Recall, that water is amphoteric. That is, it can act as both an acid and a base.

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1 C4S: ACID BASE EQULIBRIUM LESSON 2 NOTES: WATER Ionization of Water Recall, that water is amphoteric. That is, it can act as both an acid and a base. HA + H2O(l) H3O + (aq) + A (aq) or B + H2O(l) BH + (aq) + OH (aq) Very sensitive instruments have shown that pure water actually dissociates into ions, or ionizes, slightly. We call this process self-ionization or autoionization. The equation for self-ionization is written as: H2O(l) + H2O(l) H3O + (aq) + OH (aq) or H2O(l) H + (aq) + OH (aq) This indicates there is an equilibrium established between hydronium and hydroxide ions. Click on the link below to view an animation of the self-ionization of water at the particles level. Ionization of Water

2 Ion Product of Water If an equilibrium is established between hydronium ions, hydroxide ions and water molecules, an equilibrium law can be written: Since water is a liquid, the product of KC and water results in the ion product for water, Kw. The equilibrium law for water becomes: Kw = [H3O + ][OH ] At 25 C, the concentration of the hydronium and hydroxide ions are equal at 1.0 x 10 7 mol/l. Therefore, at 25 C, the value of Kw is 1.0 x Effect of Adding Acid and Base to Water If the ionization of water occurs by the equation H2O(l) + H2O(l) H3O + (aq) + OH (aq) we can predict the effect of dissolving an acid or base on hydronium and hydroxide ion concentrations by using Le Chatelier's Principle. Adding Acid When an acid is dissolved in water, the acid produces a large amount of H3O + ions. If the H3O + ion concentration increases, the equilibrium will shift to the left to use up some of the added hydronium and maintain Kw at 1.0 x

3 Since equilibrium shifts left, the hydroxide ion concentration is reduced. Therefore, adding a strong acid to water increases the hydronium ion concentration and reduces the hydroxide ion concentration. Adding Base When a base is dissolved in water, the hydroxide ion concentration increases. According to Le Chatelier's Principle, the equilibrium shifts left to use up some of the added hydroxide and maintain Kw at 1.0 x Since equilibrium shifts left, the hydronium ion concentration is reduced. Therefore, adding a strong base to water increases the hydroxide ion concentration and reduces the hydronium ion concentration. This means that hydronium (hydrogen) ions AND hydroxide ions are BOTH present in any solution - whether they are acidic or basic. Calculating Hydroxide Concentration Example 1. If 2.5 moles of hydrochloric acid is dissolved in 5.0 L of water, what is the concentration of the hydroxide ions? Assume the volume remains unchanged. Solution If 2.5 moles is dissolved in 5.0 L, the concentration of the HCl would be Since HCl is a strong acid, the [H3O + ] = 0.50 mol/l.

4 Use the equilibrium law for water to find the concentration of the hydroxide ion: Calculating Hydronium Concentration Example g of NaOH is dissolved in water to make a solution with a volume of 1.0 L. What is the hydronium ion concentration in this solution? Solution We need to determine the concentration of the hydroxide ion. NaOH is an ionic base so it will dissociate complteley. This means the [NaOH] = [OH ]. To calculate the concentration, we need to convert the mass of NaOH to moles: NaOH = 40.0 g/mol The solution is 1.0 L so [NaOH] = [OH ] = mol/l Substitute into the equilibrium law and solve for hydronium ion concentration:

5 Exercise 1. What is the hydroxide ion concentration in a solution with an hydronium concentration of mol/l? 2. What is the [H3O + ] in a solution with [OH ] of ? 3. If the [H3O + ] in a nitric acid solution is mol/l, what is the [OH ]? 4. If the [OH ] in a sodium hydroxide solution is mol/l, what is [H3O + ]? moles of hydrogen chloride gas is dissolved in 2.0 L of water. Write the dissociation equation for this gas and calculate both [H3O + ] and [OH ] g of lithium hydroxide is dissolved in 750 ml of water. Write the dissociation equation and calculate both [H3O + ] and [OH ] g of calcium hydroxide is dissolved in ml of solution. Write the dissociation equation and calculate both [H3O + ] and [OH ]. 8. If the [H3O + ] of a barium hydroxide solution is mol/l, calculate the [OH ]. How many grams of barium hydroxide must have been used to make a Litre of this solution? 9. Calculate the [H3O + ] in milk of magnesia (magnesium hydroxide) that has an [OH ] of mol/l.

6 Answer Key HNO3 is a strong acid, so [H3O + ] = mol/l HCl(g) + H2O(l) H3O + (aq) + Cl (aq) HCl is a strong acid so [H3O + ] = mol/l 6. LiOH Li + + OH LiOH = 23.9 g/mol

7 [OH ] = [LiOH] = mol/l 7. Ca(OH)2(s) Ca 2+ (aq) + 2 OH (aq) Ca(OH)2 = 74.1 g/mol [OH ] = 2 [Ca(OH)2] = mol/l = mol/l OH 8. Ba(OH)2(s) Ba 2+ (aq) + 2 OH (aq) 9.