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Strong Acids The strongest common acids are HCl, HBr, HI, HNO 3 , HClO 3 , HClO 4 , and H 2 SO 4

16.5: Strong Acids and Bases. 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.

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Strong Acids The strongest common acids are HCl, HBr, HI, HNO 3 , HClO 3 , HClO 4 , and H 2 SO 4

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  1. 16.5: Strong Acids and Bases • Strong Acids • The strongest common acids are HCl, HBr, HI, HNO3, HClO3, HClO4, and H2SO4 • Strong electrolytes • Ionize completely in solution: • HNO3(aq) + H2O(l)  H3O+(aq) + NO3-(aq) • Remember, H+ and H3O+ are used interchangeably Chapter 16

  2. In solutions the strong acid is usually the only source of H+ • If the molarity of the acid is less than 10-6M 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 16

  3. 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: • O2-(aq) + H2O(l)  2OH-(aq) • H-(aq) + H2O(l)  H2(g) + OH-(aq) • N3-(aq) + H2O(l)  NH3(aq) + 3OH-(aq) • Bases are proton acceptors! Chapter 16

  4. 16.6: Weak Acids • 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: Chapter 16

  5. Text, P. 628 • Ka is the acid dissociation constant • The larger the Ka the stronger the acid • more ions are present at equilibrium relative to unionized molecules • If Ka >> 1, then the acid is completely ionized and the acid is a strong acid Chapter 16

  6. Calculating Ka from pH • The pH gives the equilibrium concentration of H+ Chapter 16

  7. Using Ka, 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 Ka • Set up the ICE table • assume that the change in concentration of H+ is x • Substitute into the Ka 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 16

  8. 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 16

  9. Using Ka to Calculate pH • Percent ionization is another method to assess acid strength • For the reaction Chapter 16

  10. 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 % ionizedwhereas 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) Chapter 16

  11. Text, P. 633

  12. 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, Ka1 > Ka2 > Ka3 etc. • As long as successive Ka values differ by a factor of 103 or more, pH can be determined by considering only Ka1 Chapter 16

  13. Text, P. 635 Chapter 16

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