Chapter 20 Acids and Bases Describing Acids and
Chapter 20 Acids and Bases
• Describing Acids and Bases 1. Properties of Acids and Bases Acids Bases Contains H+ Contains OHTurns blue litmus red Turns red litmus blue Taste sour Taste bitter Can be electrolytes Reacts with bases to Reacts with acids to form water
2. Quick review of naming acids Anion ending Example Acid name -ide Cl- chloride Hydro- (stem) –ic acid -Ite SO 3 -2 (stem)-ous acid -ate SO 4 -2 (stem)-ic acid Example Hydrogen ions and acidity 1. Hydrogen Ions from water a. When water molecules lose a hydrogen ion it becomes OH-
b. When water molecules gain a hydrogen ion it becomes H 3 O+ (called the hydronium ion) 2. Self-ionization a. When two water molecules produce ions b. H 2 O (l) H+ (aq) + OH- (aq)
c. [H+] = 1. 0 x 10 -7 M d. [OH-] = 1. 0 x 10 -7 M e. When [H+] and [OH-] are equal it is a neutral solution f. When they are independent (not equal) [H+] increases, [OH-] decreases [H+] decreases, [OH-] increases
3. Ion-product constant a. kw : product of concentration of H+ and OHin water b. Kw = [H+] [OH-] = 1. 0 x 10 -14 M 2 c. Acidic solution: one where [H+] is greater than [OH-] [H+] > 1. 0 x 10 -7 M
d. Basic solution: one where [H+] is less than [OH-] [H+] < 1. 0 x 10 -7 M e. Basic solution also known as Alkaline solution 4. The p. H concept a. Better expressed using the p. H scale
b. p. H + p. OH = 14 p. H = -log[H+] p. OH = -log[OH-] c. In a neutral solution [H+] = 1. 0 x 10 -7 M p. H = -log (1 x 10 -7) = -(log 1 + log 10 -7) = -(0. 0 + (-7. 0)) = 7. 0
d.
e. 5. Example problems: a. What is the p. H of a solution with a hydrogen-ion concentration of 1. 0 x 10 -10 M?
b. The p. H of an unknown solution is 6. 00. What is its hydrogen-ion concentration? c. What is the p. OH of a solution if [OH-] = 4. 0 x 10 -11 M? d. What is [H+] of a solution if the p. H = 3. 70?
6. Measuring p. H a. Acid-base Indicator 1. Indicator (In) is an acid or base that undergoes dissociation in a known p. H range 2. Reaction form: HIn (aq) H+ (aq) + In- (aq) Acid form Base form
3. Types: Thymol blue Bromphenol blue Bromcresol green methyl red Alizarin Bromthymol blue Phenol red Phenolphthalein p. H color 1. 2 -3. 0 red yellow 8. 0 -9. 5 yellow blue 3. 0 -4. 6 yellow blue 3. 7 -5. 3 yellow blue 4. 2 -6. 2 red yellow 4. 5 -6. 0 yellow red 6. 0 -7. 5 yellow blue 6. 9 -8. 2 yellow orange 8. 0 -10. 0 colorless pink
alizarin yellow R 8. 0 – 12. 2 yellow red 4. Useful at room temperature (25 °C) b. p. H meter 1. Useful to make rapid, accurate p. H measurements 2. more practical than liquid indicators
Acid-Base Theories 1. Arrhenius Acids and Bases a. Acids are hydrogen containing compounds that ionize to yield H+ in aq solutions b. Bases are compounds that ionize to yield OH- in aq solutions Acids c. Monoprotic acids have one hydrogen HCl
d. Diprotic acids : have two hydrogens H 2 SO 4 e. Triprotic acids: have three hydrogens H 3 PO 4 f. Only very polar bonds will dissociate Hδ+--Clδ- H+ (aq) + Cl- (aq)
g. C-H bonds weakly polar will not dissociate ex. Ethanoic acid (CH 3 COOH):
Bases h. Na. OH (s) Na+ (aq) + OH- (aq) i. Common bases: KOH, Na. OH, Ca(OH)2, Mg(OH)2
2. Bronsted-Lowery Acids and Bases a. Acid is a hydrogen-ion donor b. Base is a hydrogen-ion acceptor c. Conjugate acid – particle formed when a base gains a hydrogen ion d. Conjugate base- particle that remains when an acid has donated a hydrogen ion
e. Conjugate acid-base pair: two substances related by the loss or gain of a single hydrogen bond f. Examples: 1. NH 3(aq) + H 2 O (l) NH 4+ (aq) + OH- (aq) acceptor donor (base) (acid) (CA) (CB) 2. HCl (g) + H 2 O (l) H 3 O+ (aq) + Cl- (aq) (acid) (base) (CA) (CB)
g. Amphoteric: a substance that can act like both an acid and base
Sample problems • 1. Classify the following as Brønsted acids, bases or both. a) H 2 O b) OHc) NH 3 d) NH 4+ • 2. What is the conjugate base of the following acids? a) HCl. O 4 b) NH 4+ c) H 2 O d) HCO 3 • 3. What is the conjugate acid of the following bases? a) CN- b) SO 42 - c) H 2 O d) HCO 3 -
3. Lewis Acids and Bases a. Acid: a substance that accepts a pair of electrons to form a covalent bond b. Base: a substance that donate a pair of electrons to form a covalent bond c. Examples: 1. H+ + acid base 2.
