Acid-Base Equilibria and Solubility Equilibria

Transcription

1 Acid-Base Equilibria and Solubility Equilibria

2 Acid-Base Equilibria and Solubility Equilibria Homogeneous versus Heterogeneous Solution Equilibria (17.1) Buffer Solutions (17.2) A Closer Look at Acid-Base Titrations (17.3) Acid-Base Indicators (17.4) Solubility Equilibria (17.5) The Common Ion Effect and Solubility (17.6) Complex Ion Equilibria and Solubility (17.7) Application of the Solubility Product Principle to Qualitative Analysis (17.8)

3 General Chemistry I Concepts Representations of matter ( ) Formula calculations and stoichiometry ( ) Precipitation reactions, strong and weak acids and bases, molar concentrations of solutions and solution stoichiometry (Chapter 4)

4 17.1 Homogeneous versus Heterogeneous Solution Equilibria Review: What is homogeneous equilibria? Using a balanced equation, explain this. Review: What is heterogeneous equilibria? Using a balanced equation, explain this. How can you tell the difference between these?

5 17.2 Buffer Solutions What is a buffer? A solution of (1) a weak acid or a weak base and (2) its salt; both components must be present. The solution has the ability to resist change in ph upon the addition of small amounts of either acid or base. What is a weak acid and its salt? What is a weak base and its salt? Using HF, what species do you have in solution? Adding NaF (its salt or conjugate base), what species do you also have in solution? What species are present in this buffer?

6 17.2 Buffer Solutions How does a buffer work? Let s review: What is the reaction of HF(aq) with water? What is the reaction of NH 3 (aq) with water? What is the reaction of the weak acid (HF) with a base (NaOH)? What is the reaction of the weak base (NH 3 ) with an acid (HCl)?

7 17.2 Buffer Solutions How does a buffer work? Considering our HF/NaF buffer: What will react with added strong acid? What will react with added strong base? How is this different from adding strong acid/base to water? What is detected in the ph reading and what is the source? Figure 17.2 p. 598

8 17.2 Buffer Solutions What is the equilibrium expression for the weak acid, HF? What is pk a? What is the process to determine pk a? How does this help us judge acid strength? What is pk b? What is the process to determine pk a? How does this help us judge base strength? How are pk a and pk b related? What is the expression to calculate the ph of a buffer?

9 17.2 Buffer Solutions What is the expression to calculate the ph of a buffer? ph pk log Practice: a conjugate base What is the ph of a buffer made by combining 25.0 ml of M HF with 28.5 ml of M NaF? The K a of HF is 7.1x10-4. acid

10 17.2 Buffer Solutions How good is a buffer at resisting the addition of added acid or base? What happens when strong acid is added to water? What happens when strong acid is added to the HF/NaF buffer? Which solution resists ph change better? What happens when strong base is added to water? What happens when strong base is added to the HF/NaF buffer? Which solution resists ph change better?

11 17.2 Buffer Solutions Check: What is the ph if 5.0 ml of M HCl was added: to 53.5 ml of water to the HF/NaF buffer What is the ph if 5.0 ml of M NaOH was added: to 53.5 ml of water to the HF/NaF buffer

12 17.2 Buffer Solutions What is the equilibrium expression for the weak base, NH 3? What is the expression to calculate the poh of a buffer? poh pk log b conjugate acid base

13 17.2 Buffer Solutions What is buffer capacity? effectiveness of the buffer solution or the ability to resist ph change Using the HF/NaF buffer, what is the maximum number of moles of HCl (or any strong acid) that can be added to change the ph by 1 ph unit? If this is 1.50 M HCl, what is the maximum volume of HCl that can be added to this buffer before its capacity is reached?

14 17.2 Buffer Solutions Practice for Lab: Using 1.0 M NaH 2 PO 4 and 1.0 M Na 2 HPO 4, how would you make ml of a ph = 6.5 buffer solution?

15 17.3 A Closer Look at Acid-Base Titrations Review: What is a titration? Review: What is the balanced net ionic equation for the reaction of: A strong acid and a strong base? A weak acid and a strong base? A strong acid and a weak base? Review: What is the equivalence point? How do we use this in titration calculations?

16 17.3 A Closer Look at Acid-Base Titrations What is the equivalence point? How do we use this in titration calculations? Review Practice: A solution of barium hydroxide was standardized and found to have a concentration of M. If 28.5 ml of this solution is titrated to the equivalence point with 25.0 ml of nitric acid, what is the molar concentration of the acid?

17 17.3 A Closer Look at Acid-Base Titrations What is the titrant? What happens at these key points in a titration: Initially, before any titrant has been added? After only a small amount of the titrant has been added? At the equivalence point? After the equivalence point after an excess of the titrant has been added?

