About this experiment
Explore the difference between standard reaction Gibbs energy and the composition-dependent driving force. Set temperature and composition, locate the equilibrium minimum and follow a declared relaxation model downhill in free energy.
Learning goals
- Compute standard Gibbs energy from enthalpy, entropy and temperature.
- Include reaction quotient in the actual thermodynamic driving force.
- Relate equilibrium composition to the Gibbs-energy minimum and equilibrium constant.
How to use it
- 1Compare the sign of standard reaction Gibbs energy above and below the crossover temperature.
- 2Choose an initial composition on the free-energy landscape and inspect its actual driving force.
- 3Relax toward equilibrium and check that mixture Gibbs energy decreases.
- 4Set equilibrium composition directly and verify zero reaction Gibbs energy even when the standard value is nonzero.
The model
Assumptions
- A and B interconvert in an ideal closed binary mixture at fixed temperature and pressure. Enthalpy and entropy are constant across the temperature range.
- Natural-log composition coordinates retain very small minority fractions. They do not make equilibrium a prediction of molecular reaction rates.
- The relaxation law is a chosen dissipative model, not an experimental rate constant or an activation-energy calculation.
Try this in class
- Q1Can the reaction driving force be zero when its standard Gibbs energy is not zero?
- Q2Why can changing composition reverse the thermodynamic direction at a fixed temperature?
Frequently asked questions
Does a negative Gibbs energy mean a reaction is fast?
No. Thermodynamic driving force and kinetic barriers are different. This model explicitly specifies an illustrative relaxation timescale.
Why are there traces of the unfavoured species at equilibrium?
Ideal mixing entropy favours a distribution. At extreme parameters the minority fraction can be tiny but remains positive in the logarithmic calculation.