1. Chemical Equilibrium

2.  In principle, every chemical reaction is reversible:  Capable of moving in the forward or backward direction. Some reactions are easily reversible…Some are not!  Ie) Making water: 2H 2 + O 2 2H 2 O Breaking it up: 2H 2 O 2H 2 + O 2

3. Chemical Equilibrium What is Chemical Equilibrium?  When the concentrations of all reactants and products of a chemical reaction remain constant over time Any Any chemical in a closed system will eventually reach chemical equilibrium

4. Kinetics  Remember Kinetics? RATE RATE  Equilibrium occurs when the RATE of the forward reaction is exactly equal to the RATE of the reverse reaction. DYNAMIC EQUILIBRIUM

5.  Let’s evaluate the reversible reaction: At room temperature, colourless N 2 O 4 decomposes to brown NO 2 -. At some time, the colour stops changing and we have a mixture of the two.

6.  The point where the colour stops changing is equilibrium.  At equilibrium, as much N 2 O 4 reacts to form NO 2 - as NO 2 - reacts to re-form N 2 O 4 dynamic  The double arrow implies the process is dynamic.  Dynamic equilibrium  Dynamic equilibrium occurs when the rate of formation of products is the same as the rate of re- formation of reactions.

7. DYNAMIC  has reactants and products that are in constant motion. (i.e. – DYNAMIC)  is reversible.  can be reached from either direction.  forward or reverse

8.  Phase equilibria physical states  A dynamic equilibrium between different physical states of a pure substance in a closed system.  Such as when the rate of evaporation equals the rate of condensation.  H 2 O (l)  H 2 O (g)

9.  Solubility equilibria solute and a solvent  A dynamic equilibrium between a solute and a solvent in a saturated solution in a closed system.  The rate of dissolving equals the rate of crystallization.  Both dissolved and undissolved solute must be present.  NaCl (s)  Na + (aq) + Cl - (aq)

10.  Chemical equilibria reactants and products  A dynamic equilibrium between reactants and products in a closed system.  The reaction must be reversible.  N 2 O 4(g) + 58 kJ/mol  2NO 2 (g) (Colourless)(Brown)

11.  In a quantitative reaction the reaction consumes all of the reactants and goes to completion.  In open systems the products are removed or escape and an equilibrium does not occur.  A steady state can be created if the supply of reactants is sufficient or replenished. Reactants Steady state

12. NH 4 Cl (s) NH 3 (g) HCl (g) NH 3 (g) HCl (g) NH 3 (g) HCl (g) A quantitative reaction in an open system results in the reactants being consumed and no equilibrium state is established. NH 4 Cl (s)  NH 3 (g) + HCl (g) NH 4 Cl  NH 3 + HCl NH 4 Cl (s)  NH 3(g) + HCl (g) The ammonium chloride completely reacts as the gas products escape into the atmosphere in an open system.

13.  When a quantitative reaction is enclosed in a closed system the products build up and may begin to react in the reverse direction.  NH 4 Cl (s)  NH 3(g) + HCl (g)  The increased quantity of ammonia and hydrogen chloride combine to produce ammonium chloride.

14.  In the closed system, a quantitative reaction may produce a competition between the forward and reverse reaction.  This competition results in a chemical equilibrium.  NH 4 Cl (s)  NH 3(g) + HCl (g)

15. NH 3 (g) HCl (g) Some quantitative reactions in a closed system result in the products becoming involved in competing reverse reaction and an equilibrium state being established. NH 4 Cl (s) ⇄ NH 3 (g) + HCl (g) NH 4 Cl (s)  + NH 4 Cl  NH 3 + HCl NH 3 + HCl  NH 4 Cl

16.  As one observes the concentrations of the reactants and products in the closed system, one finds at equilibrium, the concentrations become stable.  This stability in concentrations defines the equilibrium state.

17. Time (s) Concentration (mol/L) NH 4 Cl (s)  NH 3(g) + HCl (g) NH 4 Cl (s) NH 3(g) HCl (g) No equilibrium is established in an open system. A quantitative reaction.

18. Time (s) Concentration (mol/L) NH 4 Cl (s)  NH 3(g) + HCl (g) NH 4 Cl (s) NH 3(g) HCl (g) NH 4 Cl (s) NH 3(g) HCl (g) Equilibrium established in a closed system

19. Time (s) Concentration (mol/L) NH 4 Cl (s)  NH 3(g) + HCl (g) NH 4 Cl (s) The reduction in NH 4 Cl is proportional to NH 3(g) HCl (g) the rise in product as predicted by the balanced chemical equation -X +X