Chapter 15. Properties of Acids. Structure of Acids 7/3/08. Acid and Bases

Transcription

1 Chapter 15 Acid and Bases Properties of Acids! Sour taste! React with active metals! React with carbonates, producing CO 2! Change color of vegetable dyes!blue litmus turns red! React with bases to form ionic salts Structure of Acids! Binary acids have acid hydrogens attached to a nonmetal atom!hcl, HF 1

2 Structure of Acids! Oxy acids have acid hydrogens attached to an oxygen atom!h 2 SO 4, HNO 3 Properties of Bases! AKA alkalis! Taste bitter! Solutions feel slippery! Change the color of vegetable dye! red litmus turns blue! React with acids to form ionic salts Structure of Bases! Most ionic bases contain OH ions! Some contain CO 3 2- ions! Molecular bases contain structures that react with H +!mostly amine groups (some form NH 3 ) 2

3 Indicators! Chemicals which change color depending on the acidity/basicity! Many vegetable dyes are indicators! Litmus! Phenolphthalein Acid/Base Theory Theory Acid Base Arrhenius Increase H + Increases OH - Bronsted/Lowry H + donor H + acceptor Lewis Electron Pair Acceptor Electron Pair Donor Bronsted-Lowry Theory! In a Brønsted-Lowry Acid-Base reaction, an H + is transferred! does not have to take place in aqueous solution! broader definition than Arrhenius! Acid is H donor, base is H acceptor! base structure must contain an atom with an unshared pair of electrons! In an acid-base reaction, the acid molecule gives an H + to the base molecule H A + :B! :A + H B + 3

4 s Amphoteric Substances! Amphoteric substances can act as either an acid or a base! have both transferable H and atom with lone pair! water acts as base, accepting H + from HCl HCl(aq) + H 2 O(l)! Cl (aq) + H 3 O + (aq)! water acts as acid, donating H + to NH 3 NH 3 (aq) + H 2 O(l)! NH 4+ (aq) + OH (aq) Acid/Base As Reversible! One of the advantages of Brønsted-Lowry theory is that it allows reactions to be reversible H A + :B! :A + H B +! The original base has an extra H + after the reaction so it will act as an acid in the reverse process! The original acid has a lone pair of electrons after the reaction so it will act as a base in the reverse process :A + H B +! H A + :B 4

5 Conjugate Pairs! In a Brønsted-Lowry Acid-Base reaction, the original base becomes an acid in the reverse reaction, and the original acid becomes a base in the reverse process! Each reactant and the product it becomes is called a conjugate pair! The original base becomes the conjugate acid; and the original acid becomes the conjugate base Brønsted-Lowry Acid-Base Reactions H A + :B! :A + H B + acid base conjugate conjugate base acid HCHO 2 + H 2 O! CHO 2 + H 3 O + acid base conjugate conjugate base acid H 2 O + NH 3! HO + NH + 4 acid base conjugate conjugate base acid Write the formula for the conjugate acid of the following H 2 O NH 3 HCO 3 2! H 2 PO 4 1! 5

6 Write the formula for the conjugate base of the following H 2 O NH 3 HCO 3 2! H 2 PO 4 1! Strong versus Weak! A strong acid is a strong electrolyte! A strong base is a strong electrolyte! A weak acid is a weak electrolyte! A weak base is a weak electrolyte! Strong completely reacts with water! Weak reacts very little with water Polyprotic Acids! Often acid molecules have more than one ionizable H these are called polyprotic acids! the ionizable H s may have different acid strengths or be equal! Polyprotic acids ionize in steps! each ionizable H removed sequentially! Removing of the first H automatically makes removal of the second H harder! H 2 SO 4 is a stronger acid than HSO 4-6

7 General Trends in Acidity! The stronger an acid is at donating H, the weaker the conjugate base is at accepting H! Higher oxidation number = stronger oxyacid! H 2 SO 4 > H 2 SO 3 ; HNO 3 > HNO 2! Cation stronger acid than neutral molecule; neutral stronger acid than anion! H 3 O +1 > H 2 O > OH -1 ; NH 4 +1 > NH 3 > NH 2-1! base trend opposite Acid Ionization Constant, K a! Acid strength measured by the size of the equilibrium constant when react with H 2 O HA + H 2 O! A -1 + H 3 O +1! The equilibrium constant is called the acid ionization constant, K a!larger K a = stronger acid 7

8 Autoionization of Water! Water is actually an extremely weak electrolyte! therefore there must be a few ions present! About 1 out of every 10 million water molecules form ions through a process called autoionization H 2 O + H 2 O! 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 pure water! [H 3 O + ] = [OH ] = C Ion Product of Water! The product of the H 3 O + and OH concentrations is always the same number! The number is called the ion product of water and has the symbol K w! [H 3 O + ] x [OH ] = K w = 1 x 25 C! As [H 3 O + ] increases the [OH ] must decrease so the product stays constant! Calculate the [OH - ] at 25 C when the [H 3 O + ] = 1.5 x 10-9 M, and determine if the solution is acidic, basic, or neutral 8

