Acids and Bases

Acids and Bases - Acids and Bases Definitions and Strengths...

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1 Acids and Bases Definitions 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 are written first in the formula. b. Examples. 1) Monoprotic - one acidic proton. HNO 3 Nitric acid HCl Hydrochloric acid HBr Hydrobromic acid HC 2 H 3 O 2 Acetic acid 2) Diprotic - two acidic protons. H 2 SO 4 Sulfuric acid H 2 SO 3 Sulfurous acid 3) Triprotic - three acidic protons H 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 general formula 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 the formula would indicate. 3. Limitations of the Arrhenius system.
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2 a. Many substances dissolve in water to give acidic or basic solutions but do not have 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 in H 2 O. However , according to the Arrhenius system K 2 O cannot be classified as a base 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 original acid. After a base accepts a proton, it is converted into an acid - the conjugate acid of the original base. b. Examples. H 2O + NH 3 NH 4 + + OH - acid base acid base H 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 base 4. Relative strengths of acids and bases. a. Strong acids will yield weak conjugate bases and weak acids will give strong conjugate bases. The following page is a table that lists some conjugate acid-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 proton transfer will be complete, that is, the reaction will essentially go to completion.
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Acids and Bases - Acids and Bases Definitions and Strengths...

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