Standard-state reaction

We have assumed throughout the previous discussion that the temperature of reaction 2.1 is 298.15 K. What if the reaction enthalpy at a different temperature is required Let us assume, for instance, that we need to evaluate Ar//(2.1) at 310 K. As shown by the cycle in figure 2.2 or by equation 2.9, the first step in this exercise is to evaluate the temperature effect on the standard state reaction 2.2. 

Information on partial molar heat capacities [1,18] is indeed very scarce, hindering the calculation of the temperature correction terms for reactions in solution. In most practical situations, we can only hope that these temperature corrections are similar to those derived for the standard state reactions. Fortunately, due to the upper limits set by the normal boiling temperatures of the solvents, the temperatures of reactions in solution are not substantially different from 298.15 K, so large ArCp(T - 298.15) corrections are uncommon. 

The combustion of organochlorine or -bromine compounds is commonly referred to the following standard state reaction (normally n = 600) ... 

Also listed in table 8.3 are the enthalpy data for the auxiliary reactions, which, in conjunction with the enthalpy of reaction 1, were used to calculate the enthalpy of the standard state reaction ... 

If the reaction is a standard state reaction where the starting materials in their standard states react to give products in their standard states, and the standard heats of formation of the elements are assumed... 

For a standard state, H+ concentration is set as 1M, which is equal to pH = 0. However, biological reactions do not occur at pH = 0, thus for standard state reactions, scientists adopted pH of 7 (10 M). For a generalized reaction involving H+ ... 

If the reaction is a standard state reaction where the starting materials in their standard states react to give products in their standard states, and the standard heats of formation (A// 7) of the elements are assumed to be zero at any given temperature, then the standard heat of reaction, A//7-(reaction), IS cxprcssed as in equation (2.18) ... 

The molar enthalpy of any substance depends on its state. The standard state of a liquid or solid substance is specified to be the pure substance at a fixed pressure of exactly 1 bar (100,000 Pa), which we denote by P°. The standard state for a gas is defined to be the corresponding ideal gas at pressure P°. The difference between the molar enthalpy of a real gas at 1 bar pressure and the corresponding ideal gas at 1 bar is numerically very small, but we will discuss this difference in a later chapter. If substance number i is in its standard state, its molar enthalpy is denoted by H (i). A standard-state reaction is one in which all substances are in their standard states before and after the reaction. The enthalpy change for a standard-state reaction is denoted by A 71°. The standard-state pressure was at one time defined to equal 1 atm (101,325 Pa). The difference in numerical values is small, and the formulas involving P° are the same with either choice. For highly accurate work, one must determine which standard pressure was used for an older set of data. 

The enthalpy change for 1 mol of any standard-state reaction in our restricted class is given by... 

Reactions Other Than Standard-State Reactions... 

If the products and reactants are not at their standard states, the enthalpy change for a reaction can have a different value from that of the standard-state reaction. For the reactions in our present class this difference is small. The enthalpy of an ideal gas does not depend on the pressure, and the enthalpies of real gases are nearly constant for moderate pressure changes. The effect on the enthalpy of pure solids and liquids due to moderate changes in pressures is also small. In a later chapter we will learn how to calculate these effects, but unless there is some need for great accuracy we will use the value of the standard-state enthalpy change at another pressure. 

The anions of ATP and ADP form complexes with positive ions such as Mg " " or Ca " ". It is customary to define a modified standard-state reaction in which the substances in the reaction equation are at unit activities but the hydrogen ions and any complexing cations are at specified activities not neeessarily equal to unity. The symbol AG° is used for the Gibbs energy change of such a modified standard-state reaction. 

For the standard-state reaction, this equation becomes... 

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