Chapter 9 Acid-base reactions

Transcription

1 CHEM, 2nd edition Cengage Learning Chapter 9 Acid-base reactions Acids and bases are chemical compounds that occur regularly in 'everyday life'. These two types of substances have opposite properties. They often occur in the foods we eat. Shawn McDonald Linn-Benton Community College

2 Types of Electrolytes Salts are water-soluble ionic compounds. All are strong electrolytes. Example: NaCl Acids form H +1 ions in water solution. Bases combine with H +1 ions in water solution. Bases increase the OH -1 concentration of the solution. May either directly release OH -1 or pull H +1 off H 2 O molecule. If the latter, this forms OH - ion and a different positively charged ion. 2

3 Properties of Acids Sour taste. Like biting into a lemon... React with active metals. I.e., Al, Zn, Fe, but not Cu, Ag or Au. 2 Al + 6 HCl AlCl H 2 Corrosive. To Al (mid-right) and skin! React with carbonates, producing CO 2. Marble, baking soda, chalk, limestone. CaCO HCl CaCl 2 + CO 2 (g) + H 2 O Change color of vegetable dyes. Blue litmus turns red. Picture at right. React with bases to form ionic salts and water. Called a neutralization. 3

4 Common Acids Chemical name Formula Uses Strength Nitric acid HNO 3 Explosive, fertilizer, dye, glue Strong Sulfuric acid H 2 SO 4 Hydrochloric acid HCl Phosphoric acid H 3 PO 4 Acetic acid HC 2 H 3 O 2 Explosive, fertilizer, dye, glue, batteries Metal cleaning, food prep, ore refining, stomach acid Fertilizer, plastics and rubber, food preservation Plastics and rubber, food preservation, vinegar Strong Strong Moderate Weak Hydrofluoric acid HF Metal cleaning, glass etching Weak Carbonic acid H 2 CO 3 Soda water Weak Boric acid H 3 BO 3 Eye wash Weak 4

5 Structures of Acids Binary acids have acid hydrogens attached to a nonmetal atom. 2 types of elements only. HCl, HF Write the H atom first, then the nonmetal atom. Dissociate in water to form H + ions and nonmetal anions (such as Cl - or F - ) Hydrofluoric acid 5

6 Structure of Acids Oxyacids have acid hydrogens attached to an oxygen atom. H 2 SO 4, HNO 3 Also write the H atom(s) first, then the polyatomic ion group. Will dissociate when put into water, to give H + ions and a polyatomic anion (like NO 3- or SO 4 2- ) 6

7 Structure of Acids Carboxylic acids have COOH group. HC 2 H 3 O 2, H 3 C 6 H 5 O 3 Only the first H in the formula is acidic. The H is on the COOH. Component of vinegar Lemons and limes Apples and wine 7

8 Properties of Bases Also known as alkalis. Taste bitter. Alkaloids = Plant product that is alkaline. Often poisonous. Potato and tomato shoots. Solutions feel slippery. Change color of vegetable dyes. Different color than acid. Red litmus turns blue. The dye in the paper reacts with OH - from base. React with acids to form ionic salts. Neutralization. Negates taste and metal dissolving power of acids. Occurs in hemlock bark, repels beetles. 8

9 Common Bases Chemical name Sodium hydroxide Potassium hydroxide Calcium hydroxide Sodium bicarbonate Magnesium hydroxide Ammonium hydroxide Formula NaOH KOH Common name Lye, caustic soda Caustic potash Uses Soap, plastic, petrol refining Soap, cotton, electroplating Strength Strong Strong Ca(OH) 2 Slaked lime Cement Strong NaHCO 3 Baking soda Cooking, antacid Weak Mg(OH) 2 NH 4 OH, {NH 3 (aq)} Milk of magnesia Ammonia water Antacid Detergent, fertilizer, explosives, fibers Weak Weak 9

10 Most ionic bases contain OH ions. NaOH, Ca(OH) 2 Some contain CO 3 2- ions. Structure of Bases CaCO 3 NaHCO 3 Molecular bases contain structures that react with H +. Mostly amine groups (N atoms). Caffeine has three amine type groups with CH 2 group attached. 10

