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Temperature Effects on Equilibrium

A Study Guide

Discussion Questions

Temperature Effects on Equilibrium

For convenience, we treat temperature, entropy, enthalpy, and Gibb's energy as general state functions and represent them by T, S, H, and G respectively, dropping the change symbol D, associated with them. The justification for doing these have been introduced in Gibb's energy.

Since S, H, and G depends on T, temperature affects chemical reactions and chemical equilibria. We discuss the temperature effects in general terms, and the strategy introduced here may be useful for solving complicated problems later.

How does temperature affect Gibb's energy?

Gibb's energy, G, is a general state function defined in terms of state functions entropy S and enthalpy H. It's relationship with the equilibrium constant K has also been defined using the formula below. This notations are widely used in the field of thermodynamics. G = H - T S
    = - R T ln K
For entropy, we have defined the standard or absolute entropy So as So = ò0298 CP dT / T The symbol CP is the heat capicity at constant pressure, it also depends on T, not a constant.

Thermodynamic data usually lists standard enthalpy Ho, and standard entropy So. When measured at temperature T other than 298 K, their values are given by

DS = So + òT298 CP dT / T
      = So + CP ln (298/T)
DH = Ho + òT298 CP dT
      = Ho + CP (T - 298)
The integrations given above are valid only if CP is a constant between 298 and T K. If the temperature range is large, an integration process either by mathmatical means or by numerical means should be carried out. Within small ranges, the assumption is reasonable, especially if average values are used. Under the circomstance, DG = Go + CP ln (298/T) + CP (T - 298)
      = - R T ln K
The above model shows that Gibb's energy depends on temperature. This formulation is true if CP is a constant between 298 and T K. In general, the heat capacity varies with temperature and the variation should not be ignored if the temperature range is very large. In some cases, an average value for the heat capacity may be used.

Gibb's free energies of a system drive both physical and chemical changes. For physical changes, the heat capacities of the system over the ranges of temperature should be considered. In cases of chemical reactions, the differences in heat capacity between products and reanctants shall be considered.

How does temperature affect equilibrium?

For chemical reactions, the differences in heat capicity between the rpoducts and the reactants DCP are to be used. Using the equation derived previously, DG = Go + DCP ln (298/T) + DCP (T - 298)
      = - R T ln K,
the effect of temperature on the equilibrium constant K can be derived, ln K = - DGo / R T - [CP / (R T)] [298 - T + ln (298/T)]
      = - ln Ko - [CP / (R T)] [298 - T + ln (298/T)]
Recall that Ko is the equilibrium constant at 298 K. The equation can be simplified to give, ln (K / Ko) = - [CP / (R T)] [298 - T + ln (298/T)]
And to generalize it, if K1 and K2 are equilibrium constants at T1 and T2 respectively, we can derive the formula, ln (K2/K1) = - [CP / R] [(1/T2) - (1/T1)] [T1 - T2 + ln (T1/T2)] By measuring the equilibrium constants at different temperatures and using this equation, we may estimate the heat capacities over ranges of temperatures. Note that CP(T1 - T2) = DH And if we ignore the term ln (T1/T2), the above equation is the same as the familiar Helmholtz equation, which is discussed in Example 1 below.

The above relationship has not been carefully checked out yet. If you detect any error, please e-mail me.

Example 1

The Gibbs-Helmholtz equation is d (DG / T)/dT = - DH / T 2. Show that
           DH
d ln K = - --- d(1/T)
            R

Solution
Since

DG = - R T   ln K,
ln K = - DG / R T
Differentiate both sides with respect to (1/T) in the above equation gives, d(ln K) / (d T) = (- 1 / R (d DG / T) / (dT)
    = - DH / R T 2

Discussion
If K1 and K2 are the equilibrium constant at T1 and T2 respectively, show further that

ln (K1 / K2) = - (DH / R) (1/T1 - 1/T2). This is achieved by definite integral. This relationship indicates that the plot of ln (K versus 1/(T is a straight line, and the slope is - (DH / R). Thus, DH can be determined by measuring the equilibrium constant at different temperatures.

Example 2

The vapour pressure of water at 298 K is 23.756 mmHg. Estimate the molar standard Gibb's energy.

Solution
The vapour pressure of 23.756 mmHg should be converted to a pressure in units of pascal (N m-2),

       101,300 N m-2
23.756 ----------- = 3166 Pa
        760 mmHg
Go = 8.312 J * 298 ln(3166)
    = 20.0 kJ / hr

Discussion
This example illustrates the evaluation of Gibb's energy when the equilibrium constant is known.

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