynamic spontaneity refers to the direction that a process will take if left alone and given enough time. 

SAQ 4.2 By inspection alone, decide whether the oxidation of sulphur dioxide is thermodynamically spontaneous or not. The stoichiometry of the reaction is V202(g)... 

A process is thermodynamically spontaneous only if the overall value of AS is positive. 

The value of AG is only ever negative, as required by a thermodynamically spontaneous process, if the initial pressure / (initial) is greater than the final pressure finai), i.e. the fraction is less than one. In other words, Equation (5.9) shows why AG is negative only if the pressure of the CO2 in the space above the liquid has a pressure that is greater than... 

We now consider the emf in more detail, and start by saying that it represents the separation in potential between the two halfcell potentials See Equation (7.22). In order for AG(cen) to remain positive for all thermodynamically spontaneous cell discharges, the emf is defined as always being positive. 

The liquid water will have a lower entropy than the gaseous water, so the process will not be thermodynamically spontaneous. 

Now, if a reaction is thermodynamically spontaneous in the direction it is written then AG for the reaction will have a negative sign. 

Redox reactions that are thermodynamically spontaneous will have a positive value of E. 

Thus the reaction of the ground-state Ru(II) species with MV2+ will not be thermodynamically spontaneous. 

Since E has a positive value, AG has a positive sign, indicating that the reaction is thermodynamically spontaneous. 

AG <0, thermodynamically spontaneous (energy released, often irreversible) AG >0, thermodynamically nonspontaneous (energy required) AG = 0, reaction at equilibrium (freely reversible) AG = energy involved under standardized conditions Decrease energy of activation, AG ... 

Although endothermic, these reactions are thermodynamically spontaneous at high temperature due to the favorable entropy change associated with the formation of random gaseous products from solid starting materials. 

The carboxylation of acetone with HCO to form acetoacetate (the reverse of Eq. 13-44) is not a thermodynamically spontaneous process (AG0 ... 

The acetylferrocene does not consume the (FeCp2+)surf., Figure Ad, because the reaction is not thermodynamically spontaneous. The conflict in our data is that steady-state photocurrents for the fast reductants is the same, but the (FeCp2+)surf. can be consumed at different rates in the dark for the various fast reductants. 

From Table 7-1, the formation of diamond from graphite (the standard state of carbon) is accompanied by a positive AH of 1.88kJ/mol at 25°C. From Problem 16.1(f), AS for the same process is negative. Since 25°C is not the transition temperature, the process is not a reversible one. In fact, it is not even a spontaneous irreversible process, and (16-2) does not apply with the inequality sign. On the contrary, the opposite process, the conversion of diamond to graphite at 1 atm, is thermodynamically spontaneous. The AS for this process would obey (16-2) with the inequality sign. This means that diamonds are NOT forever The term spontaneous does not cover the speed... 

Thus the latter equation AGsystem = — 7j A Vllnjverse shows how the two criteria which can be used to assess the thermodynamic spontaneity of reactions or processes are basically identical since if ... 

Calculate the entropy change, in calories/degree, that accompanies the separation of four components from one another in an ideal solution containing one mole of each. When separated, each component occupies one-quarter of the original volume. Deduce from your results whether or not the separation is thermodynamically spontaneous. 

Example 2.10 Is the reaction between metallic Fe and a 1 M solution of a strong (non-oxidizing) acid to produce Fe2+ ions thermodynamically spontaneous ... 

As you may recall from physical chemistry, the thermodynamic spontaneity of a reaction is governed by the Gibbs free energy, G. For a phase transformation from an initial to final state, the AG (Eq. 10 Gfinai — Gjnitiai) must be < 0 for the process to be spontaneous ... 

Since the overall electrochemical process is thermodynamically spontaneous, the free energy change associated with the electrochemical reaction is directly converted to dc electrical energy. 

The thermodynamically spontaneous protein-mediated transmembrane movement of a substance from high to low concentration is called... 

---