Download Understanding Conjugate Acids & Bases: Proton Transfer & Bronsted-Lowry Theory and more Summaries Chemistry in PDF only on Docsity! Indian River State College 10/09, rev 7/12, 4/13, 5/13, 6/13, 11/13, 3/14 mds/cs Conjugate Acids and Bases A. The heart of the Bronsted-Lowey concept of acids and bases is that acid-base reactions are proton transfer reactions. An acid is a proton donor. A base is a proton acceptor. Example: HCl + H2O (aq) → H3 +O + Cl¯ (aq) HCl is an acid, it donates a H+ and H2O is a base, it accepts the H+. Example: HCHO2 (aq) + H2O (aq) H3 +O (aq) + CHO2 ¯ (aq) The formic acid molecule is donating a proton to a water molecule and behaves as an acid. Water accepts the proton and behaves as a base (proton acceptor). B. The reverse reaction: CHO2 ¯ (aq) + H3 +O (aq) → HCHO2 (aq) + H2O (aq) H3 +O behaves as in acid, as it donates a proton to the CHO2 ¯ ion. The CHO2 ¯ ion behaves as a base accepting the proton. Accordingly, there are two acids and two bases in this reaction. Substances that differ from each other by a proton are referred to as a conjugate acid base pair (CJ Pair). CJ Pair HCHO2 + H2O H3O + + CHO2 − Acid Base Acid Base CJ Pair C. One method of solving these problems involves identifying the acid in the left side of the equation as the other component will be the base on the left side of the equation. On the right side of this equation look for the material that has a difference of a proton from the left side. Therefore H3 +O is a CJ acid of H2O and CHO2 ¯ is conjugate base of HCHO2, since it cannot provide another hydrogen for H2O. D. Determining the conjugate acid or conjugate base of existing compound. Question 1: Find the conjugate base of HNO3 and conjugate acid of HSO4 ¯. 1. The first step is to recognize that members of any conjugate acid-base pair differ by one H+. The member having the greater number of hydrogen ions is the acid. Indian River State College 10/09, rev 7/12, 4/13, 5/13, 6/13, 11/13, 3/14 mds/cs 2. The second step is to recognize that the conjugate base (CB) of an acid will have one less hydrogen ion and that the conjugate acid (CA) of a pair will have one more hydrogen ion. Another option for expressing this statement is to note that the H+ ion is positive and will involve addition while the OH- ion is negative and will involve a subtraction of a proton. H+ represents acid and OH- represents base. HNO3 (Subtract CB H+ from HNO3) NO3 ¯ + H+ Acid CB HSO4 ¯ (add H+ to HSO4 ¯) H2SO4 Base CA HNO3 + HSO4 − NO3 − + H2SO4 Reaction of the acid and base 3. Charges on these substances also change so that solutions remain electrically neutral. Question 2: Identify the two acids and the two bases in this reaction. HCO3 ¯ + H2 PO4 ¯ H2CO3 (aq) + HPO4 −2 1. Acids have more H+ than bases: H2PO4 ¯ (acid) H2CO3 (acid) HPO4 ¯ (base) HCO3 ¯ (base) E. Determine if a substance is amphoteric a. When a negative charge is associated with a compound and a hydrogen atom is available within the compound, it will always be amphoteric. b. If a compound has no H present within the structure, it will not be amphoteric. There is no proton (i.e. hydrogen) to donate. c. Otherwise, a Lewis structure must be drawn to determine if the central atom of a compound can accept a hydrogen atom. Are there unpaired electrons that can accept a hydrogen atom? i. If there are no unpaired electrons, then no hydrogen can be added and the material is not amphoteric. ii. If there are unpaired electrons, draw a Lewis structure and determine if the structure is stable and satisfies valence electron criteria. d. Strong acids cannot accept H+.
