Tham khảo sách 'chemistry part 28, julia burdge,2e (2009)', khoa học tự nhiên, vật lý phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 682 CHAPTER 17 Acid-Base Equilibria and Solubility Equilibria The Common Ion Effect Up until now we have discussed the properties of solutions containing a single solute. In this section we will examine how the proper lies of a solution change when a second solute is introduced. Recall that a system at equilibrium will shift in response to being stressed and that stress can be applied in a variety of ways including the addition of a reactant or a product 1 Section . Consider a liter of solution containing 0 mole of acetic acid. Using the Ka for acetic acid X 10-3 and an equilibrium table 1 Section the pH of this solution at 25 C can be determined Initial concentration A Change in concentration A Equilibrium concentration A The percent ionization of acetic acid is x 10 3M x 1qo 1 3o W CH3COOH a H a CH3COO o 0 0 x x Assuming that x M M and solving for x we get X 10 Therefore CHjCOOH M H CH3COCT X 10 1 M and pH . Now consider what happens when we add mole of sodium acetate CH3COONa to the solution. Sodium acetate dissociates completely in aqueous solution to give sodium ions and acetate ions By adding sodium acetate we also add sodium ions to the solution. However sodium ions do not interact with water or with any of the other species present M4 Section CH3COONa tz Na i CH3COO Thus by adding sodium acetate we have increased the concentration of acetate ion. Because acetate ion is a product in the ionization of acetic acid the addition of acetate ion causes the equilibrium to shift to the left. The net result is a reduction in the percent ionization of acetic acid. Addition CH3COOH H aq CH3COO Equilibrium is driven toward reactant. Shifting the equilibrium to the left consumes not only some of the added acetate ion but also some of the hydrogen ion. This causes the pH to change in this case the pH increases . Sample Problem shows how an equilibrium table can be used to calculate
