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Unit Twelve Acids and Bases. Strengths. What is an electrolyte? A solution that contains ions and will conduct electrical current Acids and bases ionize (molecular) or dissociate (ionic) into ions Dissociation – when ionic compounds break apart to form ions Examples: NaCl, NaOH, Na 2 SO 4
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Strengths • What is an electrolyte? • A solution that contains ions and will conduct electrical current • Acids and bases ionize (molecular) or dissociate (ionic) into ions • Dissociation – when ionic compounds break apart to form ions • Examples: NaCl, NaOH, Na2SO4 • Ionization – when molecular compounds break apart to form ions (acids) • Examples: HCl, H2SO4 • Dissolving – when solute molecules are surrounded by solvent molecules and go into solution
Strengths • Strengths depend on how much ionization/dissociation takes place • Strong acids and bases completely ionize/dissociate in solution • 100% dissociated (lots of H+ and OH-) • Strong electrolytes • Weak acids and bases do not completely ionize/dissociate in solution • < 100% dissociated (only a few H+ and OH-) • Weak electrolytes
Strong versus weak • For strong acids and bases we use a single arrow to indicate the forward reaction is favored • For weak acids and bases we use a double arrow to indicate the partial ionization (forward and reverse reactions take place)
Strong Acids and Bases • There are six strong acids • HClO4 • H2SO4 • HNO3 • HCl • HBr • HI • There are eight strong bases • LiOH • NaOH • KOH • RbOH • CsOH • Ca(OH)2 • Sr(OH)2 • Ba(OH)2
Water • Amphoteric – water can act as an acid or a base • Self-ionizations – water can also act as an acid and a base with itself! (Even neutral, pure, distilled water)
Kw • In pure water at 25°C, the preceding reaction occurs only to a very small extent, resulting in equal, small concentrations of H+ and OH–. • [H+] = [OH–] = 1.0 x 10-7 M • Ion product constant for water (Kw) – the product of the concentration of H+ and OH–in aqueous solutions • Kw = [H+][OH–] • Kw = (1.0 x 10-7M)(1.0 x 10-7M) • Kw = 1.0 x 10-14 M • Can be used for aqueous solutions at 25°C (↑temp, ↑ movement, ↑ dissociation) • Kw will not change when the concentrations change because strengths are based on the amount of ionization.
Kw • Since Kwis constant, and Kw = [H+] [OH‾], it follows that: • If [H+] increases, then [OH‾] decreases, and • If [H+] decreases, then [OH‾] increases. • In a neutral solution: [H+] = [OH-] • In an acidic solution: [H+] > [OH-] • In a basic solution: [H+] < [OH-]
pH and pOH Scale • The pH scale relates to the strengths • pH scale measures the hydrogen ion concentration and the pOH measures the hydroxide ion concentration • Logarithmic scale – a change in 1 pH unit corresponds to a tenfold change in [H+] (lime (pH=2) versus plum(pH=3)) • pH < 7 : acidic solution • pH = 7 : neutral • pH > 7 : basic solution
pOH 13 12 11 10 9 8 7 6 5 4 3 2 1
Calculations • Kw = [H+] [OH‾] • pH = -log[H+] • pOH = -log[OH‾] • 14 = pH + pOH • [H+] = 10-pH • [OH‾] = 10-pOH
Practice Problems • What are the [H+] and [OH‾] concentrations in a 0.01 M HCl solution? • What are the [H+] and [OH‾] concentrations in a 0.0001 M NaOH solution? • What are the [H+] and [OH‾] concentrations in a 0.00001 M HNO3 solution? • What is the pH of a 0.0001 M HNO3 solution? • What is the pOH of a 0.001 M KOH solution?
Practice Problems • Calculate the [H+] of a solution with a pH of 8.37. • What is the concentration of OH- in a solution with a pOH of 4.80? • Calculate the pH of a solution with [OH-] = 1.3 x 10-2 M. Is the solution acidic or basic? Hint: Start with KW first to find [H+]. • Calculate the OH- concentration for a solution with a pH of 3.66.
