How Does Gibbs Free Energy Predict Chemical Reaction Spontaneity?
Specific reactions are known to be spontaneous as they give off energy in the form of heat (H < 0). Interestingly, a few other spontaneous reactions result in an increase in the disorder of a system (S > 0). Therefore, calculating H and S can identify the actual forces behind such reactions.
Furthermore, when one such force behind a reaction is favoured, and others are not, Gibbs free energy (G) is used to identify those results. Moreover, it also reflects the balance between these reactions.
What is Gibbs Free Energy?
In thermodynamics, Gibbs free energy is known as a thermodynamic potential. Moreover, this potential is used to calculate the optimum of reversible work that one thermodynamic system can perform at constant pressure and temperature. Additionally, the measuring unit of Gibbs free energy is Joules in SI.
Furthermore, when a thermodynamic system transforms reversibly from its initial state towards its final state; the decrease in Gibbs energy is similar to the work done by this system and its surroundings. However, the work of pressure forces is not considered here.
Besides, this thermodynamic potential is minimised when a system reaches its chemical equilibrium at a constant temperature and pressure. Furthermore, the derivative of this system with respect to its reaction coordinate vanishes at this equilibrium point. Hence, a reduction in Gibbs free energy is needed to make such reactions spontaneous.
History of Gibbs Free Energy
The quantity termed as “free energy” is an advanced and accurate replacement for the archaic term “affinity”. This term was used by chemists in the initial years of physical chemistry to portray the forces behind chemical reactions.
Furthermore, in 1873, Josiah Willard Gibbs published his paper, “A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces”. In this journal, he mentioned the principles behind his new equation, which can predict or estimate the propensities of a different natural process which follows when these systems or bodies come in contact with each other.
Moreover, in this paper, Gibbs identified three states of the above mentioned equilibrium. They are “necessarily stable”, “unstable”, and “neutral”. Additionally, he also mentioned whether any changes would follow or not. He arrived at this conclusion by understanding the interactions between homogeneous substances in contact. Furthermore, he used a three-dimensional volume-entropy-internal graph to study substances, which are part-solid, part-liquid and part-vapour.
Additionally, in another paper “Graphical Methods in the Thermodynamics of Fluids,” Gibbs outlined how his equation has the capability to assume the behaviour of systems when they are mixed. Moreover, this quantity here is associated with chemical reactions that can do the work. It also represents the sum of its enthalpy and product of its temperature and entropy.
The Gibbs energy formula that defines this quantity is G=H -TS, or more completely as G=U+PV-TS. In this equation –
U is internal energy with SI unit joule.
P is pressure where the SI unit is pascal.
V is a volume with SI unit m3T is temperature, and the SI unit is Kelvin.
S is entropy with SI unit kelvin/joule.
H is enthalpy where SI unit is a joule.
Gibbs Energy Reactions
Spontaneous Process
In chemistry, Gibbs energy change spontaneity of a process is the one that does not require any external energy. Moreover, it is considered natural as it occurs itself, without any outside influence. The spontaneous process can be quick or slow because it is not associated with kinetics rate. A prominent example of spontaneous reaction is diamonds turning into graphite.
Additionally, over a long period, the carbon in the diamond slowly becomes more stable and less shiny, graphite. However, this process takes a long time, and it is hard for any human being to survive and witness this phenomenon.
Furthermore, another point to remember here is that this process can be endothermic, as well as exothermic.
How to Determine a Spontaneous Reaction?
The easiest way to understand this situation while solving an equation is if G is negative, then it is spontaneous. Otherwise, it is non-spontaneous, as it requires a continuous supply of external energy. Therefore, the Gibbs free energy symbol, i.e. G, can be ideally considered as “standard free energy charge”.
Furthermore, as per the second law of thermodynamics, every spontaneous process raises the entropy of the universe. However, using this law to calculate a spontaneous reaction can be a little difficult. Chemists are usually interested in changes happening around them. Typically, it is a reaction in a beaker. Therefore, there is no need to investigate the entire universe, t understand a small change.
