Surely the answer is simple. The Ellingham curves relate to an oxygen
activiity of unity. These are not the conditions you have in a crucible in
a
reducing atmosphere. If the oxygen activity is kept continuously low then
the activity of the copper oxide will be low too. At equilibrium the copper
and tin oxides will be in equilibrium with carbon monoxide. To quote my
school certificate chemistry book if you remove one of the products a
reversible reaction will proceed to completion
Peter Hutchison
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