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Chapter 2 – Acids & Bases Dr.Gergens

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Chapter 2 – Acids & Bases Dr.Gergens Chapter 2 - Acids & Bases Dr. Gergens – SD Mesa College Mantra – We say ACID, and as Citizens of Science we say … Ammork & HClindy: Nano, Nano … Particle Demonstration A. “I am Ammork from Ammonium Hydroxide. I smell like fish. I am a little unstable. Greetings, I leave in pieces." 1 NH4OH (aq) ------> NH3 (g) + H2O (l) NH4OH aqueous aqueous solution Ammork solution B. “Hi, I am HClindy. I am bubbly …. out of a solution of HCl (aq) I turn blue litmus paper red, acid 1 HCl (aq) ------> HCl (g) + H O (l) 2 HCl aqueous solution HClindy aqueous solution C. “And together, we are Ammork & HClindy” Nano, Nano 1 NH3 (g) + 1 HCl (g) ------> particles NH4OH HCl aqueous aqueous solution solution Ammork & HClindy: Nano, Nano … Particle Demonstration A. “I am Ammork from Ammonium Hydroxide. Greetings, I leave in peaces." B. Nano Particle Demonstration - What are those particles forming in mid-air ???? 1 NH3 (g) + 1 HCl (g) ------> C. Draw the Lewis Dot for each substance • ICAO on each substance • follow all rules for drawing Lewis Dot structure • indicated full and partial charges D. Law of Electrostatics, Opposites Attract & FONClBrISCH E. Complete the reaction: • atoms • electrons • formal charges • ALL must balance • opposites attract F. Draw an electron arrow push between substances to show chemical reactivity. 2 Acid-Base Trends, Conjugates, and Reactions Dr. Gergens - SD Mesa College Periodic Trend ACIDS conjugate bases acidity increases basicity increases – – - - CH4 NH3 H2O H-F CH3 NH2 OH F – - - PH3 H2S H-Cl PH2 SH Cl acidity basicity increases increases - H-Br Br - H-I I Evaluating Acid-Base Reactions Definitions (relative strength to produce weaker acid) — – H—I + OH I + H2O – - SH + H2O H2S + OH + - NH3 + H-Cl NH4 + Cl •• •••• N H Predicting Proton-Transfer Products H O H H - H CH3 + H2O - HO + H2S H Å •• •••• - N O NH2 + HBr H H H H NH3 + HF Can you name all the ions on molecules on this handout? Bronsted-Lowry Acid-Base Theory Practice - Dr.Gergens Identify the specified conjugate proton donor conjugate base •• + •• remove proton H from C=O O H C A1 •• C H •• O •• H H H C C H H •• •• •• •• O O •• A2 •• + C C remove proton H from C H C H H C H H enolate ion H + remove proton H from N •• H N CH B1 •• 3 N-methyl amide ion •• H N CH3 H •• •• H N CH2 + B2 remove proton H from C H + •• •• •• •• remove proton H from C O O •• H •• C •• H •• C1 O C H O C C H •• •• O •• H H enolate ion H H •• O C C H •• •• H •• O •• •• H O •• H •• •• O C H •• + C2 O C H remove proton H from O •• •• H H carboxylate ion •• •• •• •• + O O •• remove proton H from C •• •• •• H N C C H C D1 2 H2N C H •• •• H O H enolate ion H •• C H2N C H •• •• H •• •• O H O •• H •• •• H N C H + D2 H N C H remove proton H from N •• H H amidate ion proton acceptor conjugate acid Å H H H + •• •• •• add proton H to C=O O •• O •• O E1 •• •• H Å •• H •• Å H H2NO H2NO H2N O •• •• •• •• •• •• O •• + O add proton H to O •• •• H •• •• H H2NO •• E2 H2NOÅ H •• O •• + E3 •• add proton H to N H H3NO Å •• + H H add proton H to inner C C C F1 H Å H C H C H H H H C C H H C C H H H H H H H C C H C C H Å H H Å H + F2 C C C add proton H to end C C H H H H + add proton H to O •• •• H2N OH2 G1 Å •• •• H2N OH •• Å •• H3N OH + G2 •• add proton H to N Acid and Base Strengths Acid Base pKa - HClO4 ClO4 -10 HI I- -10 - Strong acid H2SO4 HSO4 -10 in water HBr Br- -9 HCl Cl- -7 - HNO3 NO3 -2 + H3O H2O -1.74 - CCl3CO2H CCl3CO2 0.52 - -2 HSO4 SO4 1.99 - Strong acid H3PO4 H2PO4 2.12 - in ammonia CH2ClCO2H CH2ClCO2 2.85 HF F- 3.17 - HNO2 NO2 3.3 Weak acids - CH3CO2H CH3CO2 4.75 in water + C5H5NH C5H5N 5.25 - H2CO3 HCO3 6.35 -- H2S HS 7.00 + NH4 NH3 9.24 - -2 HCO3 CO3 10.33 + CH3NH3 CH3NH2 10.56 - H2O OH 15.74 Weak acid - CH3OH CH3O 18 in ammonia Not an acid HCCH HCC- 25 - in water NH3 NH2 34 - CH2CH2 CH2CH 36 Not an acid - CH3CH3 CH3CH2 42 in ammonia Relative Acid Base strengths are expressed by the pKa scale. A given acid will give up its proton to the base of an acid with a higher pKa value. The base of a given acid will deprotonate an acid with a lower pKa value. Examples: Consider the solvent water. + Water, H2O, is the base of the hydronium ion, H3O , pka -1.74. This value is greater than the pKa of HCl, -7. This means that HCl will give