Strong and Weak Acids/Bases Chapter 20 Notes, part IV Strong and Weak Acids/Bases

What makes an acid acidic? Increasing hydrogen ions in solution! For example, hydrofluoric acid: As drawn, is there a free hydrogen ion? H F

What makes an acid acidic? No!! In order to give off a hydrogen ion, it must dissociate, or break apart into separate ions.

Strong/Weak Acids and Bases Whether an acid or base is considered strong is based on how much they dissociate, or break down into ions, when in solutions.

Strong/Weak Acids and Bases A strong acid is completely ionized in aqueous solution. A weak acid is only partly ionized in aqueous solution. A strong base is completely ionized in aqueous solution. A weak base is only partly ionized in aqueous solution.

Here draw HF dissolved as acid together and as ions.

Strong/Weak Acids and Bases Strong acids include: Binary acids—HCl, HBr, HI Ternary acids—HNO3, HClO3, HClO4, H2SO4 Strong bases include: All hydroxides with metals in the first two groups on the periodic table except Be and Mg. Talk about old ternary rule.

Dissociation Constants The acid dissociation constant tells how much an acid will break into ions in water, giving us an index for strength of an acid. For the reaction of an acid: HA + H2O D H3O+ + A- The formula is: Ka = This is just a Keq problem. Remember, Keq tells you the ratio of products over reactants. This means that the more acidic, the higher the Ka value, because the more H+ ions. Generally there are no Ka values in the middle—they are either really big (over 1000) or really small 1E-3 or less. Usually the Ka is only considered for the weak acids because the acid is considered to be completely ionized. Ka is useful for finding pH of a weak acid, because you can find the H+ concentration even though it doesn’t completely break apart. [H3O+][A-] [HA]

Dissociation Constants Acids that have more than one hydrogen ion (diprotic and triprotic acids) will have more than one Ka. For example, H3PO4 D H+ + H2PO4- Ka = 7.5x10-3 H2PO4- D H+ + HPO42- Ka = 6.2x10-8 HPO42- D H+ + PO43- Ka = 4.8x10-13

Dissociation Constants Kb works the same way, except it measures dissociation of bases. For the equation: XOH D X+ + OH- The formula is: Kb = [X+][OH-] [XOH]

% Ionization Another way in which dissociation can be written is as a percent ionization, which tells you what percentage of the hydrogen ions will dissociate.

Practice Problem #1 If the equilibrium concentrations for a chlorous acid solution are as follows, what is the Ka for the reaction? [H+] = 0.00465M [HClO2] = 0.0120M

Practice Problem #2 Find the pH of copper (I) hydroxide if the Kb value is 1.5x10-6 and the equilibrium concentration [CuOH] = 0.025M.

Given beginning concentration: If the equilibrium concentrations aren’t given, you have to use some system to account for what happens as the compound dissociates.

Practice Problem #3 A 0.100M solution of acetic acid (HC2H3O2) is only partially ionized. From measurements of the pH of the solution, [H+] is 1.34x10-3M. What is Ka for acetic acid? Here explain the 5% rule, and show how it works if needed.

Practice Problem #4 A 0.0250M solution of HNO2 has a pH of 3.50. What is the Ka for nitrous acid?

Practice Problem #5 What is the pH of a 0.315M HMnO4 solution if its Ka is 1.52x10-5?

Practice Problem #6 What is the Ka of a 0.200M HBrO2 solution if has a 3.4% ionization?