18 17.3 A Closer Look at Acid-Base Titrations Our process with titrations will be: 1. We will first consider the neutralization reaction (the reaction of the acid and the base). We will pay close attention to: - If we have an excess of strong acid or base (as this will dominate the ph - The formation of a conjugate base or acid - The total volume of the solution 2. We will then consider any equilibrium occurring for any conjugate weak acid or base.

19 17.3 A Closer Look at Acid-Base Titrations Our calculations with titrations will be: What is the ph at these 4 points: 1. Initially, before any titrant has been added? 2. After only a small amount of the titrant has been added? 3. At the equivalence point? 4. After the equivalence point after an excess of the titrant has been added? What does this look like, plotting ph versus the volume of the titrant added?

20 17.3 A Closer Look at Acid-Base Titrations We will work through three types of titrations: A strong acid and a strong base A weak acid and a strong base A strong acid and a weak base

21 17.3 A Closer Look at Acid-Base Titrations A strong acid and a strong base What is the balanced net ionic equation? ml of M nitric acid is titrated with a M standardized solution of sodium hydroxide. What is the ph/volume of NaOH at these points? 1. Initially, before any NaOH has been added? 2. After 13.2 ml of NaOH has been added? 3. What is the volume of NaOH added at the equivalence and what is the ph at this point? 4. After 35.8 ml of NaOH has been added?

22 17.3 A Closer Look at Acid-Base Titrations Figure 17.4 p. 601

23 Titration 17.3 of a A strong Closer acid (25.00 Look ml at of Acid-Base M HNO 3 ) with a strong base ( M NaOH) Titrations ph = For our titration: ph ph = ph = 1.42 ph = 7.00 This is the equivalence point. At this point, ml of the base has been added Volume (ml) of NaOH added

24 17.3 A Closer Look at Acid-Base Titrations A weak acid and a strong base What is the balanced net ionic equation? ml of M formic acid is titrated with a M standardized solution of sodium hydroxide. What is the ph/volume of NaOH at these points? 1. Initially, before any NaOH has been added? 2. After 13.2 ml of NaOH has been added? 3. What is the volume of NaOH added at the equivalence and what is the ph at this point? 4. After 35.8 ml of NaOH has been added?

25 17.3 A Closer Look at Acid-Base Titrations Figure 17.5 p. 603

26 Titration of a weak acid (25.00 ml of M HCOOH) with a strong base ( M NaOH) ph = ph ph = 2.37 ph = 3.67 ph = 8.22 This is the equivalence point. This is the buffer region (where ph pk a ) At this point, ml of the base has been added Volume (ml) of NaOH added

27 17.3 A Closer Look at Acid-Base Titrations A strong acid and a weak base What is the balanced net ionic equation? ml of M methyl amine is titrated with a M standardized solution of nitric acid. What is the ph/volume of HNO 3 at these points? 1. Initially, before any HNO 3 has been added? 2. After 13.2 ml of HNO 3 has been added? 3. What is the volume of HNO 3 added at the equivalence and what is the ph at this point? 4. After 35.8 ml of HNO 3 has been added?

28 17.3 A Closer Look at Acid-Base Titrations Figure 17.6 p. 606

29 Titration of a weak base (25.00 ml of M CH 3 NH 2 ) with a strong acid ( M HNO 3 ) ph ph = ph = This is the buffer region (where ph pk a ) ph = 5.98 This is the equivalence point. At this point, ml of the acid has been added ph = 2.08 Volume (ml) of HNO 3 added

30 17.4 Acid-Base Indicators How can we tell when we reach an equivalence point? Using a ph probe Using an indicator What is an indicator? What is the structure of an indicator? How does it work? Over what ph range will the indicator work?

31 17.4 Acid-Base Indicators O Ionizable H H Ionization only occurs after the acid in the solution has been ionized (immediately after the equivalence point) O H + (neutralized by OH - ) SO 3 SO 3 red O yellow O

32 17.4 Acid-Base Indicators Table 17.1 p. 608

33 17.5 Solubility Equilibria What is a soluble salt? Describe NaCl using the balanced equation. What is an insoluble salt? Describe AgCl using the balanced equation. What does insoluble mean? Are all salts equally soluble? Are all salts equally insoluble?

34 17.5 Solubility Equilibria What is an insoluble salt? Are all salts equally insoluble? What is the equilibrium expression for an insoluble salt? Can we always compare the value of the equilibrium constant (the solubility product constant, K sp ) for any two salts to be able to gauge solubility?