11 9-1b What is an acid or a base? An acid base reaction is any reaction in which an H + is transferred. Does not have to take place in aqueous solution. Broader definition than Arrhenius. An acid is a H + donor; A base is a H + acceptor. Either can be a molecule or an ion. Since H + is a proton, acid is a proton donor and base is a proton acceptor. Base structure must contain an atom with an unshared pair of electrons to bond to H +. In the reaction, the acid molecule gives an H + to the base molecule. H A + :B :A + H B + 11

12 Amphoteric Substances Amphoteric substances can act as either an acid or a base. They have both a transferable H atom and an atom with a lone pair. HCl(aq) is acidic because HCl transfers an H + to H 2 O, forming H 3 O + ions. Water acts as base, accepting H +. HCl(aq) + H 2 O(l) Cl (aq) + H 3 O + (aq) NH 3 (aq) is basic because NH 3 accepts an H + from H 2 O, forming OH (aq). Water acts as acid, donating H +. NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH (aq) Thus water is amphoteric, it can act as a base with an acid, or as an acid with a base. Its nature is the opposite of the compound with which it is interacting. 12

13 An example acid-base reaction In the reaction H 2 O + NH 3 HO + NH +: 4 water ammonia hydroxide ion ammonium ion H 2 O and HO constitute an acid/conjugate base pair. If hydroxide ion accepts a proton it will revert to a water molecule. NH 3 and NH 4+ constitute a base/conjugate acid pair. If the ammonium ion donates a proton to a base, it will revert to the ammonia molecule. 13

14 Example Identify the Brønsted Lowry acids and bases and their conjugates in this reaction. C. HNO 3 (aq) + H 2 O(l) H 3 O + (aq) + NO 3- (aq) 14

15 Neutralization Reactions H + + OH - H 2 O net ionic eqn. Acid + base salt + water Double-displacement reactions. Salt = cation from base + anion from acid. Sometimes the salt in insoluble. Cation and anion charges stay constant. H 2 SO 4 + Ca(OH) 2 CaSO H 2 O Some neutralization reactions are gas evolving, where H 2 CO 3 (carbonic acid) decomposes into CO 2 and H 2 O. H 2 SO NaHCO 3 Na 2 SO H 2 O + 2 CO 2 sulfuric acid sodium bicarbonate sodium sulfate water carbon dioxide gas 15

16 Example Write the equation for the reaction of aqueous perchloric acid with strontium hydroxide. 1. Write the formulas of the reactants. HClO 4 (aq) + Sr(OH) 2 (aq) 2. Determine the ions present when each reactant dissociates. (H + + ClO 4 ) + (Sr 2+ + OH ) 3. Exchange the ions. H +1 combines with OH -1 to make H 2 O(l). Like other double displacements. (H + + ClO 4 ) + (Sr 2+ + OH ) (Sr 2+ + ClO 4 ) + H 2 O(l) 16

17 Write the equation for reaction of aqueous perchloric acid with strontium hydroxide, Continued. 4. Write the formulas of the products. Cross charges and reduce subscripts if possible. HClO 4 (aq) + Sr(OH) 2 (aq) Sr(ClO 4 ) 2 + H 2 O(l) 5. Balance the equation. Each atom and group on left vs. right side of the equation. May be quickly balanced by matching the numbers of H and OH to make H 2 O. Coefficient of the salt is always 1. 2 HClO 4 (aq) + Sr(OH) 2 (aq) Sr(ClO 4 ) H 2 O(l) 17

18 Write the Equation for reaction of aqueous perchloric acid with strontium hydroxide.continued 6. Determine the solubility of the salt. Sr(ClO 4 ) 2 is soluble (look up in solubility table). 7. Write an (s) after the insoluble products and an (aq) after the soluble products. 2 HClO 4 (aq) + Sr(OH) 2 (aq) Sr(ClO 4 ) 2 (aq) + 2 H 2 O(l) The reaction occurs since one of the products formed is water, and water molecules mainly stay as molecules and don't ionize very much. 18

19 9-2 Strong or Weak A strong acid is a strong electrolyte. Practically all the acid molecules ionize,. completely A strong base is a strong electrolyte. Practically all the base molecules form OH ions, either through dissociation or reaction with water,. completely A weak acid is a weak electrolyte. Only a small percentage of the molecules ionize,. A weak base is a weak electrolyte. Only a small percentage of the base molecules form OH ions, either through dissociation or reaction with water,. 19

