Refer to Ellingham Diagram supplied with problem sheet.
1.What is the best reducing agent at all temperatures up to
about 2000ºC?
2. Write the chemical equations corresponding to the lines ending with Al2O3 and ZnO. Show how they can be combined to give a spontaneous reaction between one of these metals and the oxide of the other.
2Zn(s) + O2(g) ---> 2ZnO
4/3Al(s) + O2(g) ---> 2/3Al2O3(s)
The lines on an Ellingham diagram correspond to the DG for reactions involving one mole of dioxygen.
Since the Al line is lower than the Zn line at all temperatures Al is the better reducing agent and ends up with the oxygen in a competition with Zn:-
4/3Al(s) + 2ZnO ---> 2/3Al2O3(s) + 2Zn(s)
or
2Al(s) + 3ZnO ---> Al2O3(s) + 3Zn(s)
3. Will Al powder react with Cr2O3? If so, at what temperature? Write a balanced reaction.
Yes, at all temperatures.
2Al(s) + Cr2O3(s) ---> 2Cr(s) + Al2O3(s)
This reaction is commonly used to make small quantities of
many metals.
4. At about what temperature is it possible to reduce MgO
with carbon? Write a balanced equation including the
physical states of all species.
At about 1700ºC - well above the boiling point of magnesium.
MgO(s) + C(s) ---> CO(g) + Mg(g)
5. What is the reaction between carbon and steam at
a. 100ºC, and b. 1000ºC.
At 100ºC: C(s) + 2H2O(g) ---> 2H2(g) + CO2(g)
At 1000ºC: C(s) + H2O(g) ---> H2(g) + CO(g)
Both reactions are endothermic, and at 100ºC it would be too
slow to be useful, but the higher temperature reaction is
used to produce an inflammable mixture of gases, known as
"water gas".
6. Which is the best oxidising agent at 1500ºC, MgO or
Al2O3?
MgO - the higher line at this temperature.
7. Why is gold not found in nature as its oxide?
Since it is above the zero line, DGformation(Au2O3) is positive, and the
compound is thermodynamically unstable.
8. Why does the line for Zn/ZnO increase its slope sharply
at about 900ºC?
The boiling point of Zn is at about 900ºC.
Below this temperature the reaction is:-
2Zn(l) + O2(g) ---> 2ZnO(s)
Above this temperature the reaction is:-
2Zn(g) + O2(g) ---> 2ZnO(s)
Above 900ºC there are three moles of gas on the left hand side of the equation rather than only one, the entropy change, DS, is approximately tripled and since gas is being lost it will be negative. The line is a plot of DG against T and the equation is:-
DG = DH - TDS
The slope is -DS so if DS gets more negative the slope will
become more positive.
(C) H.J Clase 1998 - Updated 1999.04.09