You are already familiar with some acid and base chemistry. According to the Arrhenius model, acids are substances that when dissolved in water ionize to yield hydrogen ion (H + ) and a negative ion. e.g. HCl(g) H + (aq) + Cl (aq) bases are substance that yields hydroxide ion (OH ) when dissolved in water. e.g. NaOH(s) Na + (aq) + OH (aq) A more encompassing model was one developed by two Swedish chemists, Bronsted and Lowry. According to this model: acid = a proton (H + ) donor base = a proton (H + ) acceptor In donating a proton an acid (HA) then becomes a base (A ). The base it becomes is termed its conjugate. What are the Bronsted Lowry conjugate acid base pairs in the following reaction? HA(s) + H 2 O(l) = H 3 O + (aq) + A (aq) The above reaction is also often represented in a more simple way: HA(s) = H + (aq) + A (aq) Keep in mind that a bare proton (i.e. an empty s orbital) is extremely reactive and in water will form a coordinate covalent bond (water contributes one of its lone pairs) resulting in a hydronium (H 3O + ) ion. H + + H 2O = H 3O + May 1 2:14 PM May 1 2:15 PM Nevertheless the symbols H + and H 3O + are used interchangeably. Problem 1: Identify the acid base conjugate pairs in the following reactions: b) NH 3 (g) + H 2 O(l) = NH 4 + (aq) + OH (aq) HCl(g) + H 2 O(l) = H 3 O + (aq) + Cl (aq) May 1 2:15 PM May 1 2:17 PM Note that in the above reactions water acted as a B L base in reaction (a) but then as a B L base in reaction (b). A substance which can act as either a B L acid (proton donor) or B L base (proton acceptor) is termed amphoteric (or amphiprotic). For example, HSO 4 Acids can have more than one hydrogen that ionizes and bases can more than one hydroxide group that dissociates. The following are examples: Acid no. H + type base no. OH type HCl 1 monoprotic KOH 1 monohydroxy H2SO4 2 diprotic Mg(OH)2 2 dihydroxy H3PO4 3 triprotic Al(OH)3 3 trihydroxy May 1 2:18 PM May 1 2:18 PM 1
Acid/Base Strength A strong acid is by definition an acid that ionizes virtually completely in water. For example, bubbling hydrogen chloride through water results in the following reaction: HCl(g) + H 2 O(l) = H 3 O + (aq) + Cl (aq) Problem 2: Calculate the hydronium ion (or hydrogen ion) concentration of a solution made by dissolving 3.65 g of hydrogen chloride in 1.00 liter of solution. Show the reaction of hydrogen chloride in water: initial 1000 0 0 99.9% change 999 +999 +999 equilibrium 1 999 999 In the above reaction the HCl donates a proton (H + ) to water and water becomes a hydronium ion. Hence the HCl is a B L acid and water acts as a B L base. May 1 2:18 PM May 1 2:20 PM Since strong acids ionize completely we would expect them to be strong electrolytes. You should memorize the following strong acids and bases (you can assume all others are weak): strong acids strong bases hydrochloric lithium hydroxide nitric sodium hydroxide sulfuric potassium hydroxide perchloric calcium hydroxide Weak acids ionize only to a small extent. Let s consider the ionization of acetic acid in water: HC 2H 3O 2(aq) + H 2O(l) = H 3O + (aq) + C 2H 3O 2 (aq) initial 1000 0 0 change 10 +10 +10 equilibrium 990 10 10 Weak acids will thus be expected to be weak electrolytes. hydroiodic strontium hydroxide hydrobromic barium hydrpxide Apr 30 9:22 AM May 1 2:20 PM May 1 2:21 PM May 1 2:23 PM 2
Properties of Acids Solutions of acids are electrolytes. Acids react with active metals to produce hydrogen gas. For example, Zn(s) + HCl(aq) CIE: NIE: Acids react with bases to form a salt and water via a neutralization reaction. For example: HNO 3(aq) + KOH(aq) Nonmetallic oxides (acid anhydrides) react with water to form acids. For example: CO 2(g) + H 2O(l) Acids have a sour taste (i.e. lemon juice) Acids cause color changes in indicators. An indicator is a substance (generally a weak acid or base) that has a different color in the unionized and ionized form. Which form predominates depends on the hydrogen (hydronium) ion concentration. May 1 2:25 PM May 1 2:26 PM The Autoionization of Water Properties of Bases Solutions of bases are electrolytes. Bases cause color changes in indicators (see acids). Bases react with acids to form a salt and water via a neutralization reaction. Metallic oxides (basic anhydrides) react with water to form bases (metallic hydroxides). For example: CaO(s) + H 2O(l) Dilute solutions of bases have a slippery feeling. Recall