CRE: Gibbs Free Energy in Reacting Systems

Gibbs Free Energy in Reacting Systems

If we consider the following reaction:

$H_2 + Cl_2 \Longleftrightarrow 2HCl$

We might be tempted to decide that the reaction will proceed until we have minimized the energy of the system. In order to examine that, we consider the bond strengths of each of the bonds:

$D_{H-H} = 4.5 eV$, $D_{Cl-Cl} = 2.5 eV$, $D_{H-Cl} = 4.5 eV$

so our three species have, respectively

$H_2 = 4.5 eV$, $Cl_2 = 2.5 eV$, $2xHCl = 2x4.5 eV = 9.0eV$

In other words, we need to put in 7$eV$ to break the bonds of the "reactants", but regain 9$eV$ when they reform as 2 molecules of $HCl$. This would suggest that the reaction would (at equilibrium) proceed completely to the right.

However, having two moles of differing material (or better yet, all three!) gives us higher entropy than if we make a system of pure $HCl$.


This entropic argument becomes particularly powerful when we either gain (higher entropy) or lose (lower entropy) moles as the reaction proceeds (i.e., $\sum_i\nu_i \ne 0$).

In reality, we need to balance our change in energy with the change in entropy (trying to minimize energy, while maximizing entropy), in much the same way that we did with phase equilibrium.

Mathematically, we can see this by considering the Gibbs Free Energy of the system (which is a mixture!):

$G = \sum_i n_i \bar G_i = \sum_i n_i \mu_i$

we recall that we need a reference state for $\bar G_i$ or $\mu_i$.


We will use the standard state as our reference point, which is chosen to be the T of interest in our problem and a pressure of 1 bar.

this gives us a $\mu_i$ expression of

$\mu_i = g_i^o + RT \ln \left [\frac{\hat f_i}{f_i^o} \right ]$

For simplicity, let's use an ideal gas for this example, so we reduce our expression to:

$\mu_i = g_i^o + RT \ln \left [\frac{p_i}{1 bar} \right ]= g_i^o + RT \ln \left [\frac{y_iP_{tot}}{1 bar} \right ]$

Plugging this into our total $G$ expression, we can expand each of the ln terms and make use of the ln product rule to get

$G = \sum_i n_i g_i^o + RT \sum_i n_i\ln \left [P_{tot} \right ] + RT \sum_i n_i\ln \left [y_i \right ]$

where we can note that:

If we plot this for the reaction in question, we get

g reaction

The minimum Gibbs Free Energy balances the entropy gained through diversifying the material present with the energy gained by changing the bond energies. We note that the minimum free energy as a function of $\xi$ is found where $\displaystyle{\frac{dG}{d\xi}=0}$.


Explain the relationship between energy and entropy in reacting systems