1 Acids and Bases Chapter 15

2 Acids and Bases The concepts acids and bases were loosely defined as substances that change some properties of water. One of the criteria that was often used was taste. Substances were classified – salty-tasting, sour-tasting, sweet-tasting, bitter- tasting Sour-tasting substances would give rise to the word 'acid', which is derived from the Greek word oxein, which mutated into the Latin verb acere, which means 'to make sour' Vinegar is a solution of acetic acid. Citrus fruits contain citric acid.

3 Acids React with certain metals to produce hydrogen gas. React with carbonates and bicarbonates to produce carbon dioxide gas Have a bitter taste Feel slippery. Many soaps contain bases. Bases

4 Properties of Acids þ Produce H + (or H 3 O + ) ions in water (the hydronium ion is a hydrogen ion attached to a water molecule) þ Taste sour þ Corrode metals þ Good Electrolytes þ React with bases to form a salt and water þ pH is less than 7 þ Turns blue litmus paper to red

5 Properties of Bases  Generally produce OH - ions in water  Taste bitter, chalky  Are electrolytes  Feel soapy, slippery  React with acids to form salts and water  pH greater than 7  Turns red litmus paper to blue

6 Arrhenius Definition Arrhenius Acid - Substances in water that increase the concentration of hydrogen ions (H + ). Base - Substances in water that increase concentration of hydroxide ions (OH - ).

7 Bronsted-Lowry Definition Acid- neutral molecule, anion, or cation that donates a proton. Base- neutral molecule, anion, or cation that accepts a proton. HA + :B  HB + + :A - HCl + H 2 O  H 3 O + + Cl - Acid Base Conj Acid Conj Base

8 Conjugate Base-The species remaining after an acid has transferred its proton. Conjugate Acid-The species produced after base has accepted a proton. HA & :A - -conjugate acid/base pair :A - -conjugate base of acid HA :B & HB + -conjugate acid/base pair HB + -conjugate acid of base :B Conjugate Acid Base Pairs

9 Note: Water can act as acid or base AcidBase Conjugate Acid Conjugate Base HCl+ H 2 O  H 3 O + +Cl - H 2 PO H 2 O   H 3 O + + HPO 4 2- NH H 2 O   H 3 O + +NH 3 BaseAcid Conjugate Acid Conjugate Base :NH 3 + H 2 O   NH 4 + +OH - PO H 2 O   HPO OH - Examples of Bronsted-Lowry Acid Base Systems

10 Lewis Acid-an electron pair acceptor Base-an electron pair donor G.N. Lewis Definition

11 pH and acidity HNO 3 + NaOH  H 2 O + NaNO 3 HC 2 H 3 O 2 + H 2 O  H3O + + C 2 H 3 O 2 - NH 3 + HCl  NH Cl - HSO HPO 4 -2  H 2 SO 4 + PO 4 -3 HSO HPO 4 -2  SO H 2 PO 4 -1

12 Acid Strength Strong Acid-Transfers all of its protons to water; - Completely ionized or dissociated; - Strong electrolyte; - The conjugate base is very weak Weak Acid-Transfers only a fraction of its protons to water; - Only partly ionizes or dissociates; - Weak electrolyte; - The conjugate base is stronger  As acid strength decreases, base strength increases.  The stronger the acid, the weaker its conjugate base  The weaker the acid, the stronger its conjugate base

13 Base Strength Strong Base-all molecules accept a proton; - completely ionizes or dissociates; - strong electrolyte; - conjugate acid is very weak Weak Base-fraction of molecules accept proton; - partly ionizes or dissociates - weak electrolyte; - the conjugate acid is stronger.  As base strength decreases, acid strength increases.  The stronger the base, the weaker its conjugate acid.  The weaker the base the stronger its conjugate acid.

14 Common Strong Acids Strong Acids Hydrochloric Acid, HCl Hydrobromic Acid, HBr Hydroioidic Acid, HI Nitric Acid, HNO 3 Sulfuric Acid, H 2 SO 4 Chloric Acid, HClO 3 Perchloric Acid, HClO 4

15 Common Strong Bases Strong Bases Any Group 1 Hydroxide and any Group 2 Hydroxide below Mg. Sodium Hydroxide, NaOH Potassium Hydroxide, KOH *Barium Hydroxide, Ba(OH) 2 *Calcium Hydroxide, Ca(OH) 2 *While strong bases they are not very soluble

16 pH and acidity The pH values of several common substances are shown at the right. Many common foods are weak acids Some medicines and many household cleaners are bases.

17 The pH Scale pH [H 3 O + ] [OH- ] pOH

18 pH and acidity 1.Acidity or Acid Strength depends on Hydronium Ion Concentration [H 3 O + ] 2.The pH system is a logarithmic representation of the Hydrogen Ion concentration (or OH - ) as a means of avoiding using large numbers and powers. pH = - log [H 3 O + ] pOH = - log [OH - ] In pure water [H 3 O + ] = 1 x M  pH = - log(1 x 10 -7) = 7 4.pH range of solutions: pH 1 x M pH > 7 (Basic) [H 3 O + ] < 1 x M

19 pH and acidity Important Equations pH = - log [H 3 O + ] [H 3 O + ] = 10 -pH pOH = - log [OH - ] [OH - ] = 10 -pOH pH + pOH = 14 K w = [H 3 O + ] [OH - ] = 1.0 x In pure water: [H 3 O + ] = [OH - ] = 1.0 x10 -7

20 Calculating the pH pH = - log [H 3 O + ] Example 1: If [H 3 O + ] = 1 X pH = - log 1 X pH = - (- 10) pH = 10 Example 2: If [H 3 O + ] = 1.8 X pH = - log 1.8 X pH = - (- 4.74) pH = 4.74

21 pH and acidity If a substance has a pH of 3.5, determine: [H 3 O + ], [OH - ], pOH and whether it is acidic, basic, or neutral. If a substance has an [OH - ] of 8.7 x 10 -5, determine: pH, pOH, [H 3 O + ] and whether it is acidic, basic, or neutral.

22 Indicators