116 PLTL Activity sheet / Intro Acid - Base equilibrium Set 8

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1 Potentially Useful or useless information: Strong and Weak Acids and Bases Definitions Arrhenius acid a compound that releases hydrogen ions ( + ) in water Brønsted acid a proton, + ion, donor Lewis acid an electron pair acceptor Arrhenius base a compound that produces hydroxide ions (O ) in water Brønsted base a proton acceptor Lewis base an electron pair donor There are basically four types of acid base problems: 1. Strong acid. Strong base } Straight-forward solution stoichiometry problems 3. Weak acid 4. Weak base } Equilibrium problems INT: You must identify your problem as one of these four types prior to a solving attempt. Potentially Useful or useless information: Strong Acids Strong Acids 100% ionized in aqueous solution For example, Cl in water ionizes completely: O, 100% Cl(g) + (aq) + Cl (aq) 100% Or equivalently, Cl(g) + O(l) 3O + (aq) + Cl (aq) There are very few strong acids, but they are extremely important in chemistry since they are excellent sources of + (aq), a highly reactive ion! Strong Acids Mineral acids Oxidizing acids plus a small number of non-metal Cl (hydrochloric NO 3 (nitric acid) oxides which react with water to give a acid) strong acid Br (hydrobromic acid SO4 (sulfuric acid) eg., SO3(g) + O SO4 I (hydroiodic acid) ClO4 (perchloric acid) Activity 1 What is the [3O + ] and p of a M NO3 solution? 1

2 Potentially Useful or useless information: Weak Acids Weak Acids Most acids are weak. Weak acids are typically less than 5% ionized in water thus the predominant species is the un-ionized form relatively small amounts of + (aq) are formed Typical weak acid ionizations in water are and C3O + O(l) 3O + (aq) + C3O (aq) or, equivalently, C3O + water + (aq) + C3O (aq) or equivalently SO(g) + O(l) SO3 + (aq) + SO3 (aq) SO(g) + O(l) 3O + (aq) + SO3 (aq) In each case above, reaction proceeds only to a very limited extent; typically over 95% of the weak acid remains un-ionized! Since the predominant form is un-ionized, chemists do not split up weak acids into ions when writing an ionic equation. Weak Acids include: Molecular compounds with an acidic hydrogen C3O = C3CO = C3COO (acetic acid) F (hydrofluoric acid) CN (hydrocyanic acid) C65COO (benzoic acid) NO (nitrous acid) Non-metal oxides SO(g) (sulfur dioxide) CO(g) (carbon dioxide) NO(g) (nitrogen oxide) Note : A few non-metal oxides such as SO3(g) (sulfur trioxide) and NO5(g) (dinitrogen pentoxide) give strong acids when dissolved in water. These acidic oxides of S and N are major contributors to acid rain Many cations are acidic ammonium ion (N4 + ) amine-type cations such as C3N3 +. metal cations with have a high charge/radius ratio are acidic such as Be +, Al +3, and Fe 3+ Activity What is the p of a M NO solution? KA for CN =

3 By what factor must the [3O + ] change in order to effect a p change of.4 units? Potentially Useful information: Percent Dissociation In addition to the acid-dissociation constant, KA, another measure of the strength of an acid is percent dissociation, determined by the following formula: [ A] dissociate d Percent dissociation = 100% A [ ] Percent dissociation increases with increasing KA. Strong acids, for which KA is very large, dissociate completely (100%). For weak acids, the percent dissociation changes with concentration. The more diluted the acid is, the greater percent dissociation. A certain weak acid, A, has a KA value of Calculate the percent dissociation of A in a 0.10M solution. Express your answer as a percent using two significant figures. initial Calculate the percent dissociation of A in a M solution. Express your answer as a percent using two significant figures. Are the results in (a) and (b) consistent with your expectations? Explain What is the [3O + ] of M NO? Assume that the KA= o C. 3

4 Potentially Useful or useless information: Strong Bases Strong Bases 100% ionized in aqueous solution to form the hydroxide ion, O, and a cation Typical ionization reactions are NaO(s) O, Na + (aq) + O (aq) and NaO(s) O, Na + (aq) + O (aq) Strong Bases include Alkali metal hydroxides and the more soluble alkaline earth hydroxides NaO(s) (sodium hydroxide) KO(s) (potassium hydroxide) Ba(O)(s) (barium hydroxide) Alkali metal oxides and the more soluble alkaline earth oxides NaO(s) (sodium oxide) KO(s) (potassium oxide) BaO(s) (barium oxide) The less soluble hydroxides and oxides of the alkaline earth cations are weak bases. Since solubility increases for these compounds as you go down Column II, the hydroxides and oxides of Ba + and Sr + are generally considered strong bases, while those for Ca + are on the borderline between strong and weak due to their limited solubility in water 0.65 grams of sodium hydroxide pellets are dissolved in water to make 4.0 liters of solution. What is the [3O + ] and p of this solution? 0.65 grams of potassium hydroxide pellets are dissolved in water to make 4.0 liters of solution. What is the [3O + ] and p of this solution? 4

5 Potentially Useful or useless information: Weak Bases Weak Bases The vast majority of bases are weak Much like weak acids, weak bases are typically less than 5% ionized their water solutions contain low concentrations of O (aq) Typical ionization reactions are N3 + O N4 + (aq) + O (aq) C3N + O C3N3 + (aq) + O (aq) Cu(O)(s) + water Cu + (aq) + O (aq) CuO(s) + O Cu + (aq) + O (aq) In each case above, reaction proceeds only to a very limited extent; typically over 95% of the weak base remains un-ionized! Weak bases are therefore not split up into ions when writing ionic equations. Weak Bases include Metal hydroxides and metal oxides other than those listed as strong bases above Mg(O)(s) (magnesium hydroxide) Cu(O)(s) (copper(ii) hydroxide) MgO(s) (magnesium oxide) Ammonia (N3) and amine-type bases such as C3N Typical amine-type bases have an ammonia-type structure, but with one or more of the atoms replaced by a hydrocarbon group Most anions are basic CO3 (aq) (hydrogen carbonate or bicarbonate ion), CO3 (aq) (carbonate ion) A M solution of quinine, a drug used in treating malaria, has a p of What are the values of KB and pkb? 5