Gibbs Free Energy
Definition and meaning of Gibbs Free Energy in chemistry.
Gibbs free energy, represented by the letter G, is a thermodynamic property of a system. It combines the values of enthalpy and entropy into a single metric. This quantity predicts whether a reaction will happen spontaneously at constant temperature and pressure.
In more detail
The mathematical definition of this property is enthalpy minus the product of temperature and entropy. This relationship is written as G = H - TS. For changes at constant temperature and pressure, the equation becomes ΔG = ΔH - TΔS.
A negative ΔG value means a reaction is spontaneous, or thermodynamically favored. A positive value means the reaction is nonspontaneous and needs energy to proceed. When ΔG equals zero, the reaction has reached equilibrium.
A common student misconception is that spontaneous reactions must happen quickly. In chemistry, spontaneous only means a reaction is energetically favored to occur. It does not describe the speed of the reaction.
For example, the conversion of diamond to graphite is spontaneous, but it takes millions of years. Another error is confusing standard free energy change with non-standard free energy change. The standard value applies only to set conditions, while the non-standard value changes as the reaction goes on.
This thermodynamic tool is vital for engineers designing chemical reactors and batteries. The free energy value also links directly to the equilibrium constant of a reaction. This connection is expressed by the equation ΔG° = -RT ln K. Here, R is the gas constant, T is temperature, and K is the equilibrium constant.
A large negative standard free energy change means the reaction will produce mostly products. Knowing this relationship helps chemists predict how far a reaction will proceed.
Key facts
| Symbol | G (change: ΔG) |
|---|---|
| Equation | ΔG = ΔH − TΔS |
| Units | joules or kilojoules per mole (kJ/mol) |
| Field | Physical Chemistry |
| State function status | Yes |
| Spontaneous condition | ΔG < 0 |
Consider the melting of ice at one atmospheric pressure. Below zero degrees Celsius, ΔG is positive, so ice does not melt on its own. Above zero degrees Celsius, ΔG is negative, which makes the melting process spontaneous. At exactly zero degrees Celsius, ΔG is zero, meaning liquid water and solid ice coexist in equilibrium.
Frequently asked questions
What does a negative ΔG mean for a reaction?
It means the reaction is spontaneous at that temperature and pressure. The reaction will release free energy that can do useful work as it goes toward equilibrium.
How is ΔG related to the equilibrium constant K?
They are linked by the standard free energy equation, ΔG° = -RT ln K. A large negative standard free energy change corresponds to a large equilibrium constant favoring products.
Can a reaction with a positive change in enthalpy be spontaneous?
Yes, it can be spontaneous if the reaction increases the entropy of the system. At high temperatures, the entropy term can overcome the positive enthalpy change.