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Chapter 4

Chapter 4 Acids and Bases.pdf

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University of Florida
CHM 2045

Chapter 4 Acids and Bases Monday, July 15, 2013 12:41 AM 1. Definitions a. 2. Water-Based Acid-Base Chemistry a. Water is amphoteric,meaning that it may act as either an acid or a base b. Neutral water naturally dissociates into hydronium and hydroxide i. c. Only a small fraction dissociates at 25 C, enough to generate 10^-7 M H3O and 10^-7MOH i. This means that neutral water has both a little hydronium and a little hydroxide d. A neutral aqueous solution is defined as one in which [H3O]=[OH], and therefor pH=pOH e. An acidic solution has more hydronium than hydroxide, and can turn blue litmus paper red 3. Acid Dissociation a. When an acid is mixed with water, it is said to dissociate b. The acid dissociation constant, K is aothing more than the equilibrium constant of an acid in water c. A moleculeof an acid when added to water, dissociates to form hydronium ion and its conjugate base (A-) i. d. Therefor, i. e. f. As the relative strength of an acid increases, it's K iacreases and its pK decaeases 4. Base Hydrolysis a. When a base is mixed with water, it is said to undergo hydrolysis b. The base hydrolysis constant K is nbthing more than the equilibrium constant for the hydrolysis reaction of a base in water c. Each molecule of a base, when added to water, hydrolyzes one water moleculeto form hydroxide ion and its conjugate acid i. d. Therefor, i. ii. e. As the relative strength of base increases, its K incbeases and its pK decrebses 5. Determinationof Reagent strength a. The strength of a reagent is determined strictly from K or pKaand tha K or pK b b a. The strength of a reagent is determined strictly from K or pa and tae K or pK b b b. Strength is there for the measure of completenessof a reaction in water c. The stronger the acid, the more "electrolytic"it is because it conducts electricitybetter due to the greater number of ions in solution d. A strong acid completelytransfers its proton to water, leaving no undissociated moleculesin solution i. Its conjugate base has a negligible tendency to be protonated (due to its stability) e. 6. Strong acids a. Dissociatefully when mixed with water b. 7. Weak Acids a. Acids that only dissociate partially when dissolved in water b. Any acid with K leas than 1 is a weak acid c. Any acid with a K lass than 10 -14as a very weak acid d. Therefore,the pK ranae for a weak acid is 0-14 e. 8. Very weak acids a. Dissociateless than water -14 b. The K value of a very weak acid is less than 10 because products divided by reactants is less than K w c. 9. Strength and the pK scale a. It may seem peculiar, but the conjugate base of a weak acid is most often a weak base 10. Haloacids a. The series of HX acids, whose X represents a halogen b. The acidity of the haloacids is predictable based on the size of the halide ion c. The large the halide, the more acidic the haloacid d. This trend in acidity is attributed to the increased stability of the conjugate base as it increases size i. The negative charge is more diffuse on the larger anion ii. The more diffuse, the morestable, the less basic, and this the moreacidic its conjugate acid e. In addition, the longer the bond, the weaker the bond i. As halogen size increases, the bond length of an H-X bond must also increase ii. As the bond length increases, the bond strength decreases, and the H+ can be removedmore readily, which indicates a stronger acid f. g. Within a period of the periodic table, it is electronegativitythat dictates the strength of an acid, not atomic radius h. Take for example H-NH2 and H-OH and H-F i. When an atom bonded to hydrogen has a greater electronegativity (electron withdrawing), the electrons are pulled more towards the atom, rather than towards hydrogen. This allows the bond to be cleaved in a heterolytic fashion rather easily ii. The strongest is there for HF ii. The strongest is there for HF iii. The atomic size does not change that noticeably between NO and F because they all have the same valence level 11. Oxyacids a. The hydrogen dissociates from an oxygen rather than a halide b. Formed from the hydration of non-metal oxides i. ii. 1) Non-metal oxides act as Lewis Ac
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