All books

Professional

Apps About Coach Log in Start reading

Chemistry · Glossary

What is Equilibrium oxygen pressure?

Definition 6.4 University Chemistry — Year 2 · Chapter 6 — Ellingham Diagrams and Metallurgy

The equilibrium oxygen pressure of a metal–oxide pair at temperature TT is the pressure of dioxygen at which metal and oxide coexist at equilibrium: RTln⁡(pOX2/p∘)=ΔrG∘(T)RT\ln(p_{\ce{O2}}/p^\circ) = \Delta_r G^\circ(T) for the reaction written per mole of OX2\ce{O2}.

Examples

Example 6.6 (The oxide of mercury)

For 2 Hg+OX2→2 HgO\ce{2Hg + O2 -> 2HgO}, the CODATA key values give ΔrH∘=−181.6 kJ/mol\Delta_r H^\circ = -181.6\,\mathrm{kJ}/\mathrm{mol} with liquid mercury; above the boiling point of mercury the metal is a gas and ΔrH∘=−304.3 kJ/mol\Delta_r H^\circ = -304.3\,\mathrm{kJ}/\mathrm{mol}, ΔrS∘=−414.6 J/(K mol)\Delta_r S^\circ = -414.6\,\mathrm{J}/(\mathrm{K}\,\mathrm{mol}). The line crosses zero at T=304.3/0.4146≈734 KT = 304.3/0.4146 \approx 734\,\mathrm{K}: heated above that temperature in air, or under 1 bar1\,\mathrm{bar} of oxygen, red mercury oxide gives off its oxygen — the way oxygen was first isolated in 1774.

Read in context →