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Gibbs free energy defines the maximum work done by a thermodynamic system under constant pressure and constant temperature. ‘G’ is used to denote Gibbs free energy, and it is expressed in Joules or Kilojoules. This energy can also be interpreted as the amount of energy present in a system that can be used to perform work on the system. These notes on Gibbs free energy and EMF of a cell will provide you with everything you need to know about this topic.
What Is Gibbs Free Energy?
Gibbs free energy is a thermodynamic potential used to calculate the maximum work done by a system in thermodynamics. So, the 2nd law of thermodynamics states that under standard conditions of temperature and pressure, there is a natural tendency to achieve a minimum of Gibbs free energy.
So you can say that change in Gibbs free energy G is equal to change in enthalpy H and entropy S when the temperature is kept constant.
ΔG = ΔH – TΔS
Where ∆ G = the change in Gibbs free energy
∆ H = change in enthalpy
T= temperature in Kelvin
∆S= change in entropy
When G<0, the reaction is spontaneous.
When G> 0, the reaction is nonspontaneous.
The reaction tends to be in equilibrium when G= 0.
Calculating the Change in Gibbs Free Energy
Gibbs free energy is temperature-dependent, but we take H and S as temperature-independent because there is no phase change in the reaction. So, the G can be calculated if the H and S are known.
Methods of calculating G are:
By estimating H the reaction using the bond enthalpies.
By calculating H using the standard heat of formation.
By calculating H and S using standard values.
Free Energy and Equilibrium Constant
∆S and ∆H are used to calculate the magnitude of ∆G, and equilibrium constant K is the ratio of the concentration of products and reactants at equilibrium. So, we can say that we can express ∆G in terms of K and vice versa.
Combining the terms given in the equation of ∆G and Q
∆G= ∆G° + RTlnQ ………. (1)
Where ∆G° tells that the reactants and products all are of standard value. For a system at equilibrium (K= Q), ∆G= 0. So, the relationship between ∆G and K is
0 = ∆G° + RTlnQ ………. (2)
∆G° = – RTlnQ…………….(3)
If we combine equations 1&3, we get
∆G = RT ln Q/ K
In standard states of products ∆G = 0 and K > 1, the products are formed over reactants at equilibrium.
Opposite to this, if ∆G = 0 and K< 1, then reactants are formed over products.
But, if ∆G = 0 and K = 1, then neither products nor reactants are favoured in the chemical reaction.
Second Law of Thermodynamics
This law is based on the ideas of spontaneity and entropy. It can be defined on the given basis:
According to the law, all spontaneous processes in thermodynamics cannot be reversed.
It also states that when the conversion of heat into work takes place, it is impossible not to waste any energy.
The entropy of the whole universe keeps on increasing.
It also defines that the total entropy change of the system is positive.
Spontaneous reactions don’t need any external work to be done on the system, while non-spontaneous reactions need a constant energy supply.
What Is EMF of a Cell?
The electromotive force of a cell or EMF is the maximum potential difference between two electrodes of a cell. It can also be interpreted as the maximum voltage between oxidation and reduction half-reactions. EMF is used to know whether a cell is electrochemical or a galvanic cell.
Emf of the cell = Eox + E red
= Ecathode + Eanode
The emf of a cell becomes zero when the chemical reaction attains equilibrium.
Relationship Between Gibbs Free Energy and EMF of a Cell
In galvanic cells, Gibbs’s free energy is related to the electromotive force of the cells.
G = -nFE(cell)
Where n= no. of moles of electrons involved
F= Faraday’s constant
= 1 Faraday
=96500 Coulombs
E= Emf of the cell
If the reactants and products are provided in their standard states,
G = -nFEcell
When charge flows through the galvanic cell in a reverse reaction, the maximum amount of work is done by the galvanic cell. The amount of this reversible work done by the cell results in a decrease of Gibbs free energy.
∆r G = -nFEcell
This equation can be used to calculate the standard cell potential. The concentration of all considerable reacting species is the same, E (cell) = E°(cell).
Conclusion
Gibbs free energy is the maximum work done by a thermodynamics system under constant pressure and concentration. This article primarily talks about the relationship between Gibbs Free Energy and EMF of a cell. You can understand the topic better by analysing the relationship between Gibbs free energy and emf of cell examples explained in this study material.
