REVERSIBLE REACTIONS

1. REVERSIBLE REACTIONS

2. Brief Outline • What is reversible reaction? • Examples of reversible reaction • Dynamic Equalibrium • Le Chatelier’s Principle • The Haber Process

3. Most reactions are not reversible. For examples: 1.2Mg + O2 2MgO • NaOH + HCl  NaCl + H2O But some reactions are reversible.

4. What is reversible reaction? • A reversible reaction is a chemical reaction in which the products can be converted back to reactants under suitable conditions. • The reaction involves two reactions. • There is a "forward" reaction and a mirror image "reverse" reaction. • A reversible reaction is shown by the sign ( ) a half-arrow to the right (forward reaction) and a half-arrow to the left (reverse reaction).

5. Decomposition of ammonium chloride • What would happen to the moist litmus papers ? • The moist red litmus paper turns blue first, then the moist blue litmus paper turn red again. (Ammonia which has a smaller molecular mass than hydrogen chloride diffuses faster.)

6. Example 1 • On heating strongly, the white solid ammonium chloride, decomposes into a mixture of two colourless gases ammonia and hydrogen chloride. • On cooling the reaction is reversed and solid ammonium chloride reforms. • Ammonium chloride + heat ammonia + hydrogen chloride • NH4Cl(s) NH3(g) + HCl(g)

7. The above reaction is also known as thermal dissociation. In a reversible reaction, the reactants are not completely converted to products. Instead an intermediate position or equilibrium is reached whereby both reactants and products are present. Reversible reactions are never complete, a mixture of reactants and products is obtained.

8. Example 2 • On heating the blue solid, hydrated copper(II) sulphate, steam is given off and the white solid of anhydrous copper(II) sulphate is formed. • When the white solid is cooled and water added, blue hydrated copper(II) sulphate is reformed. • Blue hydrated copper(II) sulphate + heat white anhydrous copper(II) sulphate + water • CuSO4.5H2O(s) CuSO4(s) + 5H2O(g)

9. Other Examples • The conversion of sulphur dioxide to sulphur trioxide by combination with oxygen: • 2SO2 + O2 2SO3 • The Haber Process for the manufacture of ammonia from hydrogen and nitrogen : • 3H2 + N2 2NH3 • Definition: A reversible reaction is one which can be made to go in either direction by changing the conditions under which it is carried out.

10. Dynamic Equilibrium • Dynamic equilibrium is the state in a reversible reaction in which the forward and reverse reactions are occurring at the same rate

11. Dynamic Equilibrium: Child ascending escalator at the same rate the escalator descends. At the balance point (the equilibrium position), the child and escalator are moving at the same rate in opposite directions. Static Equilibrium: Children on see-saw. At the balance point (the equilibrium position), there is no movement of the children or the see-saw (the opposing processes) . Dynamic equilibrium VS Static Equilibrium

12. Static Equilibrium Dynamic Equilibrium Dynamic equilibrium VS Static Equilibrium

13. Characteristics of Dynamic Equilibrium • The concentrations of the reactants and products (macroscopic properties) remain the same but the reactions don't stop! • The reactants are still reacting to form the product and the product is still being converted back to the reactants (microscopic processes).

14. Characteristics of Dynamic Equilibrium • The rate at which the reactants change into products is exactly equal to the rate at which the products change back to the original reactants. • Thus it appears as if the reaction has stopped, but in actual fact, it is still going on.

15. Characteristics of Dynamic Equilibrium • Dynamic equilibrium can only be achieved in a closed reaction (i.e an enclosed reactor) • A closed system is one in which there is absolutely no loss/gain of material to/from the surroundings. • An open system may allow matter to escape or to enter.

16. Explanation • In a reversible reaction, the reactants react to form products. • As the amount of reactant decreases, products increase at the samerate the reactants are decreasing. • At the same time, as the amount of reactant decreases, the forward reaction slows down.

17. Explanation • As products increase, the rate of reverse reaction increases. • This means that the moment products are formed, reverse reaction is taking place. • Ultimately, when the forward and reverse reactions are equal, dynamic equilibrium is achieved.

18. Example 1 • If steam is passed over red-hot iron, hydrogen is formed and collected over water. • If the hydrogen is passed over heated triion tetroxide (Fe3O4), water can be condensed in a cooled tube. • 3Fe + 4H2O Fe3O4 + 4H2 • In each case, the reaction can go to completion. • The changing conditions in these two experiments consist of making the products of one reaction the reactants of the reverse reaction.

19. Example 1 • If the experiment of reacting steam with red-hot iron was carried out in a closed vessel: • Hydrogen and triion tetroxide would be formed; these would subsequently react to produce steam and iron. • Eventually, an equilibrium would be formed, where there would be iron, steam, triion tetroxide and hydrogen. • The relative proportions of the four substances constitute the equilibrium mixture for the reaction.

20. Summary • A reversible reaction is one which can be made to go in either direction by changing the conditions under which it is carried out. • Dynamic equilibrium is the state in a reversible reaction in which the forward and reverse reactions are occurring at the same rate. • It is like running up a descending escalator, at the same rate the escalator is descending. • It can only be achieved in a closed system, and at dynamic equilibrium, microscopic processes continue but macroscopic properties remain constant.