Chap 16 Chemical Equilibrium HSU FUYIN

Transcription

1 Chap 16 Chemical Equilibrium HSU FUYIN 1

2 Definitions: Arrhenius & Brønsted Lowry acid and base Arrhenius theory: An acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions. Hydrogen ions react with water molecules to produce complex ions, mainly hydronium ion, H 3 O + H + + H 2 O H 3 O + A base is a substance that, when dissolved in water, increases the concentration of hydroxide ions. Note: (acid or base?) 2

3 Definitions: Arrhenius & Brønsted Lowry acid and base Brønsted Lowry theory defines acids and bases in terms of proton (H + ) transfer. an acid is a proton donor a base is a proton acceptor. 3

4 Conjugate Acids and Bases The term conjugate means joined together as a pair. Reactions between acids and bases always yield their conjugate bases and acids. 4

5 Conjugate Acids and Bases H 2 S + NH 3 NH 4+ + HS acid base Conjugated acid Conjugated base conjugate pair: (H 2 S, HS ), (NH 3, NH 4+ ) OH + H 2 PO 4 H 2 O + HPO 4 2 base acid Conjugated acid Conjugated base conjugate pair: (OH, H 2 O), (H 2 PO 4, HPO 4 2 ) 5

6 Water Is Amphiprotic ( 兩性 ) 6

7 Exercise 16.1 Identifying Conjugate Acids and Bases (a) What is the conjugate base of HClO 4,H 2 S, PH 4+, HCO 3? Sol: If we remove a proton from HClO 4, we obtain ClO 4, which is its conjugate base. The other conjugate bases are HS, PH 3, and CO 2 3. (b) What is the conjugate acid of CN, SO 4 2, H 2 O, HCO 3? Sol: If we add a proton to CN, we get HCN, its conjugate acid. The other conjugate acids are HSO 4, H 3 O +, and H 2 CO 3. Notice : hydrogen carbonate ion (HCO 3 ) is amphiprotic. It can act as either an acid or a base. 7

8 Exercise: Identify the Bronsted Lowry acid, the Bronsted Lowry base, the conjugate acid, and the conjugate base. Sol: 8

9 Relative Strengths of Acids and Bases A stronger acid can donate H+ more readily than a weaker acid. The stronger an acid, the weaker is its conjugate base. The stronger a base, the weaker is its conjugate acid. An acid base reaction is favored in the direction from the stronger member to the weaker member of each conjugate acid base pair. 9

10 Exercise 16.3 Predicting the Position of a Proton-Transfer Equilibrium For the following proton-transfer reaction use Figure 16.4 to predict whether the equilibrium lies to the left (K c < 1) or to the right (K c > 1): HSO 4 (aq) + CO 2 3 (aq) SO 2 4 (aq) + HCO 3 (aq) Solution HSO 4 (aq) + CO 3 2 (aq) SO 4 2 (aq) + HCO 3 (aq) A B CB CA A CB CA B 10 The resulting equilibrium lies to the right, favoring products (that is, K c > 1)

11 Autoionization of Water Water is amphoteric. In pure water, a few molecules act as bases and a few act as acids. This is referred to as autoionization. 11

12 The Ion Product of Water The equilibrium constant for this process is called the ion product ( 離子積 ) of water and is represented by the symbol Kw The equilibrium expression for this process is K c = [H 3 O + ][OH ] All aqueous solutions contain both H 3 O + and OH. The concentration of H 3 O + and OH are equal in water. [H 3 O + ] = [OH ] = 10 7 M at 25 C 12

13 Aqueous Solutions Can Be Acidic, Basic, or Neutral If a solution is neutral, [H + ] = [OH ]. If a solution is acidic, [H + ] > [OH ]. If a solution is basic, [H + ] < [OH ]. 13