water is an amphoteric substance it can donate a proton (acid) or accept a proton (base). In a pure sample of water, water autoionizes one water molecule donates a proton to another water molecule: H2O(l) + H2O(l) = Write the equilibrium expression for this reaction: (the Keq is called Kw) Kw = The numeric value of Kw at 25 o C = 1 X 10 14 What does this value of Kw tell us about the extent of the reaction? Why does pure water not conduct electricity even though there are hydronium and hydroxide ions present? Calculate the molar concentrations of hydronium ion and hydroxide ion in a pure sample of water at 25 o C: How do the concentrations compare to each other? May 1 2:26 PM May 1 2:26 PM Now let s get quantitative! Suppose we had some hydrochloric acid to water. The hydrochloric acid will donate a proton to water to form hydronium ion, and thus the hydronium ion concentration will increase. Again the autoionization of water represents an equilibrium condition. What is the concentration of hydronium ion in water before the addition of any acid? H2O(l) + H2O(l) = H3O + (aq) + OH (aq) Use Le Chatelier s principle to predict what will happen if we add acid [H3O + ] to water: Suppose after addition of acid, the hydronium ion concentration is 1.0 X 10 3 M. By what magnitude did the concentration of hydronium ion change? What is the concentration of hydroxide ion in water before the addition of any acid? Use Le Chatelier s principle to predict what will happen if we add base [OH ] to water: H2O(l) + H2O(l) = H3O + (aq) + OH (aq) What will happen to the concentration of hydroxide ion after addition of acid to the water? By what magnitude did the concentration of hydroxide ion change? Calculate the new concentration of hydroxide ion after the addition of the acid: May 1 2:27 PM May 1 2:27 PM 3
Another problem: Calculate the concentration of hydronium ion in a solution after the addition of base results in a hydroxide ion concentration = 1.0 X 10 4 M. By what magnitude did the concentration of hydronium ion change? In pure water the following reaction occurs: H 2O(l) + H 2O(l) = or H 2O(l) = In any aqueous solution, the following is always true: K w = [H 3O + ][OH ] = 1 X 10 14 ph Scale Chemists have come up with the idea of ph to avoid writing numbers with exponents when talking about the acidity (or basicity) of a solution. px = log[x] or Note: [H + ] = 10 ph for example, what is [H + ] if ph = 3? Note: ph + poh = 14 Note: Since ph is a logarithmic scale, a difference of 1 unit in ph corresponds to a factor of 10. ph = log[h + ] and poh = log[oh ] [H3O + ] 1 10 7 10 14 ph 0 7 14 [H3O + ] > [OH ] [H3O + ] = [OH ] [H3O + ] < [OH ] Problem 3 a) What is the ph of a solution whose hydronium ion concentration = 3.70 X 10 4 M b) Calculate the ph and hydroxide ion concentration of a solution whose hydronium ion concentration = 5.60 X 10 3 M c) Calculate the ph, poh, and hydronium ion concentration in a solution whose hydroxide ion concentration = 1.20 X 10 6 M. d) Determine the hydronium ion and hydroxide ion concentrations in a solution that has a ph = 8.53. e) Determine the hydronium ion and hydroxide ion concentrations in a solution that has a ph = 3.80. f) What is the difference in hydronium ion concentration between a solution having a ph = 2 and a solution having a ph = 6 May 1 2:29 PM May 1 2:30 PM 4
Problem 6. Calculate the hydronium ion, hydroxide ion, ph and poh of a solution of acetic acid that is 2.75% ionized. Problem 4. Calculate the ph of a 0.800M solution of nitrous acid that is 3.50% ionized. Problem 7. A 0.650M solution of a monoprotic acid (HA) has a ph = 4.55. Determine its Ka. Problem 5. Calculate the ph of a 0.500M solution of ammonia that is 4.25% ionized. May 1 2:31 PM May 1 2:31 PM Neutralization and Hydrolysis Some terms: Titration a quantitative process in which a standard solution is used to determine the concentration of another solution. It involves adding a measured volume of standard acid or base to an acid or base of unknown molarity (neutralization) until the equivalence point is reached. The equivalence point can be determined by addition of an indicator whose endpoint is close to the equivalence point. Neutralization Problem 8: What is the molarity of a sodium hydroxide solution if it takes 25.0 ml of 0.50 M HCl to exactly neutralize 50 ml of the base? What will be the ph at equivalence? Reaction: May 1 2:32 PM May 1 2:36 PM Standard Solution one whose concentration is precisely known. Equivalence Point the ph at which the moles of acid = moles of base in a titration. At equivalence: moles acid = moles base Problem 9: What is the molarity of 50.0 ml a magnesium hydroxide solution if it takes 50.0 ml of a 1.0 M hydrofluoric acid solution to completely neutralize it. What will be the relative ph at equivalence? Reaction: so, M av ai a = M bv bi b where i = number of hydrogen ions in the acid or number of hydroxide ions in the base so, HCl i = 1 NaOH i =1 H2SO4 i = 2 Ca(OH)2 i = 2 H3PO4 i = 3 Al(OH)3 i = 3 Endpoint the ph at which the indicator changes color. Neutralization Reaction: acid + base salt + water Hydrolysis Reaction: salt + water acid + base Apr 30 9:23 AM May 1 2:37 PM 5
Indicators Indicators are weak acids or bases which have different colors in the ionized and unionized forms. Indicators do not change color instantaneously at a given ph. The color change is due to a changing proportion of the indicator molecules in the acid or base forms. For example, below ph 3.2 virtually all of methyl orange molecules have H + attached and are in the acid form (red). Above that ph the methyl orange molecules begin to donate protons to other bases in the solution, leaving methyl orange in the base form, which is yellow (ph 4.4 and above). Types of Titrations Strong acid & strong base solution neutral BTB, phenolpthalein Strong acid & weak base a hydrolysis reaction will methyl orange make equivalence point slightly acidic Weak acid & strong base a hydrolysis reaction will phenolpthalein make equivalence point slightly basic May 1 2:37 PM May 1 2:37 PM 2. Salts whose cation has neutral properties (i.e. Na +, K + ) and whose anion is the conjugate base of a weak acid will produce a basic solution. Example: NaC 2H 3O 2 Acid Base Properties of Salts Salts are ionic compounds. They dissociate in water and may exhibit acid base behavior. The key question in deciding whether a salt will act as an acidic, basic, or neutral species in solution is What are the acid base properties, and strengths, of each component of the salt? Salts that consist of the cations of strong bases and the anions of strong acids have no effect on [H + ]. The ions of these salts do not react with water; they have no acid base properties. Example: KCl 3. Salts in which the cation is the conjugate acid of a weak base will produce an acidic solution. Example: NH 4Cl May 1 2:38 PM Apr 30 9:24 AM 2. Salts that consist of the cations of a strong base and the anions of a weak acid will produce a basic solution. Example: NaC 2H 3O 2 3. Salts that consist of the cations of a weak base and the anions of a strong acid will producer an acidic solution Example: NH 4Cl May 10 1:29 PM Apr 30 9:24 AM 6
Acid Rain Acid rain is precipitation with an acidic ph (5.6 or lower). Most of the acid rain is produced as the result of the combustion of fossils fuels. We will consider three sources of acid rain. The reaction of a salt in water is called hydrolysis and is the opposite of neutralization. Show how the following salts hydrolyze in water and predict whether they will give acidic, basic, or neutral solutions: K2CO3 Combustion of fossil fuels yields water vapor and carbon dioxide. The carbon dioxide can then react with water vapor in the atmosphere to produce carbonic acid: CO 2(g) + H 2O(g) H 2CO 3(aq) The combustion of gasoline in car engines produces temperatures high enough to allow nitrogen to also undergo combustion: N 2(g) + O 2(g) NO 2(g) NaCl Na3PO4 The NO 2(g) then combines with water vapor in our atmosphere to produce nitrous and nitric acids: NO 2(g) + H 2O(g) HNO 2(aq)/HNO 3(aq) The combustion of coal containing sulfur impurities results in the production of sulfur dioxide gas: S(s) + O 2(g) SO 2(g) The sulfur dioxide gas then reacts with water vapor in our atmosphere to produce sulfurous/sulfuric acids: SO 2(g) + H 2O(g) H 2SO 3(aq)/H 2SO 4(aq) Lakes with limestone (CaCO 3) beds are able to withstand the effects of acid rain more so than those without because the limestone has a buffering effect of the acid: CaCO 3(s) + H 3O + (aq) Ca 2+ (aq) + H 2CO 3(aq) Landscapers typically will sprinkle lyme (CaO) on grass to counter the effects of acid rain: Apr 30 9:25 AM CaO(s) + H 3O + (aq) Ca 2+ (aq) + H 2O(l) May 1 2:38 PM May 1 2:39 PM May 1 2:42 PM Apr 30 9:25 AM 7