20 Strong Acids The stronger the acid, the more willing it is to donate H. Use water as the standard base. Strong acids donate practically all their H s. 100% ionized in water. Strong electrolyte. [H 3 O + ] = [strong acid]. [ ] = molarity. HCl H + + Cl - HCl + H 2 O H 3 O + + Cl - No HCl is left after you dissolve the compound in water. 20

21 Strong Acids, Continued Hydrochloric acid Hydrobromic acid Hydroiodic acid HCl HBr HI Nitric acid HNO 3 Perchloric acid HClO 4 Sulfuric acid H 2 SO 4 21

22 Strong Acids, Continued Pure water Water is not an electrolyte and will not conduct current. HCl solution HCl is a strong electrolyte. Its solution will conduct current. 22

23 Weak Acids Weak acids donate a small fraction of their Hs. Most of the weak acid molecules do not donate H to water. Often less than 1% ionized in water. [H 3 O + ] << [weak acid]. HF H + + F - HF + H 2 O H 3 O + + F - Most HF stays in this molecular form, does not ionize in water to form H + or H 3 O + 23

24 Weak Acids, Continued Hydrofluoric acid HF Acetic acid HC 2 H 3 O 2 Formic acid HCHO 2 Sulfurous acid H 2 SO 3 Carbonic acid H 2 CO 3 Phosphoric acid H 3 PO 4 Stronger attraction between H and F than between H + and water 24

25 Weak Acids, Continued HF in water is a weak electrolyte and only conducts electricity poorly note dimness of light bulb. Pure water HF solution 25

26 Degree of Ionization The extent to which an acid ionizes in water depends in part on the strength of the bond between the acid H + and anion compared to the strength of the bond between the acid H + and the O of water. HA(aq) + H 2 O(l) A (aq) + H 3 O + (aq) In other words, which is stronger? the H-A bond or the H-O bond in the hydronium ion. If the former, then the acid will only ionize slightly. Fluorine for example forms a stronger bond with H than does Cl, thus HF is a weak acid and HCl is a strong acid. 26

27 Strong Bases The stronger the base, the more willing it is to accept H. Use water as the standard acid. Strong bases, practically all molecules are dissociated into OH or accept Hs. Strong electrolyte. Multi-OH bases completely dissociated. [HO ] = [strong base] x (# OH). Molarity will be discussed shortly... NaOH Na + + OH - All the dissolved NaOH has dissociated into sodium and hydroxide ions. 27

28 Strong Bases, Continued Lithium hydroxide Sodium hydroxide Potassium hydroxide LiOH NaOH KOH Calcium hydroxide Ca(OH) 2 Strontium hydroxide Sr(OH) 2 Barium hydroxide Ba(OH) 2 28

29 Weak Bases In weak bases, only a small fraction of molecules accept Hs. Weak electrolyte. Most of the weak base molecules do not take H from water. Much less than 1% ionization in water. [OH ] << [weak base]. NH 3 + H 2 O NH 4+ + OH - Most of the ammonium hydroxide molecules do not accept H from water molecules. 29

30 Weak Bases, Continued Ammonia Pyridine Methyl amine Ethyl amine Bicarbonate NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH (aq) C 5 H 5 N(aq) + H 2 O(l) C 5 H 5 NH + (aq) + OH (aq) CH 3 NH 2 (aq) + H 2 O(l) CH 3 NH 3+ (aq) + OH (aq) C 2 H 5 NH 2 (aq) + H 2 O(l) C 2 H 5 NH 3+ (aq) + OH (aq) HCO 3 (aq) + H 2 O(l) H 2 CO 3 (aq) + OH (aq) Most of the base molecules or species stay in the unprotonated form. 30

31 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 DI water. [H 3 O + ] = [OH ] = 1 x 10-7 M at 25 C in pure water. These are important concentrations to remember (related to ph that will we study shortly). 31

32 Ion Product of Water The product of the H 3 O + and OH concentrations is always the same number for solutions at 25 Celsius. The number is called the ion product of water and has the symbol K w. [H 3 O + ] x [OH ] = 1 x = K w. As [H 3 O + ] increases, the [OH ] must decrease so the product stays constant. Inversely proportional. 32