up its protons to water essentially completely to + form the H3O cation. We call HCl a strong acid in water. One can assume that all of the HCl in a water solution is 100 percent dissociated meaning that both the hydronium ion concentration and the chloride ion concentration correspond directly to the amount of added HCl. Acetic acid, CH3CO2H, has a pKa of 4.75, greater than that of the hydronium ion, but less than that of water itself, 15.74. This means that acetic acid can dissociate in water, but only to a small extent. We call acetic acid a weak acid. To calculate the actual concentration of the hydronium ion and the acetate anion in an acetic acid water solution one must carry out an equilibrium calculation. If one adds sufficient sodium hydroxide, NaOH, to a solution of acetic acid in water then the hydroxide anion will totally deprotonate the acetic acid, because the hydroxide ion is a strong base in water. The final product will be a sodium acetate solution. Acetylene, HCCH, has a pKa value of 25 greater than that of water, 15.74. This means that HCCH can not act as an acid in water. In water there will be essentially no dissociation of HCCH, the concentration of the HCC- anion can be assumed to be zero. Consider the solvent ammonia. + Ammonia, NH3, is the base of the ammonium ion, NH4 , pka 9.24. This value is higher than the pKa of acetic acid, 4.75. This means that acetic acid will give up its protons to ammonia + essentially completely to form the NH4 cation. We call acetic acid a strong acid in ammonia. One can assume that all of the acetic acid in a ammonia solution is 100 percent dissociated meaning that both the ammonium ion concentration and the acetate ion concentration corresponds directly to the amount of added acetic acid. Acetylene, HCCH, has a pKa value of 25, greater than that of the ammonium ion, but less than that of ammona itself, 34. This means that acetylene can dissociate in ammonia, but only to a small extent. We call acetylene a weak acid in ammonia. To calculate the actual concentration of the ammonium ion and the HCC- acetylide anion in an acetylene ammonia solution one must carry out an equilibrium calculation. If one adds sufficient sodium amide, NaNH2, to a solution of acetylene in ammonia then the amide anion will totally deprotonate the acetylene, because the amide ion is a strong base in ammonia. The final product will be a sodium acetylide solution. Ethane, CH3CH3, has a pKa value of 42 greater than that of ammonia, 34. This means that CH3CH3 can not act as an acid in ammonia. In ammonia there will be essentially no dissociation - of CH3CH3, the concentration of the CH3CH2 anion can be assumed to be zero. Acidity and Basicity Trends for Organic Compounds Dr. Gergens - Mesa College 1. Memorize the following general acidity trend for the following organic compounds given below, and give an approximate pKa for each acidic proton. Become familiar with following sequence in acidity strength: + H3O >> carboxylic acid > phenol > water > alcohol > terminal alkyne> amine > hydrocarbon structure: can be written ascan be written as RCOOH Ph-OH HO-H RO-H R C C H RNH R-H •• 2 H OÅ H RCO H Æ-OH 2 R = alkyl R = alkyl R = alkyl R = alkyl H O OH R C OH pKa ~ 5 10 15.7 16 -19 25 ~33 ~50 Explanation for the observed acidity trend: The acidity trend depends on the extent to which a proton can be separated from a molecule, and the stability of its conjugate base. The greater acidity of carboxylic acids relative to alcohols are based on two effects: (1) inductive effects, (2) resonance effects. Addition of an oxygen atom from the carbonyl carbon, C=O, adjacent to the hydroxyl group, -OH, causes an inductive electron withdrawal weakening of the O-H bond making it easier for the proton to be removed. Although structurally similar to an alcohol, phenol is a million times more acidic due primarily to resonance stabilization in the conjugate base, phenoxide ion, formed upon loss of a proton. Like phenoxide ion, resonance stabilization in the conjugate base of the carboxylate ion for a carboxylic acid is also observed. As one might guess, between a carboxylic acid and an alcohol, inductive effects have a greater influence over resonance effects on the extent to which a proton can be separated from a molecule. Alcohols in general are more acidic than amines and hydrocarbons.
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