Ellingham diagram.?

Question

Ellingham diagram is a plot of change in STANDARD free energy change
with temperature for various reactions like the formation of
oxide,sulphides etc.
STANDARD FREE ENERGE CHANGE is the change in energy measured at
S.T.P.Hence its a constant.
My question is how can it be compared at different temperatures as done in ellingham diagrams??
A constant cannot be varied with any variable...

Answer 1

Check out:
http://books.google.com/books?id=E_KHeqwctTQC&pg=PA121&lpg=PA121&dq=ellingham+plot&source=web&ots=ciif26VFlx&sig=9WxB2EfF6SLvfeTaz70YFoRkoMA