Thermodynamic Output

5.2 Thermodynamic Output

When given a collection of molecular properties, thermo uses statistical mechanics formulae to calculate the corresponding thermodynamics parameters. Thermo can also calculate canonical transition state theory rate constants. Much of the output is obvious, but some items are explained here.

Standard State: the standard state must be selected. The conventional standard state for most tabulations is 1 bar, ideal gas. The numerical value of the equilibrium constant for a reaction ( Kequil) depends on the selection of the standard state. When comparing forward and reverse reaction rate constants, for example, it is often more convenient to choose the standard state of 1

molecule cm -3 . Molar Enthalpy: The enthalpy for formation at 0 K is required input. Thermo output echoes the

input and also reports the enthalpy of formation at 298.15 K and the standard free energy of formation ( DelG(298)).

Equilibrium constant: Kequil is reported for every temperature. In addition, it is reported as a function of temperature: Kequil = A(T)*exp(B(T)/T). The parameters A(T) and B(T), which are obtained by finite differences, are in general functions of temperature. The accuracy of these parameters is less than the accuracy of Kequil itself. Note that the numerical value of the equilibrium constant depends on the standard state.

Canonical Transition State Theory Rate Constant: " RATE k(T)" is reported for every temperature. In addition, it is reported as a function of temperature: k(T) = A(T)*exp(B(T)/T). The parameters A(T) and B(T), which are obtained by finite differences, are in general functions of temperature. The accuracy of these parameters is less than Canonical Transition State Theory Rate Constant: " RATE k(T)" is reported for every temperature. In addition, it is reported as a function of temperature: k(T) = A(T)*exp(B(T)/T). The parameters A(T) and B(T), which are obtained by finite differences, are in general functions of temperature. The accuracy of these parameters is less than

Molar Entropy: the numerical value for the entropy depends on the standard state that is selected, as well as on the energy units selected.

Molar Heat Capacity: Cp depends on energy units selected. Enthalpy Function: [H(T)-H(0)] depends on the energy units selected. Accuracy:

All accuracies depend on the accuracy of the input data. For a given set of input data, the accuracies achieved by Thermo are relative to a benchmark based on the same input data. The

benchmarks most commonly used are taken from the NIST-JANAF Thermochemical Tables. 35 Note that when rotations are treated classically, the entropy, heat capacity, free energy, and

equilibrium constant are less accurate at low temperatures. • molecular weights are accurate to 0.002 g mol -1 , or better.

• enthalpies and Gibbs free energy for individual species generally agree with the JANAF tables to within 0.05 kJ mol -1 , or better.

• entropies for individual species are generally accurate to 0.1 J K -1 mol or better. • electronic partition function is accurate to 0.1% or better.

• enthalpy, entropy, heat capacity, and Gibbs free energy differences for reaction ( DelS(rxn), DelH(rxn), DelCp(rxn) and DelG(rxn), respectively) are generally more accurate than the corresponding quantities for the individual species.