Ch 18 Acids and Bases Big Idea: Acids and Bases can be defined in terms of hydrogen ions and hydroxide ions or in terms of electron pairs.

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1 Ch 18 Acids and Bases Big Idea: Acids and Bases can be defined in terms of hydrogen ions and hydroxide ions or in terms of electron pairs. Ch 18 - Acids and Bases I CAN: 1) Compare properties of acids and bases. 2) Compare and contrast the Arrhenius and Bronsted-Lowry models of acids and bases. 3) Relate the strength of an acid or base to its degree of ionization. 4) Define ph and poh and calculate them in solutions. 5) Write balanced equations for neutralization reactions. 6) Determine whether salt solutions are acidic, basic or neutral in salt hydrolysis reactions. 1

2 Properties of Acids and Bases Acids Bases 1. Latin "acidus" sour taste Bitter taste lemons citric acid Slippery feel grapefruit ascorbic acid 2. React w/ metals to produce H 2 May be caustic (cause burns) React w/ carbonates to produce CO 2 3. Litmus paper turns pink Litmus turns blue Phenolphthalein colorless Phenolphthalein pink 4. Strong or weak electrolytes Strong or weak conduct electricity electrolytes conduct electricity 5. ph < 7 ph > 7 [H +1 ] > [OH 1 ] [OH 1 ] > [H +1 ] All water solutions contain hydrogen ions (H + ) and hydroxide ions (OH ). Arrhenius Model of Acids and Bases (1883) Arrhenius Acids: ionize in H 2 O to produce H +1 ions HCl(aq) H +1 (aq) + Cl 1 (aq) Arrhenius Bases: ionize in H 2 O to produce OH 1 ions NaOH (aq) Na +1 (aq) + OH 1 (aq) Shortcoming: NH 3 does not contain OH group but is a wellknown base 2

3 Bronsted Lowry Model (1923) Acid: donates a H +1 ion to another substance Base: accepts a H +1 ion from another substance Example: HCl + H 2 O H 3 O +1 + Cl 1 Acid Base Conjugate acid base pair: 2 substances that differ only by H +1 HCl and Cl 1 are a conjugate acid base pair: they differ only by H +1 H 2 O and H 3 O +1 are also a conjugate acid base pair Example: NH 3 + H 2 O NH OH 1 Amphoteric: a substance that can act as either an acid or a base (H 2 O) 3

4 Monoprotic and Polyprotic Acids Monoprotic Acid can donate only ONE hydrogen ion (H + ) Examples: HCl, HBr, HNO 3, CH 3 COOH CH 3 COOH + H 2 O H 3 O + + CH 3 COO Polyprotic Acid can donate MORE THAN ONE hydrogen ion (H + ) Examples: H 2 SO 4, H 3 PO 4, H 2 CO 3 All polyprotic acids ionize in steps: H 2 SO 4 + H 2 O H 3 O + + HSO 4 HSO 4 + H 2 O H 3 O + + SO 4 2 4

5 Practice: Write the ionization equations for H 3 PO 4 Strengths of Acids and Bases Strong Acids ionize completely in water Examples: HCl, HNO 3, H 2 SO 4 HCl + H 2 O H 3 O + + Cl 1 no reverse rxn occurs Weak Acids ionize only partially in water Ex: CH 3 COOH, HF, H 2 CO 3, H 3 PO 4 CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO 1 (aq) this reaction reaches equilibrium: forward rate = reverse rate K a = [H 3 O + ] [CH 3 COO ] = 1.8 x 10 5 [CH 3 COOH] K a is the acid ionization constant. The weaker the acid, the smaller the K a. 5

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7 Strong Bases ionize completely in water Examples: metallic hydroxides (group 1 & 2 metals with OH) NaOH(s) Na +1 (aq) + OH 1 (aq) Weak Bases ionize only partially in water CH 3 NH 2 (aq) + H 2 O (l) CH 3 NH 3 +1 (aq) + OH 1 (aq) K b = [CH 3 NH 3 +1 ] [OH 1 ] = 4.3 x 10 4 [CH 3 NH 2 ] 7

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9 Strong, Weak, Concentrated, Dilute What is the difference? Strong acid or base completely separated into ions Weak acid or base contain few ions Dilute or concentrated refers to the molarity of the solution 0.1 M HCl 6 M CH 3 COOH ph pouvoir hydrogen (hydrogen power) ph concept Soren Sorensen 1909 ph Scale l l stronger acids neutral stronger bases Pure H 2 O ionizes slightly: H 2 O(l) H + (aq) + OH 1 (aq) K w = ion product constant for water K w = [H + ] [OH ] For pure H 2 O at 298 K (25 C): [H + ] = [OH ] = 1.0 x 10 7 M so K w = (1.0 x 10 7 ) (1.0 x 10 7 ) and K w = 1.0 x

10 ph Scale [H + ] are very small numbers easier to use ph scale Def: ph = log [H + ] MEMORIZE THIS!!! Ex: [H + ] = 1.0 x 10 7 ph = [H + ] = 1.0 x 10 6 ph = [H + ] = 1.0 x 10 5 ph = As [H + ] increases, ph. As [H + ] decreases, ph. 10

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12 [H + ] [OH ] ph poh [H + ][OH ] = 1.0 x [H + ] [OH ] ph = log [H + ] [H + ] = antilog ( ph) [OH ] = antilog ( poh) poh = log [OH ] ph ph + poh = poh 12

13 poh poh = log [OH ] ph + poh = Ex: [H + ] of a soft drink is 7.3 x 10 4 M. What are the ph and poh? Ex: [OH ] of a solution is 5.6 x 10 8 M. What are the ph and poh? Calculating [H + ] and [OH ] from ph or poh Ex: An antacid solution has a ph of What are [H + ] and [OH ]? Since ph = log [H + ] ph = log [H + ] And antilog ( ph) = [H + ] So antilog ( poh) = [OH ] 13

14 ph of strong acids and strong bases Recall: strong acids and bases are essentially 100% ionized HCl (aq) H + (aq) + Cl (aq) 0.1 M so ph = NaOH (aq) Na + (aq) + OH (aq) 0.1 M so poh = Ca(OH) M 14

15 Neutralization Reactions Acid + Base Salt + H 2 O Ex: HNO 3 (aq) + Ca(OH) 2 (aq) Acid Base Titration a method for determining the concentration of a solution by reacting it with another solution of known concentration, called a standard solution. Ex: You have 25 ml of 1.0 M NaOH(aq). Find the concentration of 15 ml of an acetic acid solution. M a x V a = M b x V b Equivalence point moles of H + from acid = moles of OH from base. End point the point at which the indicator changes color during the titration 15

16 Titration Experiment 16

17 Salt Hydrolysis (salt reacting with H 2 O) Salt ionic compound obtained by a neutralization reaction in an aqueous solution. The resulting salt solution may be neutral, acidic or basic depending on the strength of the parent acid and parent base. Rules: 1. Strong Acid + Strong Base Neutral solution 2. Strong Acid + Weak Base Acidic solution 3. Weak Acid + Strong Base Basic Solution 4. Weak Acid + Weak Base depends on relative strengths of acid and base Salt Parent Acid Parent Base ph 17