The concept of Gibbs energy was developed by the American scientist Josiah Willard Gibbs in the 1870s.
The change in Gibbs energy of a system is used to measure the spontaneity of reactions at constant temperature and pressure. As mentioned in the previous article, it is defined as:
or under standard conditions:
According to eq12 of the previous article, ΔGsyso < 0 for a spontaneous reaction to occur. With reference to eq14 and eq12, the values of ΔHsyso, ΔSsyso and T influencing the spontaneity of a reaction are summarised as follows:
| ΔHsyso | ΔSsyso | T |
| – | + | Reaction is spontaneous at any T |
| – | – | Reaction is spontaneous when |
| + | – | Reaction is not spontaneous at any T |
| + | + | Reaction is spontaneous when |
The remarks in the above table is consistent with Le Chatelier’s principle and our knowledge of chemical equilibria, where the increase in temperature of an endothermic and an exothermic reaction shifts the position of equilibrium of a reversible reaction to the right and left respectively, while the decrease in temperature of an endothermic and an exothermic reaction shifts the position of equilibrium to the left and right respectively.
Note that Gibbs energy is previously called Gibbs free energy where the term ‘free’ refers to energy that is “free to do non-pV work”. However in 1988, IUPAC dropped the term to avoid confusion.
Question
Calculate ΔGsyso for the reaction H2(g) + Cl2(g) → 2HCl(g) given Smo[H2(g)] = 130.6 JK-1mol-1, Smo[Cl2(g)] = 165.0 JK-1mol-1, Smo[HCl(g)] = 186.8 JK-1mol-1 and ΔHsyso = -184.6 kJmol-1.
Answer
Using eq14,
Since ΔGsys < 0, the reaction is spontaneous.