1. CHE1102, Chapter 15 Learn, 1 Chapter 15 Acids and Bases, A Molecular Look

2. CHE1102, Chapter 15 Learn, 2 Brønsted–Lowry Acid/Base Definition Johannes Brønsted 1879 – 1947 Thomas Lowry 1874 – 1936

3. CHE1102, Chapter 15 Learn, 3 Some Definitions Brønsted–Lowry –Acid – chemical substance that acts a proton donor (species that produces H + ) –Base – chemical subtance that acts as a proton acceptor (species that takes in H +) HCl H + (aq) + Cl - (aq) H2OH2O HCl + H 2 O H 3 O + (aq) + Cl - (aq) H + = H 3 O + hydronium ion

4. CHE1102, Chapter 15 Learn, 4 A Brønsted–Lowry acid… …must have a removable (acidic) proton. A Brønsted–Lowry base… …must have a pair of nonbonding electrons.

5. CHE1102, Chapter 15 Learn, 5 If it can be either… …it is amphoteric. H 2 CO 3 HCO 3  CO 3 2- H 2 SO 4 HSO 4  SO 4 2  H 3 O + H 2 O OH -

6. CHE1102, Chapter 15 Learn, 6 What Happens When an Acid Dissolves in H 2 O? Water acts as a Brønsted–Lowry base and abstracts a proton (H + ) from the acid. As a result, the conjugate base of the acid and a hydronium ion are formed.

7. CHE1102, Chapter 15 Learn, 7 H + = H 3 O + proton = hydronium ion

8. CHE1102, Chapter 15 Learn, 8 Conjugates HNO 2 / NO 2 − are conjugate acid/base pair H 3 O + / H 2 O are conjugate acid/base pair – an acid/base pair that differs only by a single H +

9. CHE1102, Chapter 15 Learn, 9 What is the conjugate base of HCl ? HCl H + (aq) + Cl − (aq) H2OH2O HCl / Cl − are conjugates What is the conjugate acid of NH 3 ? NH 3 (aq) + H 2 O NH 4 + (aq) + OH − (aq) NH 3 / NH 4 + are conjugates (base/acid) (acid/base)

10. CHE1102, Chapter 15 Learn, 10 What is the conjugate acid of HS − ? HS − / H 2 S are conjugates (base/acid) (acid/base) What is the conjugate base of HS − ? HS − / S 2− are conjugates Is SO 4 2 − the conjugate base of H 2 SO 4 ? No way…… HSO 4 − is the conjugate base

11. CHE1102, Chapter 15 Learn, 11 Strong Electrolyte NaCl (aq) Na + (aq) + Cl - (aq) H2OH2O – substance that completely dissociates into ions in aqueous solution

12. CHE1102, Chapter 15 Learn, 12 Strong Acid – an acid that completely, 100% dissociates into ions in aqueous solution 6 common strong acids: HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 4 HCl (g) + H 2 O H 3 O + (aq) + Cl - (aq) Note the single arrow.

13. CHE1102, Chapter 15 Learn, 13 Strong Base – a base that completely, 100% dissociates into ions in aqueous solution 6 common strong bases: LiOH, NaOH, KOH, RbOH, CsOH, Ba(OH) 2 NaOH (s) Na + (aq) + OH - (aq) H2OH2O

14. CHE1102, Chapter 15 Learn, 14 Weak Acid – an acid that only slightly dissociates into ions in aqueous solution CH 3 COOH + H 2 O CH 3 COO - (aq) + H 3 O + (aq) Note the double arrow.

15. CHE1102, Chapter 15 Learn, 15 Weak Base – a base that only slightly dissociates into ions in aqueous solution – reacts with water to produce hydroxide ion

16. CHE1102, Chapter 15 Learn, 16 Acid and Base Strength

17. CHE1102, Chapter 15 Learn, 17 Acid and Base Strength -The more easily a substance gives up a proton, the less easily its conjugate base accepts a proton. -The more easily a base accepts a proton, the less easily its conjugate acid gives up a proton. The stronger an acid, the weaker its conjugate base, and the stronger a base, the weaker its conjugate acid.

18. CHE1102, Chapter 15 Learn, 18  The conjugate base of a strong acid exhibits NO basic properties whatsoever  The conjugate acid of a strong base exhibits NO acidic properties whatsoever  The conjugate base of a weak acid exhibits weak basic properties  The conjugate acid of a weak base exhibits weak acidic properties

19. CHE1102, Chapter 15 Learn, 19 Salt Solutions: Ions as Weak Acids and Bases Conjugate acids of weak molecular bases are weak acids and can affect pH of solution (make pH < 7.00). e.g. NH 4 +, HC 17 H 19 O 3 N +, etc. The cation of a strong base is too weak to influence the pH of solution e.g. NaOH ionizes 100% Cations as Acids

20. CHE1102, Chapter 15 Learn, 20 Anions as Bases Conjugate bases of weak acids are weak bases and can influence the pH of solution. It will tend to make the solution basic (pH > 7.00) The anion of a strong acid is too weak a base to influence the pH of solution e.g. HCl ionizes 100%, conjugate base extremely weak Little or no tendency to attract H + to it Salt Solutions: Ions as Weak Acids and Bases

