Chapter 1611 Strong Acids The strongest common acids are HCl, HBr, HI, HNO 3, HClO 3, HClO 4, and H 2 SO 4 Strong electrolytes Ionize completely in solution: HNO 3 (aq) + H 2 O(l)  H 3 O + (aq) + NO 3 - (aq) Remember, H + and H 3 O + are used interchangeably 16.5: Strong Acids and Bases

Chapter 1621 In solutions the strong acid is usually the only source of H + If the molarity of the acid is less than M then the autoionization of water needs to be taken into account Therefore, the pH of the solution is the initial molarity of the acid

Chapter 1631 Strong Bases Most soluble ionic hydroxides are strong bases Group 1 hydroxides, the heavy group 2 metal hydroxides Strong electrolytes, dissociate completely The pOH (and hence pH) of a strong base is given by the initial molarity of the base (stoichiometry!) Bases do not have to contain the OH - ion: O 2- (aq) + H 2 O(l)  2OH - (aq) H - (aq) + H 2 O(l)  H 2 (g) + OH - (aq) N 3 - (aq) + H 2 O(l)  NH 3 (aq) + 3OH - (aq) Bases are proton acceptors!

Chapter 1641 Weak acids are only partially ionized in solution There is a mixture of ions and unionized acid in solution Therefore, weak acids are in equilibrium: 16.6: Weak Acids

Chapter 1651 K a is the acid dissociation constant The larger the K a the stronger the acid more ions are present at equilibrium relative to unionized molecules If K a >> 1, then the acid is completely ionized and the acid is a strong acid Text, P. 628

Chapter 1661 Calculating K a from pH The pH gives the equilibrium concentration of H +

Chapter 1671 Using K a, the concentration of H + (and hence the pH) can be calculated –Write the balanced chemical equation (equilibrium) –Write the equilibrium expression. Find the value for K a –Set up the ICE table –assume that the change in concentration of H + is x –Substitute into the K a expression and solve, change to pH if necessary Use the quadratic equation OR Assume x is very small compared to the initial concentration and drop it from the equilibrium concentration expression (check S.F. and x) If x is more than 5% of the initial value, it is better to use the quadratic formula

Chapter 1681 Remember: weak acids are partially ionized [H + ] (aq) is only a fraction of the concentration of the acid solution Physical properties of the acid solution reflect this: Poor conductors of current React slowly with metals

Chapter 1691 Using K a to Calculate pH Percent ionization is another method to assess acid strength For the reaction

Chapter Percent ionization relates the equilibrium H + concentration, [H + ] eqm, to the initial HA concentration, [HA] 0 The higher percent ionization, the stronger the acid Percent ionization of a weak acid decreases as the molarity of the solution increases For acetic acid, a 0.15 M solution is 1.0 % ionized whereas a 0.05 M solution is 2.0 % ionized For dilution, remember LeChâtelier: The reaction will shift in the direction of the larger # of particles Counters the effect of the decreasing concentration of the particles (more molecules are ionized)

Text, P. 633

Chapter Polyprotic Acids Polyprotic acids have more than one ionizable proton The protons are removed in steps: It is always easier to remove the first proton in a polyprotic acid than the second Therefore, K a1 > K a2 > K a3 etc. As long as successive K a values differ by a factor of 10 3 or more, pH can be determined by considering only K a1

Chapter Text, P. 635