CHEM 162: Gilbert Chapter 16 Part I W2013 page 1 of 29 Chapter 17: Equilibrium in the Aqueous Phase Problems: 17.3-17.80 17.1 ACIDS AND BASES: THE BRØNSTED-LOWRY MODEL PROPERTIES OF ACIDS & BASES Acids – produce hydrogen ions, H+ – taste sour – turn blue litmus paper red – e.g. citrus fruits, vinegar, coffee Bases – produce hydroxide ions, OH– – taste bitter; feels soapy, slippery – turn red litmus paper blue – e.g. bleach, drain cleaner, baking soda Arrhenius Definitions: Arrhenius acid: A substance that releases H+ when dissolved in water – Some acids are monoprotic (release only one H+ per molecule) – e.g. HCl, HBr, HI, HNO3, HClO4 – Some acids are polyprotic (release more than one H+ per molecule) – e.g. H2SO4 and H2CO3 are both diprotic; H3PO4 is triprotic. HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq) HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2–(aq) Note: H3O+ is the hydronium ion, a hydrated proton: H+ + H2O = H3O+ Since H3O+ ⇔ H+ + H2O, we will often simply use H+, but recognize that it actually exists as H3O+. HCl(aq) → H+(aq) + Cl–(aq) HC2H3O2(aq) H+(aq) + C2H3O2–(aq) Arrhenius base: A substance that releases OH– when dissolved in water NaOH(aq) → Na+(aq) + OH–(aq) The general equation for an Arrhenius acid-base neutralization reaction is shown below: HX(aq) + MOH(aq) → H2O(l) + MX(aq) acid base water saltCHEM 162: Gilbert Chapter 16 Part I W2013 page 2 of 29 Brønsted-Lowry (B-L) Definitions: Danish chemist Johannes Brønsted and English chemist Thomas Lowry developed the following model for acids and bases in 1923: Brønsted-Lowry acid: A substance that donates a proton (H+)—i.e., a proton donor Brønsted-Lowry base: A substance that accepts a proton (H+)—i.e., a proton acceptor – Unlike an Arrhenius base, a B-L base need not contain OH–. According to Brønsted-Lowry, an acid-base reaction simply involves a proton (H+) transfer, not necessarily the formation of water and a salt. For example, the ionization of HCl can be viewed at the molecular level as follows: In this example, the HCl donates a H+ ion to the H2O molecule. → HCl is the proton donor = acid and H2O is the proton aceptor = base Note: The Brønsted-Lowry base (or proton acceptor) must have a lone pair of electrons to form the bond with the H+ ion. conjugate acid-base pairs: a Brønsted-Lowry acid/base and its conjugate differ by a H+ – For the reaction above, when HCl donates H+ to H2O, it leaves behind Cl–. – An acid and base that differ only in the presence/absence of H+ are called conjugate acid-base pairs. – Thus, HCl is the conjugate acid of Cl–, and Cl– is the conjugate base of HCl. Example: Indicate the other conjugate acid-base pair in the reaction above: ________________CHEM 162: Gilbert Chapter 16 Part I W2013 page 3 of 29 In the general reaction for the dissociation (or ionization) of an acid below the double-arrow indicates both the forward and reverse reactions can occur: HA(aq) + H2O(l) H3O+(aq) + A–(aq) Brønsted-Lowry acid = ____________ Brønsted-Lowry base= ____________ Also, H2O is the conjugate ________ of H3O+, and A– is the conjugate ________ of _______. acid or base acid or base Example: Label the Brønsted-Lowry acid and base in each of the following reactions, and fill in the blanks below to indicate the conjugate acid/base pairs shown: a. F–(aq) + H2O(l) OH–(aq) + HF(aq) F– is the conjugate _______ of __________; H2O is the conjugate _______ of ____________. acid or base acid or base b. NH3(aq) + H2O(l) NH4+(aq) + OH–(aq) NH4+ is the conjugate _______ of __________; OH– is the conjugate _______ of _________. acid or base acid or base c. CH3COOH(aq) + H2O(l)  H3O+(aq) + CH3COO–(aq) CH3COO– is the conjugate _____ of ___________; H3O+ is the conjugate _____ of ________. acid or base acid or baseCHEM 162: Gilbert Chapter 16 Part I W2013 page 4 of 29 A substance that can accept as well as donate a proton —i.e. act as an acid or as a base—is amphoteric. – e.g., H2O is amphoteric. HNO3(aq) + H2O(l) → H3O+(aq) + NO3–(aq) NH3(aq) + H2O(l) NH4+(aq) + OH–(aq) RELATIVE STRENGTHS OF ACIDS AND BASES The strength of an acid is defined by the equilibrium position for its dissociation (or ionization) reaction: HA(aq) + H2O(l) H3O+(aq) + A–(aq). A strong acid ionizes almost completely. → The equilibrium lies far to the right. → A forward arrow (not double arrow) is used for the dissociation of a strong acid like HNO3 since all the HNO3 molecules ionize to H+ and NO3–, leaving no HNO3 molecules in water. HNO3(aq) → H+(aq) + NO3–(aq) All strong acids ionize completely in water to produce H3O+ ions, the strongest acid. → Water has a leveling effect for strong acids. → 1 mole of any strong acid in water would produce 1 mole of H3O+. → Strong acids of equal concentration are indistinguishable—i.e. level–in water. In contrast, a weak acid ionizes only to a very small extent. → The equilibrium lies far to the left. → Thus, a double arrow is used for the dissociation of a weak acid like HF(aq) since mostly HF exists at equilibrium, with very few H+ or F– ions in solution. HF(aq) H+(aq) + F–(aq) Similarly, a strong base dissociates (breaks up) almost completely. → The equilibrium lies far to the right. → A forward arrow is used for the dissociation of a strong base like Ca(OH)2 (aq) since virtually all the Ca(OH)2 ionizes to Ca2+ and OH–, leaving almost not Ca(OH)2. Ca(OH)2(aq) → Ca2+(aq) + 2 OH–(aq)CHEM 162: Gilbert Chapter 16 Part I W2013 page 5 of 29 Know the following acids and bases. All other acids and bases are weak! Strong Acids Strong Bases HCl, HBr , HI, HNO3, HClO4, H2SO4 LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2, Ba(OH)2 Note: H2SO4(aq) is a strong acid and diprotic (able to release 2 H+ ions), but it generally ionizes to release