NSW HSC Chemistry

## Reactions that do not go to completion Many reactions are treated as going one way until a reactant runs out, but a great many can also run backwards. A reaction that can proceed in both directions is **reversible**, and it is written with a double arrow, as in $\ce{N2O4(g) <=> 2NO2(g)}$. If a reversible reaction is left with nothing entering or leaving, the forward reaction gradually slows as reactants are used up and the reverse reaction speeds up as products build, until the two rates are equal. From then on the amounts of every substance stay constant, even though both reactions are still going. This is **dynamic equilibrium**. The idea shows up in simple laboratory observations. Hydrated cobalt(II) chloride is pink and loses water on heating to give blue anhydrous cobalt(II) chloride, and adding water to the blue solid turns it pink again, so the change can be reversed by changing conditions. Iron(III) nitrate solution and potassium thiocyanate solution give a blood-red complex, $\ce{FeSCN^2+}$, but the colour does not deepen indefinitely however much of one reagent is added, because the reaction settles at a mixture of reactants and product. Burning magnesium and burning steel wool are the contrasting cases, because neither product turns back into its reactants under ordinary conditions: the magnesium oxide or iron oxide stays as it is, and the steel wool even gains mass as oxygen from the air joins the solid. ## Open and closed systems A **closed system**…