Why the values of enthalpy change and entropy change for any chemical reaction remain nearly constant even on varying temperature? I was studying Ellingham Diagram in Metallurgy. The following line is given from my textbook:
"It is interesting to note that $ΔH$ (enthalpy change) and $ΔS$ (entropy change) values for any chemical reaction remain nearly constant even on varying temperature. So the only dominant variable in the equation $ΔG=ΔH-TΔS$ becomes $T$."
My question is why do the values of enthalpy change and entropy change remain nearly constant for a chemical reaction even on varying temperature?
 A: Reflecting the typical context of metallurgy and oxidation/reduction, let's consider constant-pressure reactions occurring at room temperature and above; we'll work in molar quantities since the number of moles is conserved in chemical reactions.
The temperature dependence of the molar enthalpy $H$ and molar entropy $S$ are related to the constant-pressure molar heat capacity $C_P$ as
$$\left(\frac{\partial H}{\partial T}\right)_P=C_P\qquad \left(\frac{\partial S}{\partial T}\right)_P=\frac{1}{T}C_P.$$
Thus, if we calculate or are given $\Delta H$ or $\Delta S$ between two materials at one temperature, to work at a different temperature is to consider whether  $\Delta C_P$, the difference in $C_P$ between the materials, is substantial or not. (Make sure not to confuse a difference between materials with a difference between temperatures; the delta symbol always means the former in this answer.)
As described by the Dulong-Petit Law, the molar heat capacity $C_P$ of condensed matter at room temperatures and higher is generally around $3R$ (where $R$ is the ideal gas constant), and those of monatomic and diatomic gases are $\frac{5}{2}R$ and $\frac{7}{2}R$, respectively.
Because these values are all similar, $\Delta H$ and $\Delta S$ don't vary much over a temperature range where a reaction occurs between consistent phases.
Equivalently, the Ellingham diagram slope  is essentially constant for consistent reactants, with its slope representing the comparatively large entropy change from generating or eliminating (high-entropy) gas in the oxidation/reduction process.
