The Van’t Hoff equation concerns how the equilibrium constant (Keq) changes with the change in temperature. This equation was proposed by the Dutch chemist Jacobus Henricus van’t Hoff in 1884.
In thermodynamics, the Van’t Hoff equation is used to explore the changes in the state function.
Derivation of Van’t Hoff equation
The derivation of this equation starts from the following equations:
ΔGº = – RT lnK → (i)
lnK = – ΔGº/RT → (ii)
Step 1: Differentiate the equation (ii) for lnK
In the first step, we differentiate the lnK with temperature. Hence, it will give,
dlnK/dT = – 1/R d((ΔGº/T)/dT
It is rearranged into the following:
d((ΔGº/T)/dT = -R dlnK/dT
Step 2: Use the Gibbs-Helmholtz equation (i) and rearrange
In this step, we use the Gibbs-Helmholtz equation in the following form:
d((ΔGº/T)/dT = – ΔHrº/T2
- ΔGº is the standard Gibbs free energy
- T is the absolute temperature
- ΔHrº = Standard reaction enthalpy
Now combine this equation with the expression we get in step 1,
R dlnK/dT = ΔHrº/T2
It rearranged into the following equation,
d(lnK)/dT = ΔHrº/RT2
Hence, this equation is known as Van’t Hoff equation:
Alternating form of Van’t Hoff equation
The Van’t Hoff equation can also be written as:
dlnK/d(1/T) = – ΔHrº/R
The integrated form of the Van’t Hoff equation
Van’t Hoff equations can be written in the following form:
dlnK= ΔHrº/RT2 × dT
By integrating the above equation we get:
lnK = – ΔHrº/RT+ I
- I = constant of integration
According to the above equation, lnK varies with temperature linearly. However, the slope is equal to – ΔHrº/R.
- The plot of lnK versus 1/T when ΔHrº is negative
- The plot of lnK versus 1/T when ΔHrº is positive
Application of the Van’t Hoff equation
There are the following conclusions drawn from Van’t Hoff equations:
For an endothermic reaction
For the endothermic reaction ΔHrº> 0. Therefore, the right-hand side of the equation is positive. Hence, ‘lnK’ increases with increasing temperature.
For exothermic reactions
For the exothermic reaction ΔHrº< 0. Therefore, the right-hand side of the equation is negative. So, lnK decreases with increasing temperature.
Is Van’t Hoff factor always 1?
The Van’t Hoff factor for the non-ionizing molecule is one. However, for the ionic compound, the value of Van’t Hoff factor equals the number of ions that dissociate.
How do you determine Van’t Hoff?
The Van’T Hoff factor can be determined by the values of calculated molar mass and experimentally observed molar mass,
What is Vant Hoff’s rule of N?
The N is the number of ions produced by the complete dissociation of one mole of the substance.
What is meant by Van’t Hoff’s rule?
Van’t Hoff’s rule states that any change in temperature shifts the equilibrium in that direction which nullifies its effect.
Which has the highest Van t Hoff factor?
The aluminum sulfate Al2(SO4)3 has the highest value of the Vant’t Hoff factor.
What is the van ‘t Hoff factor of H2O?
Although water undergoes self-ionization, but is dissociation is very weak. Therefore, it has a value of Van’t Hoff factor one.
What is Van’t Hoff isotherm?
According to Van’t Hoff isotherm, at the constant temperature the osmotic pressure of the liquid is inversely proportional to the volume.
- Physical Chemistry by Peter f. Atkins
- A textbook of physical chemistry Thermodynamic and Chemical equilibrium by K.L Kapoor
- Chap 9 (Binghamanton.edu)