14. Acid-Base Def & Strengths


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Acids and Bases Definitions and Strengths I. Systems of acids and bases. A. Arrhenius system. 1. Acid: any substance that dissociates in H2O to give hydronium ions (H3O+). a. Must have H in formula. The general formula is HnX. The acidic hydrogens are written first in the formula. b. Examples. 1) Monoprotic - one acidic proton. HNO3 Nitric acid HCl HBr HC2H3O2 Hydrochloric acid Hydrobromic acid Acetic acid 2) Diprotic - two acidic protons. H2SO4 Sulfuric acid H2SO3 Sulfurous acid 3) Triprotic -
    Acids and BasesDefinitions and Strengths  I. Systems of acids and bases.A. Arrhenius system.1. Acid : any substance that dissociates in H 2 O to give hydronium ions (H 3 O + ).a. Must have H in formula. The general formula is H n X. The acidic hydrogens arewritten first in the formula.b. Examples.1) Monoprotic - one acidic proton.HNO 3 Nitric acidHCl Hydrochloric acidHBr Hydrobromic acidHC 2 H 3 O 2 Acetic acid2) Diprotic - two acidic protons.H 2 SO 4 Sulfuric acidH 2 SO 3 Sulfurous acid3) Triprotic - three acidic protonsH 3 PO 4 Orthophosphoric acid (Phosphoric acid).c. Acids are covalent substances that react with a water molecule to give H 3 O +  HCl + H 2 O H 3 O + ( aq )+ Cl - ( aq ) Shorthand notation : HCl H + + Cl - (H 3 O + written as H + ).d. The acids can be either strong acids ( 100% dissociated) or weak acids(partially dissociated).Strong: HCl + H 2 O H 3 O + ( aq ) + Cl  – ( aq ) (100%)Weak: HNO 2 ( aq )+ H 2 O H 3 O +  ( aq )+ NO 2 ! ( aq )( not  100%)  K a =[H 3 O + ][NO – 2 ][HNO 2 ]= 7.2x10  –4  2. Base : any substance that dissociates in H 2 O to give OH - ions. The generalformula is M(OH) n .a. Arrhenius bases are ionic compounds where OH - is the anion.b. All are strong but many have low solubility.c. Examples: NaOH, KOH, Ca(OH) 2 , Mg(OH) 2 , Al(OH) 3  Some bases, such as Al(OH) 3 , are more complex in structure and composition than theformula would indicate.3. Limitations of the Arrhenius system.   a. Many substances dissolve in water to give acidic or basic solutions but do nothave the classical acid or base formulas.b. Example: A solution of K 2 O in H 2 O is identical with a solution of KOH inH 2 O. However , according to the Arrhenius system K 2 O cannot be classified as abase in that it does not have OH - 's in its formula.B. Brönsted-Lowry System.1. Acid = proton donor. Must have a H in its formula.2. Base = proton acceptor.a. Must possess a lone pair of electrons to form a coordinate covalent bond with a H + .b. An acid-base reaction is the transfer of a proton from an acid to a base.3. Conjugate acid-base pairs.a. After an acid transfers a proton, the molecule or ion left has a lone pair of electrons and is therefore a base - the conjugate base of the srcinal acid.After a base accepts a proton, it is converted into an acid - the conjugate acid of thesrcinal base.b. Examples.H 2 O+ NH 3 NH 4+ + OH - acid base acid baseH 2 O + H 2 O H 3 O + + OH - (Self dissociation reaction   acid base acid base of water)H 2 O + HCl H 3 O + + Cl - base acid acid base4. Relative strengths of acids and bases.a. Strong acids will yield weak conjugate bases and weak acids will givestrong conjugate bases. The following page is a table that lists some conjugateacid-base pairs, arranged in order of decreasing acid strength.b. In a reaction of an acid with a base below it in the table, the protontransfer will be complete, that is, the reaction will essentially go to completion.The reaction of an acid with a base above will result in an equilibrium reaction;the larger the separation, the more unfavorable is the equilibrium.   Relative Strengths of Acid-Base PairsConjugate Acid Conjugate BaseStrongest Acid HClO 4 ClO 4- Weakest BaseHI I -  HBr Br -  HCl Cl -  H 2 SO 4 HSO 4- HClO 3 ClO 3-  HNO 3 NO 3-  H 3 O + H 2 OHSO 4- SO 42-  H 2 SO 3 HSO 3-  HClO 2 ClO 2- HF F -  HC 2 H 3 O 2 C 2 H 3 O 2-  HSO 3- SO 32- HClO ClO -  NH 4+ NH 3 H 2 O 2 HO 2-  H 2 O OH -  NH 3 NH 2-  Weakest Acid OH - O 2- Strongest Basec. Solvent leveling effect.1)The first seven acids in the Table are all above the base H 2 O and willtransfer their protons 100% to H 2 O. Thus, these are the strong acids that arecompletely dissociated in water.2) Note that in an aqueous solution the strongest acid that can exist is H 3 O + .All acids stronger will dissociate 100%. Therefore, in aqueous solution,there are no experimental differences in the strengths of the first seven acids;they are all 100% dissociated.3) In the same way, the strongest base that can exist in water is OH - , the anionicspecies in the self dissociation. Therefore, NH 2- and O 2- can not exist inaqueous solution.   d. Classical weak acids are those that fall between H 3 O + and H 2 O in the above list.Examples:Hydrogen sulfate HSO 4 – + H 2 O H 3 O + + SO 42– K a = 1.3x10  –2  Hydrofluoric acid HF + H 2 O H 3 O + + F  – K a = 7.1x10  –4  Acetic acid HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O 2 – K a = 1.8x10  –5  Hydrogen sulfite HSO 3 – + H 2 O H 3 O + + SO 32– K a = 6.3x10  –8  Note: The closer the conjugate acid is to the base, H 2 O, the larger the value of K a  C. Lewis System.1. Acid = any substance that can accept an electron pair in forming a coordinate covalentbond.a. A Lewis acid must have a fairly low energy vacant, or easily vacated orbital.b. Examples.H + (vacant 1s orbital)BF 3 (vacant 2p orbital on B)Ni 2+ (vacant 3d, 4s, and 4p orbitals)SO 3 (the S—O  π bond is easily broken)c. Acid are electrophiles because they react readily with electron pair donors.2. Base = any substance that can donate an electron pair in forming a coordinate covalentbond.a. A Lewis base must have a lone pair of electrons. This is the same as a Brönsted-Lowry base.b. The base is a nucleophile .3. The reaction of an acid and a base is the formation of a coordinate covalent bond.a. H + + NH 3 -------->    NH 4+ c. Ni 2+ + 6NH 3 -------> Ni(NH 3 ) 62+  acid base adduct acid + base adductb. BF 3 + NH 3 ------->    F 3 B-NH 3 d. SO 3 + H 2 O --------> H 2 SO 4  acid base adduct acid base S|O||O|OHOH|O S O||O||O|HHO S O||O|HH|O|  II. Acid - Base Strength.A. Strength of Brönsted-Lowry oxyacids.|1. In an oxyacid, the acidic proton is bonded to an oxygen; the group is − X − O − H|
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