# Understanding isosteric enthalpy (heat) of adsorption

I am trying to understand what is isosteric heat of adsorption. Based on van't Hoff equation:

$$\left(\frac{\partial \ln K}{\partial T}\right)_θ = \frac{ΔH^\circ}{RT^2}$$

and $$ΔH^\circ$$ is defined as the isosteric enthalpy of adsorption. The equilibrium constant for adsorption is:

$$K = \frac{a_{ads}}{a_g \cdot a_s}$$

and it is given by:

$$K = \exp\left(-\frac{ΔG^\circ}{RT}\right)$$

What is the point of differentiating with respect to temperature while keeping surface coverage $$θ$$ constant. I mean equilibrium constant depends only on $$T$$, that is $$K=f(T)$$.

I can't understand even how we are able to force surface coverage to have a constant value.