Products Minus Reactants

Many of the reactions that chemists study are reactions that occur at constant pressure. During the discussion of the coffee-cup calorimeter, the heat change at constant temperature was defined as qp. Because this constant-pressure situation is so common in chemistry, a special thermodynamic term is used to describe this energy enthalpy. The enthalpy change, AH, is equal to the heat gained or lost by the system under constant-pressure conditions. The following sign conventions apply  

If a reaction is involved, AH is sometimes called Ai eaction. AH is often given in association with a particular reaction. For example, the enthalpy change associated with the formation of water from hydrogen and oxygen gases can be shown in this fashion  

The negative sign indicates that this reaction is exothermic. This value of AH is for the production of 2 mol of water. If 4 mol were produced, AH would be twice -483.6 kj. The techniques developed in working reaction stoichiometry problems (see the Stoichiometry chapter) also apply here. 

If the reaction above for the formation of water were reversed, the sign of A//would be reversed. That would indicate that it would take 483-6 kj of energy to decompose 2 mol of water. This would then become an endothermic process. 

AH is dependent upon the state of matter. The enthalpy change would be different for the formation of liquid water instead of gaseous water. 

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We now turn specifically to the thermodynamics and kinetics of reactions (5. EE) and (5.FF). The criterion for spontaneity in thermodynamics is AG <0 with AG = AH - T AS for an isothermal process. Thus it is both the sign and magnitude of AH and AS and the magnitude of T that determine whether a reaction is thermodynamically favored or not. As usual in thermodynamics, the A s are taken as products minus reactants, so the conclusions apply to the reactions as written. If a reaction is reversed, products and reactants are interchanged and the sign of the AG is reversed also. 

Resonance stabilization energies are generally assessed from thermodynamic data. If we define to be the resonance stabilization energy of species i, then the heat of formation of that species will be less by an amount ej than for an otherwise equivalent molecule without resonance. Likewise, the AH for a reaction which is influenced by resonance effects is less by an amount Ae (A is the usual difference products minus reactants) than the AH for a reaction which is otherwise identical except for resonance effects ... 

Here CPj is the molar heat capacity of species At at constant pressure and ACP is the overall change in heat capacity (products minus reactants)... 

B is correct. The change in energy is energy of products minus reactants. 

The difference (measured as products minus reactants ) in potential energy is negative. Thus the enthalpy change will be negative and the elementary reaction is exothermic. 

A chemical reaction involves making and breaking bonds, with the gain, loss, or transformation of a functional group. Enthalpy is the term used to measure bond dissociation energy, and AH° for a reaction is the difference in bond dissociation energy for bonds made (products) minus that for bonds broken (starting materials). This is products minus reactants. 

The products-minus-reactants tactic is a very useful one in thermodynamics. It is also a useful idea to carry along with respect to other state functions the change in any state function is the final value minus the initial value. In Example 2.18 above, the state function of interest was enthalpy, and by applying Hess s law and the definition of formation reactions, we were able to develop a procedure for determining the changes in enthalpy and internal energy for a chemical process. 

What is the relationship between AH and AC/ for a chemical reaction If one knows the Af C7 and AfH values for the products and reactants, one can simply compare them using the products-minus-reactants scheme of equations 2.55 and 2.56. There is another way to relate these two state functions. Recall the original definition ofH from equation 2.16 ... 

For the second step, we need to evaluate AH for the reaction at 298 K. Using the products-minus-reactants approach, we find... 

This is the classic products minus reactants approach used for all state hinctions. Units are simplified from the beginning for clarity. 

Chemical potential is a measure of how much a species wants to undergo a physical or chemical change. If two or more substances exist in a system and have different chemical potentials, some process would occur to equalize the chemical potentials. Thus, chemical potential allows us to begin a consideration of chemical reactions and chemical equilibrium. Although we have considered chemical reactions in some examples (mostly from a products-minus-reactants change in energy or entropy), we have not focused on them. This will change in the next chapter. 

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