14 Exercise 16.5 Calculating [H + ]from [OH - ] Calculate the concentration of H + (aq) in (a) a solution in which [OH ] is M, (b) a solution in which [OH ] is M. Note: In this problem and all that follow, we assume, unless stated otherwise, that the temperature is 25. Solve (a) Using Equation 16.16] This solution is basic because (b) This solution is acidic because 14

15 The ph Scale ph is a method of reporting hydrogen ion concentration. ph = log[h+] Neutral ph is Acidic ph is below Basic ph is above Other p Scales The p in ph tells us to take the log of a quantity (in this case, hydrogen ions). Some other p systems are poh: log[oh - ] pkw: log Kw 15

16 Exercise 16.7 Calculating [H + ] from poh A sample of freshly pressed apple juice has a poh of Calculate [H + ]. Sol: 16

17 How Do We Measure ph? Indicators, including litmus paper, are used for less accurate measurements; an indicator is one color in its acid form and another color in its basic form. ph meters are used for accurate measurement of ph; electrodes indicate small changes in voltage to detect ph. 17

18 16.5 Strong Acids and Bases The seven strong acids are HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 3, and HClO 4. By definition: these are strong electrolytes and exist totally as ions in aqueous solution; e.g., HA + H 2 O H 3 O + + A Strong bases are the soluble hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca 2+, Sr 2+, and Ba 2+ ). These substances dissociate completely in aqueous solution MOH (aq) M + (aq) + OH (aq) or M(OH) 2 (aq) M 2+ (aq) +2 OH (aq) 18

19 Exercise 16.8 Calculating the ph of a Strong Acid What is the ph of a M solution of HClO 4? Sol: Because HClO 4 is a strong acid, it is completely ionized, giving [H + ] = [ClO 4- ] = M. ph = -log[0.040] =

20 Exercise 16.9 Calculating the ph of a Strong Base What is the ph of (a) a M solution of NaOH, (b) a M solution of Ca(OH 2 )? (a) NaOH (aq) Na + (aq) + OH (aq) (b) Ca(OH) 2 (aq) Ca 2+ (aq) +2 OH (aq) 20

21 16.6 Weak Acids For a weak acid, the equation for its dissociation is K a is called the acid-dissociation constant for acid HA. 21

22 Exercise Calculating K a from Measured ph A student prepared a 0.10 M solution of formic acid (HCOOH) and found its ph at 25 C to be Calculate K a for formic acid at this temperature. Sol: HCOOH(aq) H + (aq) + HCOO (aq) ph = log [H + ] = 2.38 [H + ] = = M 22

23 Percent Ionization Percent ionization defined as EX: 23

24 Exercise Using Ka to Calculate ph Calculate the ph of a 0.20 M solution of HCN. Sol: 24

25 Using the Quadratic Equation to Calculate ph and Percent Ionization Calculate the ph and percentage of HF molecules ionized in a 0.10 M HF solution. (HF K a = ) Sol: Step 1 : x<<0.10 可忽略 x= 8.2x10-3 M Percent Ionization = (8.2x10-3 )/0.1 = 8.2% > 5% 不可忽略 25

26 Polyprotic Acids Polyprotic acids have more than one acidic proton. It is always easier to remove the first proton than any successive proton. 26

27 Exercise Calculating the ph of a Solution of a Polyprotic Acid The solubility of CO 2 in water at 25 and 0.1 atm is M. The common practice is to assume that all the dissolved CO 2 is in the form of carbonic acid (H 2 CO 3 ), which is produced in the reaction CO 2 (aq) + H 2 O(l) H 2 CO 3 (aq) What is the ph of a M solution of H 2 CO 3? Sol: H 2 CO 3 is a diprotic acid; the two acid-dissociation constants, K a1 and K a2 (Table 16.3), differ by more than a factor of the ph can be determined by considering only K a1 27

28 Sol: 28

29 Q: What is the [CO 3 2- ] Sol: 29

30 Weak Bases Ammonia, NH 3, is a weak base. Like weak acids, weak bases have an equilibrium constant called the base dissociation constant. Equilibrium calculations work the same as for acids, using the base dissociation constant instead. 30