33 Acidic and Basic Solutions Neutral solutions have equal [H 3 O + ] and [OH ]. [H 3 O + ] = [OH ] = 1 x 10-7 Acidic solutions have a larger [H 3 O + ] than [OH ]. [H 3 O + ] > 1 x 10-7 ; [OH ] < 1 x 10-7 Basic solutions have a larger [OH ] than [H 3 O + ]. [H 3 O + ] < 1 x 10-7 ; [OH ] > 1 x 10-7 We can measure the concentration of hydronium ions in a solution by using a ph meter. These devices are used in the general chemistry courses to study acid-base behavior and reactions. ph paper also works to give us the rough ph of a given aqueous solution. 33

34 Example Determine the [H 3 O + ] for a M Ba(OH) 2 solution and Determine Whether the Solution Is Acidic, Basic, or Neutral. Ba(OH) 2 = Ba OH therefore: [OH ] = 2 x = = 4.0 x 10 4 M H 3 O K w H O OH K OH 3 w [H 3 O + ] = 2.5 x M. Since [H 3 O + ] < 1 x 10 7, the solution is basic. 34

35 9-3a The ph scale The acidity/basicity of a solution is often expressed as ph. ph = log[h 3 O + ], [H 3 O + ] = 10 ph The exponent on 10, but with a positive sign. ph water = log[10-7 ] = 7. Need to know the [H 3 O + ] concentration to find ph. 3 cases: case 1: ph < 7 is acidic; case 2: ph > 7 is basic; case 3: ph = 7 is neutral. 35

36 ph, Continued The lower the ph, the more acidic the solution; the higher the ph, the more basic the solution. 1 ph unit corresponds to a factor of 10 fold difference in acidity of that solution. Normal range is 0 to 14. ph 0 is [H + ] = 1 M, ph 14 is [OH ] = 1 M. ph can be negative (very acidic) or larger than 14 (very alkaline, at high concentration). 36

37 ph of Common Substances Substance ph 1.0 M HCl M HCl 1.0 Stomach acid 1.0 to 3.0 Lemons 2.2 to 2.4 Soft drinks 2.0 to 4.0 Plums 2.8 to 3.0 Apples 2.9 to 3.3 Cherries 3.2 to 4.0 Unpolluted rainwater 5.6 Human blood 7.3 to 7.4 Egg whites 7.6 to 8.0 Milk of magnesia (saturated Mg(OH) 2 ) 10.5 Household ammonia 10.5 to M NaOH 14 37

38 Example Calculate the ph of a M Ba(OH) 2 Solution and Determine if It Is Acidic, Basic, or Neutral. Ba(OH) 2 = Ba OH therefore, [OH - ] = 2 x = = 2.0 x 10-3 M. [H 3 O + ] = 1 x x 10-3 = 5.0 x M ph = log [H 3 O + ] = log (5.0 x ) ph = 11.3 ph > 7 therefore, basic. 38

39 Practice Calculate the ph of the Following Strong Acid or Base M HCl Solutions M Ca(OH) M HNO 3 39

40 Practice Calculate the ph of the Following Strong Acid or Base Solutions, Continued M HCl strong acid therefore, [H 3 O + ] = M. ph = log (2.0 x 10-3 ) = 2.70 acidic M Ca(OH) 2 strong base, [OH ] = M. [H 3 O + ] = 1 x = 1.0 x x 10 2 ph = log (1.0 x ) = basic 0.25 M HNO 3 a strong acid, therefore, [H 3 O + ] = 0.25 M. ph = log (2.5 x 10 1 ) = 0.60 acidic 40

41 9-4 Acid-base buffers Buffers are solutions that resist changing ph when small amounts of acid or base are added. They resist changing ph by neutralizing added acid or base. Buffers are made by mixing together a weak acid and its conjugate base. Or weak base and its conjugate acid. For example, a mixture of ammonia and ammonium chloride is a buffer. 41

42 How Buffers Work The weak acid present in the buffer mixture can neutralize added base. The conjugate base present in the buffer mixture can neutralize added acid. The net result is little to no change in the solution ph. As long as you don t over come the capacity of the buffer. Then the ph will change drastically. Can change by several ph units after capacity is overcome. 42

43 Acetic Acid/Acetate Buffer The conjugate base neutralizes any added acid. makes weak acid (acetic acid) The weak acid neutralizes the added base, forms H 2 O. 43