35 17.5 Solubility Equilibria Table 17.2 p. 610

36 17.5 Solubility Equilibria Which solution will have a higher concentration of the silver ion: AgI K sp = 8.3x10-17 AgCl K sp = 1.6x10-10 Which solution will have a higher concentration of the silver ion: Ag 2 CO 3 K sp = 8.1x10-12 AgCl K sp = 1.6x10-10

37 17.5 Solubility Equilibria Figure 17.9 p. 611

38 17.5 Solubility Equilibria Which solution has the highest molar solubility? What is the molar solubility of: AgI K sp = 8.3x10-17 AgCl K sp = 1.6x10-10 What is the molar solubility of: Ag 2 CO 3 K sp = 8.1x10-12 AgCl K sp = 1.6x10-10

39 17.5 Solubility Equilibria Which solution has the highest molar solubility? What other quantities are used to express a compound s molar solubility? Macroscopically what does a solution look like: Below the molar solubility? Above the molar solubility? Which is saturated and which is unsaturated? Which solution has the highest concentration of ions a saturated solution or an unsaturated solution?

40 17.5 Solubility Equilibria How do we use solubility product constants to predict precipitation reactions? What happens when Q < K? What happens when Q > K?

41 17.5 Solubility Equilibria How do we use solubility product constants to predict precipitation reactions? Practice: What will happen when equal volumes of M silver nitrate and M sodium chloride are combined?

42 17.5 Solubility Equilibria How do we use solubility product constants to predict precipitation reactions? Practice: What will happen when equal volumes of M silver nitrate and M sodium carbonate are combined?

43 17.5 Solubility Equilibria More practice: What mass of calcium carbonate will dissolve in 1.5 L of water? If 5.0 g of barium fluoride is added to 1.0 L of water. After a sufficient amount of time, the saturated solution is filtered and the solid is dried and weighed and found to have a mass of 3.6 g. What is the solubility product constant, K sp (BaF 2 )?

44 17.6 The Common Ion Effect In general (review, 15.4): What happens to an equilibrium if a reactant is added or removed? What happens to an equilibrium if a product is added or removed?

45 17.6 The Common Ion Effect Consider: Is the concentration of the silver ion the same in either: A solution made by putting 10.0 g of silver bromide in ml of water? A solution made by putting 10.0 g of silver bromide in ml of M sodium bromide?

46 17.6 The Common Ion Effect Consider: Is the concentration of the silver ion the same in either: A solution made by putting 10.0 g of silver bromide in ml of water? A solution made by putting 10.0 g of silver bromide in ml of M sodium bromide? What is the common ion?

47 17.6 The Common Ion Effect Consider: Is the concentration of the silver ion the same in either: A solution made by putting 10.0 g of silver bromide in ml of water? A solution made by putting 10.0 g of silver bromide in ml of M sodium bromide? What is the concentration of the silver ion?

48 17.6 The Common Ion Effect Consider: What is the concentration of all ions when 75.0 ml of M silver nitrate is mixed with 25.0 of M sodium chloride? What does the solution look like?

49 17.6 The Common Ion Effect Can also use the common ion effect for fractional precipitation. You have a solution of silver ions and copper(ii) ions would like to separate. If you have both sodium bromide and sodium sulfide, how would you separate these ions? Can barium ions and calcium ions be separated with sodium carbonate?

50 17.6 The Common Ion Effect Can also use the common ion effect for considering solubility in acidic solutions. Calcium carbonate (lime scale) can be removed with water or (much better) by adding acid. How would either of these solvents do this?

51 17.7 Complex Ion Equilibria and Solubility What is a complex ion? In the context of Lewis acid/base, what is the best way to represent cobalt(ii) in solution? What happens when sodium chloride is added? What is the equilibrium expression for this? What is the equilibrium constant called?

52 17.7 Complex Ion Equilibria and Solubility Table 17.4 p. 619

53 17.7 Complex Ion Equilibria and Solubility What would happen if cobalt(ii) nitrate were added to a saturated solution of silver chloride?

54 17.8 Application of the Solubility Product Principle to Qualitative Analysis How are the cations grouped? How are the cation groups separated? Can cations within the same group be separated? Can cations within the same group be identified?

55 17.8 Application of the Solubility Product Principle to Qualitative Analysis Table 17.5 p. 621

56 17.8 Qualitative Analysis Figure p. 623

57 17.8 Application of the Solubility Product Principle to Qualitative Analysis You are given a sample of one unknown cation which could be Pb 2+, Ag +, Ba 2+, Fe 2+, Mg 2+, Al 3+, or Na +. Describe carefully how you would determine the presence of your cation and based on positive or negative precipitation results. This may be described by a flowchart if you wish.