21. CHE1102, Chapter 15 Learn, 21 Predicting Acid-Base Properties of Salt 1.If neither cation nor anion can affect pH, solution will be neutral. e.g. NaCl 2.If only cation is acidic, solution is acidic. e.g. NH 4 I 3.If only anion is basic, solution is basic. e.g. NaCHO 2 (sodium formate) 4.If both cation is acidic and anion is basic, pH depends on relative strengths of acid and base. e.g. NH 4 CHO 2 (ammonium formate)

22. CHE1102, Chapter 15 Learn, 22 Learning Check Predict whether a 0.10 M solution of NH 4 Br will be acidic, basic or neutral. NH 4 + is conjugate acid of weak base ammonia –It’s a weak acid –Will tend to make solution acidic Br – is conjugate base of strong acid HBr –Extremely weak base –Will not affect the pH Conclusion: Solution acidic –pH <7.00

23. CHE1102, Chapter 15 Learn, 23 Is a Soln of N 2 H 5 OCl Acidic, Basic or Neutral? –For N 2 H 5 + : –For OCl – : Comparing equilibrium constants we see that: The base OCl – is stronger than the acid N 2 H 5 + Solution will be basic

24. CHE1102, Chapter 15 Learn, 24

25. CHE1102, Chapter 15 Learn, 25 Binary Acids = H n X (two elements) X = Cl, Br, P, As, S, Se, etc. 1.Acid strength increases from left to right within same period (across row) –Acid strength increases as electronegativity of X increases e.g. HCl is stronger acid than H 2 S which is stronger acid than PH 3 – or PH 3 < H 2 S < HCl Trends in Binary Acid Strength

26. CHE1102, Chapter 15 Learn, 26 Trends in Binary Acid Strength Binary Acids = H n X X = Cl, Br, P, As, S, Se, etc. 2. Acid strength increase from top to bottom within group –Acid strength increases as size of X and bond length increases e.g. HCl is weaker acid than HBr which is weaker acid than HI –or HCl < HBr < HI

27. CHE1102, Chapter 15 Learn, 27 Synopsis: Factors Affecting Acid Strength The more polar the H–X bond and/or the weaker the H–X bond, the more acidic the compound. Acidity increases from left to right across a row and from top to bottom down a group.

28. CHE1102, Chapter 15 Learn, 28 Learning Check H 2 S or H 2 O CH 4 or NH 3 HF or HI H 2 S NH 3 HI Which is stronger?

29. CHE1102, Chapter 15 Learn, 29 Oxoacids Oxoacids (H n X O m ) –Acids of H, O, and one other element –HClO, HIO 4, H 2 SO 3, H 2 SO 4, etc.

30. CHE1102, Chapter 15 Learn, 30 In oxoacids, in which an –OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid. Factors Affecting Oxoacid Strength

31. CHE1102, Chapter 15 Learn, 31 For a series of oxyacids, acidity increases as the number of oxygens increases. Factors Affecting Oxoacid Strength

32. CHE1102, Chapter 15 Learn, 32 Learning Check Which is the stronger acid in each pair? H 2 SO 4 or H 3 PO 4 HNO 3 or H 3 PO 3 H 2 SO 4 or H 2 SO 3 HNO 3 or HNO 2 H 2 SO 4 HNO 3 H 2 SO 4 HNO 3

33. CHE1102, Chapter 15 Learn, 33 Learning Check Which corresponds to the correct order of acidity from weakest to strongest acid ? A. HBrO 3, HBrO, HBrO 2 B. HBrO, HBrO 2, HBrO 3 C. HBrO, HBrO 3, HBrO 2 D. HBrO 3, HBrO 2, HBrO

34. CHE1102, Chapter 15 Learn, 34 Learning Check Arrange the following in order of increasing acid strength: HBr, AsH 3, H 2 Se –AsH 3 < H 2 Se < HBr H 2 SeO 4, H 2 SO 4, H 2 TeO 4 –H 2 TeO 4 < H 2 SeO 4 < H 2 SO 4 HBrO 3, HBrO, HBrO 4, HBrO 2 –HBrO < HBrO 2 < HBrO 3 < HBrO 4

35. CHE1102, Chapter 15 Learn, 35 Lewis acids are defined as electron-pair acceptors. Atoms with an empty valence shells can be Lewis acids. Lewis Acids A Coordinate Covalent Bond 

36. CHE1102, Chapter 15 Learn, 36 Lewis bases are defined as electron-pair donors. Anything that could be a Brønsted–Lowry base is a Lewis base. Lewis bases can interact with things other than protons, however. Lewis Bases

37. CHE1102, Chapter 15 Learn, 37 Learning Check Identify the Lewis acid and base in the following: NH 3 + H + NH 4 + Base Acid F – + BF 3 BF 4 – Base Acid SeO 3 + O 2– SeO 4 2– Acid Base