Acid Base Notes.notebook May 13, 2015 Acids, Bases and Salts A more encompassing model was one developed by two Swedish chemists, Bronsted and Lowry. According to this model: + You are already familiar with some acid and base chemistry. According to the Arrhenius model, acid = a proton (H ) donor + acids are substances that when dissolved in water ionize to yield hydrogen ion (H+) and a negative ion. base = a proton (H ) acceptor ­ e.g. HCl(g) à H+(aq) + Cl­(aq) In donating a proton an acid (HA) then becomes a base (A ). The base it becomes is termed its conjugate. What are the Bronsted­Lowry conjugate acid­base pairs in the following reaction? bases are substance that yields hydroxide ion (OH­) when dissolved in water. + ­ HA(s) + H2O(l) = H3O (aq) + A (aq) e.g. NaOH(s) à Na+(aq) + OH­(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 (H3O ) ion. + + H + H2O = H3O + + Nevertheless the symbols H and H3O are used interchangeably. May 1­7:34 AM May 1­7:35 AM Problem 1: Identify the acid­base conjugate pairs in the following reactions: + ­ 1. HCl(g) + H2O(l) = H3O (aq) + Cl (aq) 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) + H2O(l) = H3O (aq) + Cl (aq) initial 1000 0 0 + ­ 1. NH3(g) + H2O(l) = NH4 (aq) + OH (aq) 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 B­L acid and water acts as a B­L base. 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 ____________ Note that in the above reactions water acted as a B­L base in reaction (a) but then as a B­L base in reaction (b). A substance which can act as either a B­L acid (proton donor) or B­L base (proton acceptor) is termed amphoteric (or amphiprotic). For example, nitric ____________ sodium hydroxide ____________ ­ sulfuric ____________ potassium hydroxide ____________ HSO4 perchloric ____________ calcium hydroxide ____________ hydroiodic ____________ strontium hydroxide ____________ hydrobromic ____________ barium hydroxide ____________ 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 1­7:35 AM May 1­7:36 AM 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. Weak acids ionize only to a small extent. Let’s consider the ionization of acetic acid in water: Show the reaction of hydrogen chloride in water: + ­ HC2H3O2(aq) + H2O(l) = H3O (aq) + C2H3O2 (aq) initial 1000 0 0 change ­10 +10 +10 equilibrium 990 10 10 Weak acids will thus be expected to be weak electrolytes. May 1­7:37 AM May 1­7:37 AM 1 Acid Base Notes.notebook May 13, 2015 Properties of Acids Properties of Bases 1. Solutions of acids are electrolytes. 1. Solutions of bases are electrolytes. 2. Acids react with active metals to produce hydrogen gas. For example, 2. Bases cause color changes in indicators (see acids). Zn(s) + HCl(aq) à 3. Bases react with acids to form a salt and water via a neutralization reaction. 4. Metallic oxides (basic anhydrides) react with water to form bases (metallic hydroxides). For example: CIE: à NIE: 1. CaO(s) + H2O(l) 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. 5. Dilute solutions of bases have a slippery feeling. 1. Acids react with bases to form a salt and water via a neutralization reaction. For example: à 1. HNO3(aq) + KOH(aq) 2. Nonmetallic oxides (acid anhydrides) react with water to form acids. For example: à 1. CO2(g) + H2O(l) 3. Acids have a sour taste (i.e. lemon juice) May 1­7:37 AM May 1­7:38 AM 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. The Autoionization of Water What is the concentration of hydronium ion in water before the addition of any acid? 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: Suppose after addition of acid, the hydronium ion concentration is 1.0 X 10­3M. By what magnitude did the concentration of hydronium ion change? H2O(l) + H2O(l) = Write the equilibrium expression for this reaction: (the Keq is called Kw) Kw = What is the concentration of hydroxide ion in water before the addition of any acid? o ­14 The numeric value of Kw at 25 C = 1 X 10 What will happen to the concentration of hydroxide ion after addition of acid to the water? What does this value of Kw tell us about the extent of the reaction? By what magnitude did the concentration of hydroxide ion change? Why does pure water not conduct electricity even though there are hydronium and hydroxide ions present? Calculate the new concentration of hydroxide ion after the addition of the acid: Calculate the molar concentrations of hydronium ion and hydroxide ion in a pure sample of water at 25oC: How do the concentrations compare to each other? Again the autoionization of water represents an equilibrium condition. Another problem: + ­(aq) H2O(l) + H2O(l) = H3O (aq) + OH Calculate the concentration of hydronium ion in a solution after the addition of base results in a hydroxide ion concentration = 1.0 X 10­4M. By what magnitude did the concentration of hydronium ion change? + Use Le Chatelier’s principle to predict what will happen if we add acid [H3O ] to water: 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) May 1­7:39 AM May 1­7:39 AM Again, in any aqueous solution, the following is always true: + ­ ­14 Kw = [H3O ][OH ] = 1 X 10 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 pH = ­log[H+] and pOH = ­log[OH­] + ­7 ­14 [H3O ] 1 10 10 pH 0 7 14 + ­ + ­ + ­ [H3O ] > [OH ] [H3O ] = [OH ] [H3O ] < [OH ] Note: [H+] = 10­pH 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. May 1­7:40 AM May 1­7:41 AM 2 Acid Base Notes.notebook May 13, 2015 Problem 3 a) What is the pH of a solution whose hydronium ion concentration = 3.70 X 10­4M b) Calculate the pH and hydroxide ion concentration of a solution whose hydronium ion concentration = 5.60 X 10­3 M Problem 4. Calculate the pH of a 0.800M solution of nitrous acid that is 3.50% ionized. c) Calculate the pH, pOH, and hydronium ion concentration in a solution whose hydroxide ion concentration = 1.20 X 10­6 M. Problem 5. Calculate the pH of a 0.500M solution of ammonia that is 4.25% ionized. 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 1­7:42 AM May 1­7:43 AM Problem 6. Calculate the hydronium ion, hydroxide ion, pH and pOH of a solution of acetic acid that is 2.75% ionized. 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. Problem 7. A 0.650M solution of a monoprotic acid (HA) has a pH = 4.55. Determine its Ka. 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 so, MaVaia = MbVbib 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.