Thermodynamic - Gibbs Free Energy (A-Level Chemistry)
Gibbs Free Energy
Gibbs Free Energy
Key Terms
Enthalpy – Heat exchanged between the system and the surroundings at a constant pressure during a chemical reaction
Entropy – Measure of disorder or randomness of a system
Gibbs Free Energy – Energy change that takes into account both the entropy change and the enthalpy change.
Gibbs Free Energy Change, ΔG
As we learnt in previous chapters, a reaction’s feasibility depends on two factors: the enthalpy change, ΔH, and the entropy change, ΔS.
We can use these factors in an equation to determine whether a reaction will occur spontaneously. This equation calculates the Gibbs Free Energy change.
Where:
ΔG = change in Gibbs free energy, kJ mol-1
ΔH = change in enthalpy, kJ mol-1
ΔSsystem = change in entropy, J K-1 mol-1
T = Temperature, Kelvin
Reaction Feasibility and ΔG
If the value of ΔG is positive, the reaction is not feasible.
If the value of ΔG is zero or negative, the reaction is feasible.
There are however some limitations to this predictions as ΔG does not take reaction rate into account: if a reaction has a very high activation energy, reaction rate would be so slow that you would not notice the reaction was taking place, even if ΔG was negative.
Gibbs Free Energy is a measure of the amount of energy available to do useful work in a chemical reaction. It is an important topic in A-Level Chemistry as it helps to understand the direction and feasibility of chemical reactions.
Gibbs Free Energy is related to chemical reactions because it determines whether a reaction is spontaneous (i.e., will occur without the input of additional energy) or non-spontaneous (i.e., will not occur without the input of additional energy).
The equation for Gibbs Free Energy is: ΔG = ΔH – TΔS, where ΔG is the change in Gibbs Free Energy, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.
Gibbs Free Energy is related to enthalpy and entropy because it takes into account both the heat released or absorbed during a chemical reaction (enthalpy) and the disorder or randomness of the system (entropy).
A negative value of ΔG means that the reaction is spontaneous and will occur without the input of additional energy.
A positive value of ΔG means that the reaction is non-spontaneous and will not occur without the input of additional energy.
The magnitude of ΔG determines the rate and extent of the reaction. The larger the magnitude of ΔG, the slower and less complete the reaction will be.
Gibbs Free Energy can be used to predict the feasibility of a reaction by comparing the ΔG of the reactants to the ΔG of the products. If the ΔG of the products is lower than the ΔG of the reactants, the reaction will be spontaneous and feasible.
Gibbs Free Energy is related to equilibrium because at equilibrium, the Gibbs Free Energy of the system is at a minimum. Therefore, the value of ΔG can be used to determine the position of equilibrium and the direction in which the reaction will proceed to reach equilibrium.
Gibbs Free Energy is important in industrial processes because it allows for the optimization of reaction conditions to maximize the yield of desired products while minimizing energy input and waste.
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