Acid-Base Chemistry. Introduction to Reaction Mechanism

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Dustin Chandler
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1 Acid-Base Chemistry Introduction to Reaction Mechanism

2 What is an acid and what is a base? Bronsted-Lowry definition Acids are proton donors (A-H) e.g. HCl, H 2 SO 4, HBr, HNO 3, HI etc. Bronsted-Lowry definition Bases are proton acceptors (B:) e.g. NaOH, Na 2 CO 3, NaNH 2, CH 3 Li etc.

3 Common acids that you have seen before: Notice that the H in each case is bonded to a more highly electronegative atom and H will therefore be d+ If the H is electron-poor (d+) then it will be attracted to electron-rich species (i.e. Bases, B:)

4 Common bases that you have seen before: Notice that each base contains at least one atom that has a lone pair and that the metal ions are balancing charges The electron-rich species (i.e. Bases, B: will be attracted to the electron-poor H (d+) of the acid

5 In a typical Acid-Base reaction we follow the electrons: A lone pair from the base is used to bond to the (d+) H of the acid The group attached to H must then break off so that H only has two electrons (i.e. the maximum it can handle)

6 In a general sense we can represent this as: The curved arrows are used to track the use of electrons in reactions

7 Put the two formats next to each other:

8 Which of the following are Bronsted-Lowry (i.e. protic) acids?

9 Which of the following are Bronsted-Lowry (i.e. protic) acids? Answer

10 Which of the following are Bronsted-Lowry bases (i.e. proton acceptors)?

11 Which of the following are Bronsted-Lowry bases (i.e. proton acceptors)? Answer

12 You need to be able to identify components quickly: Which is the acid, the base, the conjugate acid and the conjugate base?

13 You need to be able to identify components quickly Once these relationships are established we are then able to consider K a for the reaction

14 Using examples from General Chemistry: We know that NaOH is a strong base and that H 2 SO 4 is a strong acid so the reaction here gives a weak conjugate acid and a weak conjugate base so the products are more stable than the reactants

15 We can make some general statements: Strong acids and strong bases are reactive Weak acids and weak bases are relatively stable The more stable material is preferred

16 So the reaction to the right is favoured But by how much? Well, we have pk a values to help answer that; pk a is related to K a (pka = -log 10 K a )

17 Some general statements about pk a Weak acids always have high pk a values; Strong acids have low (or negative) pk a values; (Conjugate) base strength is inversely proportional to acid strength.

18 So in this reaction the right side is favoured The difference in pk a values is a guide to Ka; Here we have a difference of ~21 units; K a therefore will be ~10 21 so the right is completely favoured.

19 Match the following: 1

20 Match the following: 1 answer

21 Match the following: 2

22 Match the following: 2 answer

23 We have a very useful list of pk a values: Strong acids: have very weak conjugate bases:

24 We have a very useful list of pk a values Weak acids: have weak to strong conjugate bases:

25 We have a very useful list of pk a values. Very weak acids: have very strong conjugate bases:

26 Match the following: 3

27 Match the following: 3 answer

28 Match the following: 4

29 Match the following: 4 answers

30 So what influences the strengths of acids and bases? Electronegativity plays a role across the periodic table: The proton becomes more positive with an electronegative atom attached and is more attracted to the base. Also, the conjugate base is more stable when the charge is on the more electronegative atom.

31 What influences the strengths of acids and bases? Bond strength and ion size play roles down the periodic table: Since the H-X bond has to break in the acid-base reaction, the strength of that bond will determine the strength of the acid; the H-X bond strength gets weaker going down the table and so is easier to break. Also, ion size - the larger the anion, the more stable the conjugate base.

32 What influences the strengths of acids? Bond strength and ion size play roles down the periodic table: The carboxylic acid will give a conjugate base with charge spread over two oxygen atoms. The phenol will produce an anion in which some of the charge must spread onto the atoms of the ring. The anion from the alcohol will have a localized negative charge.

33 Influences on the strengths of acids.. Inductive effect plays a role: The difference in pk a values is attributable to the replacement of the CH 3 group with a highly electronegative CF 3 group which will pull a large amount of electron density away from the H of the acid, and also stabilize the negative charge in the conjugate base anion.

34 Choose the correct products from the following reaction:

35 Choose the correct products from the following reaction: answer

36 What is K for the following conversion?

37 What is K for the following conversion? Answer

38 What are the expected products here?

39 What are the expected products here? Answer..

40 What is K for the following reaction?

41 What is K for the following reaction? Answer..

42 Which products are formed in the following reaction?

43 Which products are formed in the following reaction? Answer

44 What is the size of K for the following reaction?

45 What is the size of K for the following reaction? Answer

46 So practice these reactions and learn how to use pk a values

Chapter 3 Acids and Bases"

Chapter 3 Acids and Bases BrØnsted-Lowry Acids and Bases A BrØnsted-Lowry acid is a proton donor. A BrØnsted-Lowry base is a proton acceptor. H + = proton Acids and Bases Reactions of BrØnsted-Lowry Acids