JEE Main 2020 · 5 September · morning · Single correct · +4 / −1
An Ellingham diagram provides information about :
- (a) the temperature dependence of the standard Gibbs energies of formation of some metal oxides
- (b) the pressure dependence of the standard electrode potentials of reduction reactions involved in the extraction of metals
- (c) the conditions of pH and potential under which a species is thermodynamically stable
- (d) the kinetics of the reduction process
Answer and solution
Answer (a) the temperature dependence of the standard Gibbs energies of formation of some…
Solution
Chemistry ma’am · 4 steps
Look at the axes: an Ellingham diagram plots Δ G° of oxide formation against temperature.
- What is plotted. On the y-axis is Δ_f G° for forming a metal oxide, usually per mole of O₂. On the x-axis is temperature. Each metal gets one line, for example 2Zn + O₂ → 2ZnO.
Δ G° = Δ H° - TΔ S°
- Why straight lines. Over a range with no phase change, Δ H° and Δ S° stay nearly constant. So Δ G° against T is a straight line with slope -Δ S°. Forming an oxide uses up gaseous O₂, so Δ S° is negative and most lines slope upward.
- How it is used. A metal whose line lies lower can reduce the oxide of a metal whose line lies higher at that temperature. The diagram also shows the temperature at which carbon or CO becomes a suitable reducing agent.
- So. The diagram shows how the standard Gibbs energies of formation of metal oxides depend on temperature, which is option (a).
The idea to keep. Ellingham diagram means Δ G° against T for oxide formation. It tells you only about feasibility, never about speed, pH or electrode potentials.
Answer from the NTA final answer key. Solution by PrepWiser.