After completing this lesson you should be able to : Many reactions are reversible, so products may be in equilibrium with reactants. This may result in costly reactants failing to be completely converted into products. In a closed system, reversible reactions attain a state of dynamic equilibrium when the rates of forward and reverse reactions are equal. At equilibrium, the concentrations of reactants and products remain constant, but are rarely equal.

Equilibrium – key ideas

Dynamic equilibrium ReactantsProducts Chemical reactions which take place in both directions are called reversible reactions. The following is an example of a reversible reaction - hydrogen and iodine reacting to form hydrogen iodide. H 2 (g) + I 2 (g) ⇄ 2HI(g) The equilibrium can be arrived at from different starting points. The position of an equilibrium does not depend on the starting position. COPY

Dynamic equilibrium A reversible reaction attains a state of equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction. In the equilibrium mixture both the forward and reverse reactions are taking place. equilibrium Reaction rate Time Forward reaction Backward reaction Graph 1 COPY

Dynamic equilibrium Since the rates of the forward and reverse reactions are equal, the concentration of reactants and products remain constant, though not necessarily equal. The system is said to be at dynamic equilibrium. Concentration Time Forward (reactants) Backward (products) Graph 2 COPY

At equilibrium, the concentration of the products and the reactants will remain constant The concentration of reactants will probably not equal the concentration of the products. Position of equilibrium COPY

Position of equilibrium concentration time products reactants Equilibrium At equilibrium the concentration of products and reactants remains the same. COPY

Equilibrium A reversible reaction can reach equilibrium in a closed system. N 2 + 3H 2 ⇄ 2NH 3 A reaction reaches equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. COPY

Direction The equilibrium position will be the same whether we start with only the products or only the reactants Iodine dissolves in both cyclohexane and water/KI. The experiment shows one boiling tube set up with 100% iodine in cyclohexane and one with 100% iodine in water/KI. COPY

Iodine equilibrium A B cyclohexane Iodine/cyclohexane KI solution Iodine/KI solution COPY

Final equilibrium is the same A B COPY

Final equilibrium is the same Teaching notes The varying colour of iodine in different solvents is an intriguing phenomenon, but for students for whom this experiment is suited, the reasons explained below may well be beyond their level of chemistry. The solubility of iodine in organic solvents such as cyclohexane produces a purple solution, similar in colour to iodine vapour, in which non-polar iodine molecules have simply been separated from each other in the original crystal structure by non-polar solvent molecules. Iodine is only slightly soluble in water. Its solubility in aqueous potassium iodide, to form a yellow-brown solution is due to the formation of a stable tri-iodide ion, I 3 −. The tri-iodide anion dissociates back to iodine molecules and iodide ions, resulting in the equilibrium: I 2 (aq) + I − (aq) ⇌ I 3 − (aq) This is a complication that teachers may well feel lies beyond what the students need to consider in this simple experiment. The important point for students in this experiment is that they grasp the idea that iodine molecules can move between the two solvents, eventually producing an equilibrium which is the same no matter which direction it is approached from, as achieved in step 8 above. In practice the small iodine crystals are unlikely to be identical in size, resulting in deeper colours in one tube than the other. The teacher needs to be aware of this, and may need to repeat up to stage 8 with more carefully selected crystals.