Carbon monoxide (co) is toxic because it binds more strongly to the iron in hemoglobin (hb) than does oxygen (o2), as indicated by these approximate standard free-energy changes in blood:
reaction a: reaction b: hb+o2hb+co⟶⟶hbo2,hbco, δg∘=−70 kj/mol δg∘=−80 kj/mol
estimate the equilibrium constant k at 298 k for the equilibrium

hbo2+co⇌hbco+o2