Thus, chemists use Gibbs free energy change to study such reactions. This new thermodynamic quantity helps researchers to determine entropy changes in the universe. Moreover, chemical reactions involving such thermodynamics quantities, variations of the following equations are often witnessed –
ΔG (change in free energy) =ΔH (change in enthalpy) –TΔS (temperature change in entropy)
Moreover, this reaction, which does not have any subscript that specifies the thermodynamics values are for the system. Nevertheless, it is still considered that the values of H and S here are of the system of interest.
Additionally, this equation is vital and exciting, as it permits to calculate the alterations using enthalpy and entropy changes. Furthermore, the G here can be used to figure out whether a reaction is spontaneous in forward or backward direction, or at equilibrium.
Moreover, when G<0, this process is exergonic. It will move forward spontaneously and produce other products.
However, if G>0, it is an endergonic process. Thus, it is not spontaneous in the forward direction. Instead, it will move freely in the reverse direction and produce other starting materials.
On the other hand, when G=0, it reaches an equilibrium. Hence, the mixture of the products and reactants remains constant.
Spontaneity and Gibbs Free Energy
Furthermore, when any reaction occurs at a constant pressure P and constant temperature T, the second law of thermodynamics can be arranged for Gibbs energy define. Moreover, while using Gibbs free energy to determine the spontaneity of a process, the focus is on G. Thus, the absolute value is not considered here. Hence, the value of G in this process is the difference between its initial value and its final value.
Remarks on Gibbs Free Energy
The title “free energy” used to determine G has led to a lot of confusion. Thus, researchers these days primarily refer to it as Gibbs energy.
Notably, this term “free” is a part of the older portrayal related to the steam engine origin of thermodynamics. It has only interest in converting heat into work. Here G stands for the optimum amount of energy, which can be extracted from this system to execute useful work. Furthermore, here ‘useful’ means any work which is not associated with the system expansion.
Another serious difficulty of Gibbs energy change is in the framework of Chemistry. Even though G is calculated in the units of energy, it does not have a vital feature of energy, i.e. conservation. Even though energy levels fall due to any spontaneous chemical reactions, there is no increase of energy anywhere else. Hence, referring to G as energy is somewhat a misleading notion.
G has no thermodynamics quantities like H and S, as it has no physical reality like property of matter. Moreover, H and S stand for the quantity and distribution of energy in molecules, respectively. Hence, this free energy is just a useful construct, which serves as a term for a change to make the calculations easier.
Furthermore, the Gibbs free energy is vital in researches as it enables one to predict the direction of a reaction. Moreover, this ability to calculate G plays a significant role in designing lab experiments. Apart from that, it is also an essential chapter of Chemistry, and students should prepare it well for their exam.
Additionally, students can seek assistance from Vedantu to prepare various chapters of Chemistry. They can download our official Vedantu app, and join the live classes by subject experts. Moreover, they can also access study materials, mocks tests, etc. to improve their preparations.
FAQs on Gibbs Free Energy Explained: Concepts, Formula & Applications
1. What is Gibbs Free Energy (G) in simple terms?
Gibbs Free Energy is a thermodynamic quantity that represents the maximum amount of useful, non-expansion work that can be extracted from a chemical reaction at a constant temperature and pressure. In essence, it tells us how much energy is 'free' or available to do work, helping to predict whether a reaction will occur spontaneously.
2. What is the formula for calculating the change in Gibbs Free Energy (ΔG)?
The change in Gibbs Free Energy for a reaction is calculated using the Gibbs-Helmholtz equation: ΔG = ΔH - TΔS. Here's what each term means:
- ΔG: The change in Gibbs Free Energy.
- ΔH: The change in enthalpy (heat of the reaction).
- T: The absolute temperature in Kelvin (K).
- ΔS: The change in entropy (measure of disorder).