9 ph! The acidity/basicity of a solution is often expressed as ph! ph = -log[h 3 O + ], [H 3 O + ] = 10 -ph! exponent on 10 with a positive sign! ph water = -log[10-7 ] = 7! need to know the [H + ] concentration to find ph! ph < 7 is acidic; ph > 7 is basic, ph = 7 is neutral Sig. Figs. & Logs! When you take the log of a number written in scientific notation, the digit(s) before the decimal point come from the exponent on 10, and the digits after the decimal point come from the decimal part of the number log(2.0 x 10 6 ) = log(10 6 ) + log(2.0) = = ! Since the part of the scientific notation number that determines the significant figures is the decimal part, the sig figs are the digits after the decimal point in the log log(2.0 x 10 6 ) = 6.30! Calculate the ph at 25 C when the [OH - ] = 1.3 x 10-2 M, and determine if the solution is acidic, basic, or neutral 9

10 poh! Another way of expressing the acidity/basicity of a solution is poh! poh = -log[oh - ], [OH - ] = 10 -poh! poh water = -log[10-7 ] = 7! need to know the [OH - ] concentration to find poh! poh < 7 is basic; poh > 7 is acidic, poh = 7 is neutral pk! A way of expressing the strength of an acid or base is pk! pk a = -log(k a ), K a = 10 -pka! pk b = -log(k b ), K b = 10 -pkb! The stronger the acid, the smaller the pk a! larger K a = smaller pk a! because it is the log Finding the ph or poh of a Strong Acid or Base! Since strong acids and bases react completely wit water then:!the molar of the acid = [H 3 O + ]!The molar of the base = [OH - ] 10

11 Finding the ph or poh of a Weak Acid or Base! Weak acids and bases are equilibrium reaction.! The [H 3 O + ] concentration can be determine from the Ka of the acid.! The [OH - ] concentration can be determine from the Kb of the base.! What is the ph of a M solution of nicotinic acid, HC 6 H 4 NO 2? (K a = 1.4 x 25 C) Finding the ph of Mixtures of Acids! Generally, you can ignore the contribution of the weaker acid to the [H 3 O + ] equil! For a mixture of a strong acid with a weak acid, the complete ionization of the strong acid provides more than enough [H 3 O + ] to shift the weak acid equilibrium to the left so far that the weak acid s added [H 3 O + ] is negligible! For mixtures of weak acids, generally only need to consider the stronger for the same reasons 11

12 ! Find the ph of a mixture of M HF(aq) solution and M HClO 2 (aq)! Calculate the ph at 25 C of a M Sr(OH) 2 solution and determine if the solution is acidic, basic, or neutral! Find the ph of M NH 3 (aq) solution 12

13 Acid-Base Properties of Salts! Salts are water soluble ionic compounds! Salts that contain the cation of a strong base and an anion that is the conjugate base of a weak acid are basic! Salts that contain cations that are the conjugate acid of a weak base and an anion of a strong acid are acidic Relationship between K a of an Acid and K b of Its Conjugate Base! Many reference books only give tables of K a values because K b values can be found from them! Find the ph of M NaCHO 2 (aq) solution 13

14 Classifying Salt Solutions as Acidic, Basic, or Neutral! If the salt cation is the counterion of a strong base and the anion is the conjugate base of a strong acid, it will form a neutral solution! NaCl Ca(NO 3 ) 2 KBr! If the salt cation is the counterion of a strong base and the anion is the conjugate base of a weak acid, it will form a basic solution! NaF Ca(C 2 H 3 O 2 ) 2 KNO 2 Classifying Salt Solutions as Acidic, Basic, or Neutral! If the salt cation is the conjugate acid of a weak base and the anion is the conjugate base of a strong acid, it will form an acidic solution! NH 4 Cl! If the salt cation is a highly charged metal ion and the anion is the conjugate base of a strong acid, it will form an acidic solution! Al(NO 3 ) 3 Classifying Salt Solutions as Acidic, Basic, or Neutral! If the salt cation is the conjugate acid of a weak base and the anion is the conjugate base of a weak acid, the ph of the solution depends on the relative strengths of the acid and base!nh 4 F since HF is a stronger acid than NH 4+, K a of NH 4 + is larger than K b of the F " ; therefore the solution will be acidic 14

15 Polyprotic acids! Since polyprotic acids ionize in steps, each H has a separate K a K a1 > K a2 > K a3! Generally, the difference in K a values is great enough so that the second ionization does not happen to a large enough extent to affect the ph! [A 2- ] = K a2 as long as the second ionization is negligible! Find the ph of M H 2 SO 4 (aq) 25 C (first H+ is strong) 15

16 Lewis Acid - Base Theory! Electron sharing! Electron donor = Lewis Base = nucleophile! must have a lone pair of electrons! Electron acceptor = Lewis Acid = electrophile! electron deficient! When Lewis Base gives electrons from lone pair to Lewis Acid, a covalent bond forms between the molecules Nucleophile: + Electrophile! Nucleophile:Electrophile! product called an adduct! other acid-base reactions also Lewis! Complete the Following Lewis Acid-Base Reactions Label the Nucleophile and Electrophile! BF 3 + HF!! CaO + SO 3!! KI + I 2! 16