Strengths of Acids and Bases 1. Strong acids and bases a. Strong acids: completely ionize (dissociate) HCl, HNO 3, H 2 SO 4, HBr, HI, HCl. O 4 b. Dissociation constant (Ka): the ratio of the concentration of the dissociated form of an acid to the concentration of the undissociated form
***See page 600 Table 20. 7 3. Equilibrium-constant expression K = [products] [reactants] ** Remember to raise the concentrations to the coefficient number.
4. Ka = [H+][A-] [HA] (Gives the ratio of ions vs molecules) Weak acid has Ka <1 Leads to small [H+] and p. H of 2 -7 5. Kb = [BH+][OH-] [B] Weak bases has Kb < 1 Leads to small [OH-] and p. H of 12 -7 **Do not use water in the [ ]
6. Examples: a. Calculate the [OH-] of a 0. 500 M solution of aqueous ammonia. The Kb is 1. 74 x 10 -5. NH 3 + H 2 O Kb = [NH 4+][OH-] [NH 3] NH 4+ + OH-
b. You have 1. 00 M acetic acid (HOAc). Calculate the equilibrium concentrations of HOAc, H+, OAc-, and the p. H. Ka = 1. 8 x 10 -5 Step 1 Define equilibrium concentrations. [HOAc] [H+] [OAc-] Initial: Change: Equilib: 1. 00 -x 1. 00 -x 0 +x x
Step 2: Write the Ka expression HC 2 H 3 O + H 20 H + + C 2 H 3 O (HOAc) (OAc-) Ka = [H+][OAc-] [Ho. Ac] 1. 8 x 10 -5 = (x)(x) = x 2 (1. 00 –x) (1. 00 – x) This is a quadratic. Solve using the quadratic formula. OR you can make an approximation if x is very small. (Rule of thumb: 10 -5 or smaller is OK)
1. 8 x 10 -5 = x 2 1. 00 x = [H+] = [OAc -] = 4. 2 x 10 -3 M p. H = -log[4. 2 x 10 -3] = 2. 37
c. You have 0. 010 M NH 3. Calculate the p. H if the Kb = 1. 8 x 10 -5. NH 3 + H 2 O NH 4+ + OH[NH 3] Initial 0. 010 Change -x Equilibrium 0. 010 -x [NH 4+] 0 x x [OH-] 0 x x
Kb = [NH 4+] [ OH-] [NH 3] 1. 8 x 10 -5 = (x)(x) 0. 010 – x x = 4. 2 x 10 -4 M At equilibrium: 0. 010 -4. 2 x 10 -4 = 0. 00958≈0. 01
Once you find [OH-], you find the p. OH p. H + p. OH = 14
p. H indicators 1. indicator (In) is an acid or base that dissociates in a known p. H range 2. HIn (aq) acid form OH H+ H+ (aq) + In- (aq) base form
• 3. Types of indicators a. Methyl red: dye that turns red in acids 0 -4. 4 : red 4. 5 -6. 1: orange 6. 2 -above: yellow b. Phenolphthalein: colorless in acids, pink in bases below p. H 8. 2: colorless above p. H 10: pink
c. Bromothymol blue: used for weak acids/bases below p. H of 6. 0 = yellow p. H of 7. 0 = green above p. H of 7. 6 = blue d. Universal indicator: used for acids and bases 0 -3 3 -6 7 8 -11 11 -14 red orange/ green blue purple yellow
• Problems with indicators 1. Only work at room temperature (will change colors at different temp) 2. Salts in the solution may change the dissociation process p. H meter: equipment used to measure p. H (best p. H measurement)
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