3. How does the sign of ΔG predict if a chemical reaction is spontaneous?
The sign of the calculated ΔG value is a direct indicator of a reaction's spontaneity at constant temperature and pressure:
- If ΔG is negative (< 0), the reaction is spontaneous and will proceed in the forward direction.
- If ΔG is positive (> 0), the reaction is non-spontaneous and requires energy input to occur. The reverse reaction, however, will be spontaneous.
- If ΔG is zero (= 0), the system is at equilibrium, and the rates of the forward and reverse reactions are equal.
4. How do enthalpy (ΔH) and entropy (ΔS) affect a reaction's spontaneity at different temperatures?
The spontaneity of a reaction depends on the interplay between enthalpy (ΔH), entropy (ΔS), and temperature (T). The outcome can be summarised in four conditions:
- ΔH is negative, ΔS is positive: The reaction is always spontaneous at all temperatures because both terms favour spontaneity.
- ΔH is positive, ΔS is negative: The reaction is never spontaneous at any temperature, as both terms oppose spontaneity.
- ΔH is negative, ΔS is negative: The reaction is spontaneous only at low temperatures, where the favourable ΔH term dominates the unfavourable TΔS term.
- ΔH is positive, ΔS is positive: The reaction is spontaneous only at high temperatures, where the favourable TΔS term overcomes the unfavourable ΔH term.
5. What are some important applications of Gibbs Free Energy in chemistry?
Gibbs Free Energy is a fundamental concept with several critical applications. Its primary importance is to:
- Predict Reaction Feasibility: Determine if a reaction will proceed spontaneously under a given set of conditions.
- Determine Equilibrium Position: Calculate the equilibrium constant (K) for a reaction, indicating how far a reaction will proceed before reaching equilibrium.
- Calculate Work in Electrochemical Cells: Relate the maximum electrical work obtainable from a galvanic cell to its cell potential.
- Understand Phase Transitions: Explain phenomena like melting, boiling, or freezing by determining the conditions under which one phase is more stable than another.
6. Can you give a real-life example of Gibbs Free Energy at work?
A common example is the melting of an ice cube at room temperature. Although melting requires heat from the surroundings (endothermic, positive ΔH), the water molecules become much more disordered than in the solid ice structure (positive ΔS). At temperatures above 0°C (273 K), the TΔS term becomes larger than the ΔH term, making the overall ΔG negative. This negative ΔG drives the spontaneous process of the ice melting into liquid water.
7. What is the difference between Gibbs Free Energy change (ΔG) and Standard Gibbs Free Energy change (ΔG°)?
The key difference lies in the conditions under which they are measured. ΔG (Gibbs Free Energy change) refers to the free energy change under any non-standard set of conditions. In contrast, ΔG° (Standard Gibbs Free Energy change) refers to the free energy change when a reaction is carried out under specific standard conditions: 1 atm pressure for gases, 1 M concentration for solutions, and a standard temperature, usually 298 K (25°C).
8. How is Gibbs Free Energy related to the cell potential (E_cell) in an electrochemical cell?
In electrochemistry, the Standard Gibbs Free Energy change (ΔG°) is directly proportional to the standard cell potential (E°cell) of a galvanic cell. The relationship is given by the formula: ΔG° = -nFE°cell.
- n is the number of moles of electrons transferred in the reaction.
- F is the Faraday constant (approx. 96,485 C/mol).
- E°cell is the standard cell potential.
A positive E°cell results in a negative ΔG°, confirming that the cell reaction is spontaneous.
9. What is the relationship between Standard Gibbs Free Energy (ΔG°) and the equilibrium constant (K)?
The Standard Gibbs Free Energy (ΔG°) is related to the equilibrium constant (K) by the equation ΔG° = -RT ln K. This formula is crucial because it links thermodynamic data (ΔG°) with the extent of a reaction at equilibrium (K). A large negative ΔG° corresponds to a large K, meaning the reaction strongly favours the formation of products at equilibrium.
