This article demonstrates its usefulness. Since reduction of ZnO with carbon, which is also much cheaper than hydrogen, can be carried at a lower temperature, it is clear that reduction using carbon is much more economical than reduction using hydrogen.The Ellingham diagram is not always thought of as a heat-treater’s tool. Similarly, with the help of Ellingham diagrams for Zn/ZnO and H 2/H 20 systems, we can find out that H 2 will reduce ZnO at a temperature, of 1400 K. Therefore, carbon will reduce zinc oxide above 1173 K - a temperature 1000 K lower than the temperature of thermal decomposition of zinc oxide. Above this temperature, ΔG 0 will become negative. This temperature corresponds to the point of intersection of the two graphs for C/CO and Zn/ZnO systems. In this particular case, ΔG 0 becomes zero at 1 173 K as can be seen from Fig. Therefore, ΔG 0 decreases with increase in temperature and will become zero at some temperature. Since, two moles of gaseous product, i.e., CO are produced during the reaction,ΔS 0 is positive.
On adding above two equations and respective ΔG 0values we get,ĢZnO(s) + 2C(s) -> 2Zn(s) + 2CO(g): ΔG 0 = + 365 kjīecause the standard free energy change for the above reaction is positive, the reaction has little tendency to occur at 298 K. 15.2 is positive because it represents the decomposition of zinc oxide. You may note that the standard free energy change in Eq. For example, we can read off from the diagram the standard free energy changes for the following two reactions at 298 K:ĢC(s) + O 2 (g) -> 2CO(g) : ΔG 0 = - 275 kjĢZnO(s) -> 2Zn(s) + O 2(g) : ΔG 0= + 640 kj
With the help of Ellingham diagrams, we can find out the standard free energy changes for a large number of reactions. This behaviour is typical for all elements except carbon at sufficiently high temperatures the oxides become unstable relative to their constituent elements. Above this temperature, ΔG 0 will become more positive, therefore ZnO will spontaneously decompose to zinc and oxygen. For example, consider theΔG 0/T graph for the reaction of zinc with oxygen:ĢZn(s) + O 2 (g) -> 2ZnO(s)Īt 273 K, the value of standard free energy change for this reaction is 400 kJ, which becomes less negative as temperature rises and eventually at 2173 K, it becomes zero. By examining the Ellingham diagram for the formation of an oxide, we can find out the temperature at which the standard free energy change for the reaction will become positive. shows the Ellingham diagrams for the formation of metal oxides from free elements. But, due to abrupt changes in ΔS 0 breaks in the graph occur at temperatures at which reactants or products melt or boil, i.e., undergo phase change.įig.
gives a graph of constant slope, which is equal to -ΔS 0. You also know that for most of the chemical reactions, ΔH 0 and Ma do not change significantly with temperature and can be regarded as constant. As said above, ΔG 0 is related to ΔH 0, ΔS 0 and T according o the following equation: Such diagrams showing the variation of ΔG 0 with Tare called Ellingham diagrams after his name. Ellingham studied the variation of standard free energy change for the formation of a number of compounds, e.g., oxides, sulphides and chlorides, with temperature and plotted ΔG O against temperature.