Acids and Bases. Unit 10

Transcription

1 Acids and Bases Unit 10 1

2 Properties of Acids and Bases Acids Bases Taste Sour Turns Litmus Dye Red Reacts with Metals to give H 2 (g) Taste Bitter Turns Litmus Dye Blue Do Not React with Metals Reacts with CaCO 3 to give CO 2 (g) Do Not React with CaCO 2 Electrolyte Neutralized by Bases Electrolyte Neutralized by Acids 2

3 Arrhenius Definition of Acids and Bases Acids dissolve in water to produce H +1 ions. HCl (aq) H + (aq) + Cl (aq) Bases dissolve in water to produce OH -1 ions. NaOH (aq) Na + (aq) + OH (aq) 3

4 Arrhenius Definition of Acids and Bases Common Acids Nitric Acid (HNO 3 ) HNO 3 (aq) H + (aq) + NO 3 - (aq) Common Bases Potassium Hydroxide KOH KOH (s) K + (aq) + OH - (aq) Sulfuric Acid (H 2 SO 4 ) H 2 SO 4 (aq) 2H + (aq)+ SO 4 2- (aq) Magnesium Hydroxide Mg(OH) 2 Mg(OH) 2 (s) Mg 2+ (aq) + 2 OH (aq) Acetic Acid (HC 2 H 3 O 2 ) HC 2 H 3 O 2 (aq) H + (aq)+ C 2 H 3 O 2 - (aq) Calcium Hydroxide Ca(OH) 2 Ca(OH) 2 (s) Ca 2+ (aq) + 2 OH (aq) 4

5 Neutralization Reactions (Arrhenius Theory) Acids and base neutralize one another to form water and a salt. (Salt = ionic cmpd) e.g. HCl (aq) + NaOH (aq) HOH (l) + NaCl (aq) H 2 SO 4 (aq) + 2 KOH (aq) 2 HOH (l) + 2 KCl (aq) 2 HNO 3 (aq) + Mg(OH) 2 (aq) 2 HOH (l) + 2 Mg(NO 3 ) 2 (aq) 5

6 Naming Acids Acids are ionic compounds. To name acids look up the name of the anion in the acid. a) Anion ends in ide Acid Name = Hydro + root of anion + ic Acid b) Anion ends in ite Acid Name = Root of anion + ous Acid c) Anion ends in ate Acid Name = Root of anion + ic Acid 6

7 Naming Acids Practice HF.... HBr... HI..... H 2 SO 3.. H 2 SO 4.. HClO 3.. HClO 2.. H 2 CrO 4. H 2 C 2 O 4. HNO 2. HNO 3.. HCl... Anion = Fluoride Anion = Bromide Anion = Iodide Anion = Sulfite Anion = Sulfate Anion = Chlorate Anion = Chlorite Anion = Chromate Anion = Oxalate Anion = Nitrite Anion = Nitrate Anion = Chloride Hydrofluoric Acid Hydrobromic Acid Hydroiodic Acid Sulfurous Acid Sulfuric Acid Chloric Acid Chlorous Acid Chromic Acid Oxalic Acid Nitrous Acid Nitric Acid Hydrochloric Acid 7

8 Acid/Base Titrations An acid/base titration is when acid and base are reacted with one another until they reach the end point or equivalence point. The end (equivalence) point occurs when the # of moles of acid = the # of moles of base. The equivalence point is usually signaled when a dye, referred to as an indicator, changes color. 8

9 Acid/Base Titrations You will titrate HCl with NaOH. You will use a HCl solution of known concentration to find the concentration of a the base: HCl (aq) + NaOH (aq) H 2 O (l) + NaCl (aq) Known Volume and Known [HCl] Known Volume and Unknown [NaOH] 9

10 Acid/Base Titrations Use burets to deliver precise volumes of acid and base. Burets can measure to the nearest 0.01 ml. Images from : uretuse/buretuse.htm wiki/image:burette.svg 10

11 Acid/Base Titrations In our lab, the equivalence point occurs when the phenolphthalei n turns from clear to pink. Images from: ustl.edu/~courses/gen chem/labs/acidbase/ phph.htm Not enough base added. Too much base added. Just right. 11

12 K w - A special Equilibrium Constant Water molecules ionize to a very small degree: H 2 O (l) H + (aq) + OH - (aq) The equilibrium constant for the reaction is: K eq = [H + ][OH - ] This equilibrium constant is so common that it is referred to as the dissociation constant of water and is given the symbol, K w. 12

