Acids & Bases

Acids and Bases an Introduction A. Properties of Acids and Bases –1. Acids Ionize when put into water React with active metals (Group I, II) to produce Hydrogen gas Neutralized with bases Have a sour taste Found in citrus fruits, vinegar, soda pH 0-7

Acids and Bases an Introduction –2. Bases Ionize when put into water Neutralized with acids Have a bitter taste Feel Slippery Found in soaps, cleaners, antacids, etc. pH 7-14

Acids and Bases an Introduction B. Arrhenius Definitions of Acids and Bases –1. Acids Produces H + in soln; formula starts with H Limited to aqueous acids with Hydrogen as the positive ion –2. Bases Produces OH - in soln; formula ends with OH Limited to aqueous bases with Hydroxide as the negative ion –3. Another definition needed Too many substances that didn’t fit the definition so it needed to be expanded

Acids and Bases an Introduction C. Bronsted-Lowry Model –1. more inclusive definition of acids and bases than Arrhenius Acid = Proton (Hydrogen) Donor Base = Proton (Hydrogen) Acceptor

Acids and Bases an Introduction –2. Conjugate acid The substance formed from the base –3. Conjugate base The substance formed from the acid

Acids and Bases an Introduction –4. Acid-Base Pairs Acid and its conjugate base -or- Base and its conjugate acid –5. Generalized equation: H—X + B X -1 + HB +1 Acid Base CB CA

Acids and Bases an Introduction 6. Example: Write the Bronsted-Lowry equations for the weak acid HNO 2 and the weak base NH 3, (each combining with water) identifying the conjugate acid-base pairs in each reaction.

Acids and Bases an Introduction 7. Example: Write the Bronsted-Lowry expressions for the reactions between NH 3 and HCO 3 1- ; and H 3 PO 4 and H 2 O, identify the conjugate acid-base pairs in each equation.

Acids and Bases an Introduction –8. Amphiprotic species Substances that can ionize as either an acid or a base depending on the properties of the other species in soln; can have properties of an acid or a base

Acids and Bases an Introduction –9. Relative strengths of acids and bases –Strong acids: Br I Cl SO NO ClO (4, 3, 4) –Strong bases: Group I except 1 st one and group II except 1 st two –All other acids and bases are considered weak –(Remember: strong means fully ionized and weak means partially ionized)

Self-Ionization of Water, The pH Scale A. Water dissociation constant –1. In water solution H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH - (aq) acid base CB CA OR:H 2 O (l)  H +1 (aq) + OH -1 (aq) –2. For any sample of water molecules: 2 H 2 O (l)  H 3 O + (aq) + OH - (aq)

Self-Ionization of Water, The pH Scale –3. K w = the ion product constant of water expression –4. K w = 1.0x K w = [H + ] [OH - ] K w = [H 3 O + ] [OH - ] 1.0x = [H + ] [OH - ] –5. Neutral solution [H + ]=[OH - ]=1.0x10 -7

–6. Acid solution [H+] > 1x Basic Solution [H+] < 1x10 -7

Example: A sample of tap water has a [H + ] = 2.8 x M. What is the [OH - ]?

The pH Scale; pH and pOH 1. pH = “The Potential of Hydrogen” pH = -log [H + ]

–2. Size of pH Ranges from 0 to 14 0~7 = ACID 7~14 = BASE 7 = Neutral –3. pOH pOH = -log [OH - ] –4. pH and pOH relationship pH + pOH = 14

5. Example: Calculate the pH and pOH of a 0.25 M phosphoric acid solution whose [H + ] is 0.040M. NOTE – the concentration of H + is much lower than the molarity, weak acids don’t completely dissociate

6. Example: Calculate the pH and pOH of a M formic acid solution whose [H + ] is 1.8 x M. NOTE – the concentration of H + is much lower than the molarity, weak acids don’t completely dissociate

–7. pH of strong acids and strong bases a. Strong acids and bases dissociate completely in aqueous solution: HCl (aq)  H +1 (aq) +Cl -1 (aq) over 99% ions Single headed arrow

Example: Calculate the [H + ], pH, and [OH - ] of a 0.15M solution of the strong acid, HNO 3.

c. Strong bases completely dissociate in aqueous solution: NaOH (aq)  Na +1 (aq) + OH -1 (aq) over 99% ions Single headed arrow

Example: State the pH, pOH, [H + ], and [OH - ] of a solution made by dissolving 5.00 g of Ba(OH) 2 - a strong base - in 1.00 L of water.

Solutions of Weak Acids and Weak Bases A. Weak Acids and Dissociation Constants –1. Weak acids are partly dissociated in water solution: CH 3 COOH (aq)  H +1 (aq) +CH 3 COO -1 (aq) over 99% molecules Double headed arrow/reversible reaction

Solutions of Weak Acids and Weak Bases B. Weak Bases and their Dissociation Constants –1. Two types of substances act like weak bases in aqueous solution: Nitrogen-containing compounds –Ex. NH 3 Anions of acids –Ex. HCO 3 -

Polyprotic Acids 1. Definition and examples –Acid that gives off more than 1 H + when put into water 1 st Proton usually given off rapidly Subsequent protons are given off with increasing difficulty (stronger bases at each step) 2. Phosphoric acid H 3 PO 4 = 3 H’s means polyprotic, specifically triprotic

Neutralization Reactions and Titration of Strong Acids and Bases 1. Definitions –Complete dissociations –Strong Acid + Strong Base --> H 2 O + Salt 2. Strategy – start with the volume of the substance of known concentration -Use Molarity as a conversion factor -Moles over Moles step - End with Molarity (conversion factor or formula)

Example: If it requires 45.7mL of 0.75M H 2 SO 4 to completely neutralize 25.0mL of NaOH, what is the Molarity of the base?