31 Exercise Using K b to Calculate OH Calculate the concentration of OH in a 0.15 M solution of NH 3. Sol: 31

32 Types of Weak Bases Two main categories Neutral substances with an Atom that has a nonbonding pair of electrons that can accept H + (like ammonia and the amines) Anions of weak acids 32

33 Exercise Using ph to Determine the Concentration of a Salt A solution made by adding solid sodium hypochlorite (NaClO) to enough water to make 2.00 L of solution has a ph of Using the information in Equation 16.37, calculate the number of moles of NaClO added to the water. Sol: poh = ph = = 3.50 [OH ] = = M 33

34 16.8 Relationship Between K a and K b For a conjugate acid base pair, K a and K b are related in this way: 34 K a K b = K w

35 Exercise Calculate (a) K b for the fluoride ion, (b) K a for the ammonium ion. Sol: (a) For the weak acid HF, K a = 6.8 * (b) For NH 3, K b = 1.8 *

36 16.9 Acid Base Properties of Salt Solutions Many ions react with water to create H + or OH. The reaction with water is often called hydrolysis. or Anions of strong acids are neutral EX: Cl will not react with water, so OH can t be formed. Anions of weak acids are conjugate bases, so they create OH in water Ex: C 2 H 3 O 2 + H 2 O HC 2 H 3 O 2 + OH Protonated anions from polyprotic acids can be acids or bases: 36

37 Exercise Predict whether the salt Na 2 HPO 4 forms an acidic solution or a basic solution when dissolved in water. Sol: K a = 4.2x10-13 H 2 PO 4 + H 2 O HPO 4- + H 3 O + K a = 6.2x10-8 K b > K a, the anion will be basic. 37

38 Cations Group I or Group II (Ca 2+, Sr 2+, or Ba 2+ ) metal cations are neutral. Polyatomic cations are typically the conjugate acids of a weak base. EX: NH 4 + Transition (group 3-12) and post-transition metal (group 13-15) cations are acidic. Cations, such as Al 3+, that have a charge of 3+ or higher are acidic. 38

39 16.10 Factors that Affect Acid Strength H A bond must be polarized with δ + on the H atom and δ on the A atom Bond strength: Weaker bonds can be broken more easily, making the acid stronger. Stability of A : More stable anion means stronger acid. Ex: H A bond strength: 299 kj/mol in HI, 366 kj/mol in HBr, 431 kj/mol in HCl, and 567 kj/mol in HF. Because HF has the highest bond strength among the hydrogen halides. Hence, the strength of acid: HI > HBr > HCl > HF 39

40 Binary Acids Binary acids consist of H and one other element. Within a group, H A bond strength is generally the most important factor. Within a period, bond polarity is the most important factor to determine acid strength. 40

41 Oxyacids Oxyacids consist of H, O, and one other element, which is a nonmetal. Oxoacids: H-O-Y Generally, as the electronegativity of the nonmetal increases, the acidity increases for acids with the same structure. 41

42 Relationship between Number of Oxygens on the Central Atom and Acidity The oxyacid with more O atoms is more acidic; 42

43 Carboxylic Acids Carboxylic acids are organic acids containing the COOH group. Factors contributing to their acidic behavior: Other O attached to C draws electron density from O H bond, increasing polarity. Its conjugate base (carboxylate anion) has resonance forms to stabilize the anion. 43

44 Lewis Acid/Base Chemistry Lewis acids are electron pair acceptors. Lewis bases are electron pair donors. The Lewis base must have a lone pair of electrons on it that it can donate. Anions are better Lewis bases than neutral atoms or molecules. Ex: N: < N: 44

45 Lewis Acids They are electron deficient, either from being attached to electronegative atom(s) or not having an octet. They must have an empty orbital willing to accept the electron pair. Many small, highly charged metal cations have empty orbitals they can use to accept electrons. 45