Reversible reactions enthalpy changes

Enthalpies of phase transitions are reported in kilojoules per mole. The enthalpy change for a reverse reaction is the negative of the enthalpy change for the forward reaction. Enthalpy changes can be added to obtain the value for an overall process. 

SOLUTION We must write the reaction of interest as a sum of the reactions for which we have thermochemical data. If a reaction is reversed, the enthalpy changes sign. 

If the direction of a chemical reaction is reversed, the enthalpy change reverses sign. Heat is required to convert CO2 to CO and O2 at constant pressure. The decomposition of CO2 into CO and O2 is difficult to perform in the laboratory, whereas the reverse reaction is straightforward. Thermodynamics allows us to predict AH of the decomposition reaction with complete confidence, even if a calorimetric experiment is never actually performed for it. 

The treatments of chemical kinetics within the frame of the Arrhenius and the Eyring approaches were essentially based on the postulates of classical statistical equilibrium thermodynamics. It was assumed that a chemical system must pass through the sequence of equilibrium states. The principle of microscopic reversibility holds true all the way from the initial to final products. This implies that the pathways of the forward and backward reactions coincide. We have mentioned above that there exists a method of verification of the validity of thermodynamic equations used for the determination of the reaction enthalpy change. The heat production, AH, can be measured directly using a calorimetric technique. This cannot be done for the activation energy, It is necessary, therefore, to scrutinize the applicability of the conventional approaches of physical chemistry for a description of biochemical processes. 

Equations (1) and (2) are the heats of formation of carbon dioxide and water respectively Equation (3) is the reverse of the combustion of methane and so the heat of reaction is equal to the heat of combustion but opposite in sign The molar heat of formation of a substance is the enthalpy change for formation of one mole of the substance from the elements For methane AH = —75 kJ/mol... 

It is more common to find that AH° and AS° have the same sign (Table 17.2, III and IV). When this happens, the enthalpy and entropy factors oppose each other. AG° changes sign as temperature increases, and the direction of spontaneity reverses. At low temperatures, AH° predominates, and the exothermic reaction, which may be either the forward or the reverse reaction, occurs. As the temperature rises, the quantity TAS° increases in magnitude and eventually exceeds AH°. At high temperatures, the reaction that leads to an increase in entropy occurs. In most cases, 25°C is a low temperature, at least at a pressure of 1 atm. This explains why exothermic reactions are usually spontaneous at room temperature and atmospheric pressure. 

In each step, we may need to reverse the equation or multiply it by a factor. Recall from Eq. 16 that, if wc want to reverse a chemical equation, wc have to change the sign of the reaction enthalpy. If we multiply the stoichiometric coefficients by a factor, we must multiply the reaction enthalpy by the same factor. 

The enthalpy change for a reverse reaction is the negative of the enthalpy change... 

We saw in Section 6.11 that the first law of thermodynamics implies that, because enthalpy is a state function, the enthalpy change for the reverse of a process is the negative of the enthalpy change of the forward process. The same relation applies to forward and reverse chemical reactions. For the reverse of reaction A, for instance, we can write... 

For example, consider a system in which metallic zinc is immersed in a solution of copper(II) ions. Copper in the solution is replaced by zinc which is dissolved and metallic copper is deposited on the zinc. The entire change of enthalpy in this process is converted to heat. If, however, this reaction is carried out by immersing a zinc rod into a solution of zinc ions and a copper rod into a solution of copper ions and the solutions are brought into contact (e.g. across a porous diaphragm, to prevent mixing), then zinc will pass into the solution of zinc ions and copper will be deposited from the solution of copper ions only when both metals are connected externally by a conductor so that there is a closed circuit. The cell can then carry out work in the external part of the circuit. In the first arrangement, reversible reaction is impossible but it becomes possible in the second, provided that the other conditions for reversibility are fulfilled. 

In addition to its constraints on the concentration dependent portions of the rate expression thermodynamics requires that the activation energies of the forward and reverse reactions be related to the enthalpy change accompanying reaction. In generalized logarithmic form equation 5.1.69 can be written as... 

The specified reaction is twice the reverse of the formation reaction, and its enthalpy change is twice negative of the enthalpy of formation of NH3(g) ... 

We can measure enthalpies of reaction using a calorimeter. However, we can also calculate the values. Hess s law states that if we express a reaction in a series of steps, then the enthalpy change for the overall reaction is simply the sum of the enthalpy changes of the individual steps. If, in adding the equations of the steps together, it is necessary to reverse one of the given reactions, then we will need to reverse the sign of the AH. In addition, we must pay particular attention if we must adjust the reaction stoichiometry. 

It may prove possible to apply titration calorimetry data in one further direction. If AG can be estimated for SAL-goethite complexation and reaction enthalpies can be obtained under equilibrium conditions, then an entropy change for this reaction can also be derived. This can only be done, however, if the adsorption reaction can be shown to be reversible. Since this has not been proven as yet in our systems, such thermodynamic extensions of titration calorimetry can only be speculative at this time. 

A given chemical equation is tripled and then reversed. What effect, if any, will there be on the enthalpy change of the reaction ... 

From left to right, the reaction is exothermic. Therefore, the enthalpy change, AH, is negative. If the enthalpy change was the only condition that determined whether a reaction is favourable, then the synthesis reaction would take place. The synthesis reaction does take place—but only at relatively moderate temperatures. Above 400 C, the reverse reaction is favourable. The decomposition of HgO(s) occurs. Thus, the direction in which this reaction proceeds depends on temperature. 

This reaction, having equal number of mols of gas reactants and products, has a negligible change in entropy and thus a negligible heat effect if carried out reversibly at constant temperature. The maximum work available from a fuel cell under these circumstances would then be approximately the enthalpy change of the reaction, i.e., the heat of combustion of the... 

X10 kJ. First, recognize that the given enthalpy change is for the reverse of the electrolysis reaction, so you must reverse its sign from -572 to 572. Second, recall that heats of reaction are proportional to the amount of substance reacting (2 mol of H2O in this case), so the calculation is... 

The standard enthalpy change for the ionization of water is +55.83 kJ mol", which means that the reverse reaction, which occurs when acids are neutralized by bases, is exothermic, i.e. ArH = —55.83 kJ mol-1. The corresponding change in standard Gibbs energy is —79.9 kJ mol - . The reaction ... 

The reactions of the thiophene derivatives in both forward and reverse directions are characterized by lower enthalpies and entropies of activation than the reactions of the selenophene analogs. In the forward reactions, enthalpy and entropy changes compensate nearly exactly and result in slightly greater rates of adduct formation for the selenophene derivatives despite the higher enthalpies of activation. The higher entropies of activation for the selenophene derivatives have been attributed to less solvated transition states as compared to the reactions of the thiophene analogs (Table XXVIII). 

---