Gibbs Free Energy Calculator

In the world of chemistry and thermodynamics, one of the most fundamental questions is: will a reaction happen on its own? The answer lies in a powerful concept known as Gibbs Free Energy (ΔG). This value tells us whether a chemical process is spontaneous, non-spontaneous, or at equilibrium under constant temperature and pressure.

The Core Components: Enthalpy and Entropy

Gibbs Free Energy masterfully combines two critical thermodynamic quantities: enthalpy and entropy.

• Enthalpy (ΔH): This represents the change in heat content of a system. A negative ΔH (an exothermic reaction) means the system releases heat, which is generally a favorable process that contributes to spontaneity.
• Entropy (ΔS): This is a measure of the disorder, randomness, or chaos in a system. A positive ΔS means the system is becoming more disordered, which is also a favorable process. Think of a clean room (low entropy) naturally becoming messy over time (high entropy). Nature tends toward disorder.

The Gibbs Free Energy Equation: ΔG = ΔH - TΔS

The equation elegantly balances the drive for lower energy (enthalpy) and higher disorder (entropy):

• ΔG: The change in Gibbs Free Energy. This is the ultimate decider.
• ΔH: The change in enthalpy.
• T: The absolute temperature in Kelvin. Temperature acts as a weighting factor for the entropy term.
• ΔS: The change in entropy.

Important Note on Units: Enthalpy (ΔH) is typically measured in kilojoules per mole (kJ/mol), while entropy (ΔS) is measured in joules per mole-kelvin (J/mol·K). It is crucial to convert them to consistent units (usually by converting ΔH to J/mol or ΔS to kJ/mol·K) before calculating ΔG.

Interpreting the ΔG Value

The sign of ΔG tells us everything we need to know about the reaction's direction:

• ΔG < 0 (Negative): The reaction is spontaneous or exergonic. It will proceed in the forward direction without the continuous input of external energy.
• ΔG > 0 (Positive): The reaction is non-spontaneous or endergonic. It will not happen on its own in the forward direction. Energy must be supplied for it to occur. However, the reverse reaction will be spontaneous.
• ΔG = 0: The system is at equilibrium. The rates of the forward and reverse reactions are equal, and there is no net change in the concentration of reactants and products.

The Role of Temperature

Temperature (T) is the deciding factor when enthalpy and entropy are in opposition. Consider the four possible scenarios:

1. ΔH is negative, ΔS is positive: Both terms favor spontaneity. ΔG will always be negative, and the reaction is spontaneous at all temperatures.
2. ΔH is positive, ΔS is negative: Neither term favors spontaneity. ΔG will always be positive, and the reaction is non-spontaneous at all temperatures.
3. ΔH is negative, ΔS is negative: The favorable enthalpy term is opposed by the unfavorable entropy term. The reaction is spontaneous only at low temperatures, where the TΔS term is small.
4. ΔH is positive, ΔS is positive: The unfavorable enthalpy term is opposed by the favorable entropy term. The reaction is spontaneous only at high temperatures, where the TΔS term becomes large enough to overcome the positive ΔH. (e.g., melting ice).

Frequently Asked Questions (FAQ)

Does "spontaneous" mean a reaction happens quickly?

No. Spontaneity (thermodynamics) is different from reaction rate (kinetics). A spontaneous reaction is one that is energetically favorable, but it might happen incredibly slowly. For example, the conversion of diamond to graphite is spontaneous, but it takes millions of years. Catalysts are needed to speed up reactions.

Why must temperature be in Kelvin?

The Kelvin scale is an absolute temperature scale, meaning 0 K is absolute zero, the point of no thermal energy. Since Gibbs Free Energy deals with fundamental energy relationships, an absolute scale is required. Using Celsius or Fahrenheit would lead to incorrect calculations and nonsensical results (like negative entropy contributions at negative temperatures).