CHEMISTRY. Chapter 16 Acid-Base Equilibria

Transcription

1 CHEMISTRY The Central Science 8 th Edition Chapter 16 Acid-Base Equilibria Kozet YAPSAKLI

2 Why study acids bases? bases are common in the everyday world as well as in the lab. Some common acidic products vinegar (dilute acetic acid), soft drinks (carbonic acid), aspirin (acetylsalicylic acid), vitamin C (ascorbic acid) Some common basic products Antacids such as milk of magnesia (magnesium hydroxide) calcium carbonate, household ammonia, oven drain cleaners (sodium hydroxide), some bleaches (sodium hydroxide)

3 Problems Some basic substances do not have OH - Confined to H 2 O solutions

4 Some Definitions Brønsted Lowry Acid: Base: Proton donor Proton acceptor Bronsted-Lowry Concept (1923) Acid-base reaction involves proton transfer HA + B HB + + A - Base: proton acceptor does not have to have OH - in formula

5 HCl + H 2 O H 3 O + + Cl - acid base NH 3 + H 2 O NH OH - base acid & bases can be molecules or ions Back reaction is also a proton transfer.

6 NH 3 + H 2 O NH OH - base acid acid base Acid-base conjugate pairs Conjugate Acid-Base Pair (HA/A -, HB + /B) Two chemical species whose formulas differ by only one proton acid conj. base + H + base + H + conj. acid

7 Acid Base Strength Strong acids are completely dissociated in water. Their conjugate bases are quite weak. Weak acids only dissociate partially in water. Their conjugate bases are weak bases. Substances with negligible acidity do not dissociate in water. Their conjugate bases are exceedingly strong.

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9 ph ph is defined as the negative base-10 logarithm of the hydronium ion concentration. In neutral water, ph = log [H 3 O + ] K w = [H 3 O + ] [OH ] = [H 3 O + ] = ( ) 1/2 =

10 ph Therefore, in neutral water, ph log[h3o ] log[h ] poh log[oh ph = log ( ) = 7.00 An acid has a higher [H 3 O + ] than neutral water, so its ph is <7 A base has a lower [H 3 O + ] than neutral water, so its ph is >7. - ]

11 These are the ph values for several common substances.

12 Other p Scales The p in ph tells us to take the negative log of the quantity (in this case, hydrogen ions). Some similar examples are poh log [OH ] pk w log K w ph + poh = pk w = 14.00

13 How Do We Measure ph? For less accurate measurements, one can use Litmus paper Red paper turns blue above ~ph = 8 Blue paper turns red below ~ph = 5 An indicator For more accurate ph meter

14 Strong Seven strong acids are HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 3, HClO 4. These are, by definition, strong electrolytes exist totally as ions in aqueous solution. HNO 3 (aq) + H 2 O(l) H 3 O + (aq) + NO 3- (aq) For the monoprotic strong acids, [H 3 O + ] = [acid].

15 Strong Most ionic hydroxides are strong bases (e.g. NaOH, KOH, Ca(OH) 2 ). Strong bases are the soluble hydroxides, which are the alkali metal heavier alkaline earth metal hydroxides (Ca 2+, Sr 2+, Ba 2+ ). Again, these substances dissociate completely in aqueous solution.

16 Weak Weak acids are only partially ionized in solution. There is a mixture of ions unionized acid in solution. Therefore, weak acids are in equilibrium: HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) K a [H O ][A - 3 ] [HA] This equilibrium constant is called the aciddissociation constant, K a.

17 Dissociation Constants The greater the value of K a, the stronger the acid. If Ka >> 1, then the acid is completely ionized the acid is a strong acid.

18 Polyprotic Have more than one acidic proton. If the difference between the K a for the first dissociation subsequent K a values is 10 3 or more, the ph generally depends only on the first dissociation.

19 Polyprotic The protons are removed in steps not all at once: H 2 SO 3 (aq) H + (aq) + HSO - 3 (aq) K a1 = 1.7 x 10-2 HSO - 3 (aq) H + (aq) + SO 2-3 (aq) K a2 = 6.4 x 10-8 It is always easier to remove the first proton in a polyprotic acid than the second. Therefore, K a1 > K a2 > K a3 etc.

20 Weak react with water to produce hydroxide ion. The base dissociation constant, Kb, is defined as [NH ][OH - 4 ] K b [NH3]

21 K a K b K a K b are related in this way: K a K b = K w Therefore, if you know one of them, you can calculate the other. Taking negative logarithms: pkw pka pkb

22 Read: Reactions of Anions Cations with Water Read: Effect of Cations Anions Not responsible for the Part Acid-Base Behavior Chemical Structure Lewis