13 [H + ] and [OH - ] in Pure Water The concentration of H + and OH - in pure water are both 1 x 10-7 M. The K w in pure water is: K w = [H + ] x [OH - ] = [1 x 10-7 M] x [1 x 10-7 M] = 1 x In all aqueous solutions: [H + ][OH - ] = 1 x

14 Calculations of H + and OH - in Aqueous Solutions If you know the concentration of H + in solution, you can always find the concentration of OH -. e.g. Find the [OH - ] in a 0.25 M solution of HCl. Ans: 1 x10-14 = [0.25 M][OH - ] ; [OH - ] = 4.0x10-14 M In the same way, you can find [H + ] from [OH - ]. 14

15 ph Scale ph is a convenient way to express the amount of acid or base in a solution without having to use exponents. ph from [H + ] ph = - log [H + ] [H + ] from ph [H + ] = 10 -ph Examples: Find the ph of a M solution of HCl. Ans: ph = -log [0.010M] = 2.00 Find the [H + ] of a solution with ph = 3.5. Ans: = M 15

16 ph Scale Image Source is 16

17 [H + ], [OH - ], ph, and poh These four relationships will allow you to solve any problems involving [H + ], [OH - ], ph, and poh: 1. [H + ] x [OH - ] = 1 x ph + poh = ph = -log [H + ] 4. poh = -log [OH - ] 17

18 Brønsted Lowry Theory The Brønsted-Lowry Theory of acids and bases is broader than the Arrhenius theory. According to the B-L theory: Acids - donate H + ions Bases accept H + ions This theory broadens the definition of a base beyond substances that contain the OH ion. 18

19 B-L Theory of Acids and Bases Examples of B-L Acid/Base Reactions: HCl (aq) + OH- (aq) Cl (aq) + HOH (aq) Acid (H + donor) Base (H + Acceptor) NH 3 (aq) + HNO 3 (aq) NH 4 + (aq) + NO 3 - (aq) Base (H + Acceptor) Acid (H + donor) 19

20 Conjugate Acids/Bases According to B-L Theory, acids become bases after they have donated an H +. The difference between an acid and its conjugate base is simply an H +... Acid Conjugate Base HCl Cl - HSO - 4 SO 2-4 H 2 SO 4 HSO - 4 NH + 4 NH 3 20

21 Strong and Weak Acids Strong acids ionize 100% while weak acids ionize less than 10% in water. Strong Acid: HCl (aq) 100% H + (aq) + Cl (aq) Weak Acid: HF (aq) ~ 2.5% H + (aq) + Cl (aq) 21

22 Weak Acids: K A s The strength of a weak acids can be expressed by its equilibrium constant K A : HA H + + A - K A = [H ][A [HA] ] The larger the equilibrium constant, the stronger the acid. 22

23 K A s of Acids Acid Ionization Reaction K A Expression K A Value HCl HCl H + + Cl - [H ][Cl ] Very Large [HCl] HNO 3 HNO 3 H + + NO - 3 [H ][NO3 ] Very Large [HNO 3] H 2 SO 4 H 2 SO 4 2 H + + SO 2-4 [H ][HSO 4 ] Large [H SO ] HSO -1 4 HSO -1 4 H + + SO 2-4 [H ][SO ] 1.3 x 10-2 HF HF H + + F - [H ][F ] 6.7 x 10-4 [HF] HC 2 H 3 O 2 HC 2 H 3 O 2 H + + C 2 H 3 O - 2 [H ][C 1.8 x H3O2 ] [HC2H3O2] H 2 S H 2 S H + + HS - [H ][HS ] 1.0 x 10-7 [HS ] 2 [HSO ] 23

24 Strong Vs. Concentrated Weak, Dilute Weak, Concentrated Strong, Dilute Strong, Concentrated 24

25 Predicting Direction of Rxn To predict the direction of a B-L acid-base rxn, compare the K A s of the acids. The reaction will proceed away from the strongest acid: HSO HF H 2 SO 4 + F (base) (acid) (acid) (base) 6.7 x 10-4 Large 25

26 Predicting Rxn Direction Predict the direction of the following reactions: NO 2- + H 2 SO 3 HNO 2 + HSO x x 10-4 HC 2 H 3 O 2 + Cl - C 2 H 3 O HCl 1.8 x 10-5 Very Large 26

27 Acids and Bases The End