Solubility and Complex Formation Equilibria

Transcription

1 Solubility and Complex Formation Equilibria Objectives 1. Write a reaction showing the dissolution of any solid compound. 2. Write the expression for K sp for any solid compound. 3. Use the value of K sp to calculate the concentration of one ion knowing the concentration of the other. 4. Use the value of K sp to determine whether a precipitate will form. 5. Use the value of K sp to calculate the solubility of a solid compound in water or in a solution containing a common ion. 6. Use the values of K sp to determine which ion will precipitate from solution first from a mixture. 7. Write reactions and calculate the value of K for dissolving a hydroxide compound in acid solution. 8. Write reactions and calculate K for dissolving a precipitate in a solution containing a complexing agent. 9. Write reactions and calculate K for dissolving a precipitate in acid solution when a weak electrolyte (weak acid) is formed. 10. Discuss and employ qualitative analysis techniques to determine what ions are present in solutions of several metal ions. Precipitation Formation: Solubility Product Constant (K sp ) Background/Terminology Precipitation is the reverse of dissolution. When studying precipitation/dissolution processes, reactions are typically represented as follows: salt (s) <==> cation (aq) + anion (aq) The presence of water as the solvent is indicated by the subscripted (aq) following the ion formulas. As indicated by the double reaction arrows, these are equilibrium processes. Therefore, they can be treated like other equilibrium systems. That is, the reactions are "governed" by an equilibrium constant, equilibrium can be approached from both directions, and equilibrium can be shifted (through the application of Le Chatelier's Principle). Let's take a look at some examples and learn some terminology. AgCl (s) <==> Ag + (aq) + Cl - (aq) K (sp) = 1.8 x K (sp) is the equilibrium constant for this reaction (at 25 o ) and it is called the solubility product constant (hence the "sp"). 113

2 Since this is an equilibrium, the equilibrium constant is related to the concentrations as in other equilibrium expressions... K (sp) = [Ag + ][Cl - ] The pure salt and water are not included because they are pure compounds. You will notice that K (sp) is rather small which indicates that not much of the salt dissolves. This will be true for all of the salts that are studied in this chapter. The reason for this will be revealed later. Another important term that you should be familiar with is solubility (given the symbol "S"). Solubility is the concentration of the salt that is dissolved in solution. It is normally expressed in terms of molarity (moles/liter) but it can be expressed in other units (g/100 g of water or g/liter or...) For the reaction above, S = [Ag + ] = [Cl - ], since the stoichiometry of the compound is one to one. See if you can determine what "S" is equal to for the following salts" Ca 3 (PO 4 ) 2 Al(OH) 3 Fe 2 (SO 4 ) 3 Ans: For Ca 3 (PO 4 ) 2, S = 1 / 3 [Ca 2+ ] = 1 / 2 [ PO 4 3- ] For Al(OH) 3, S = [Al 3+ ] = 1 / 3 [OH - ] For Fe 2 (SO 4 ) 3, S = 1 / 2 [Fe 3+ ] = 1 / 3 [ SO 4 2- ] Do you see the relationship between the compound's stoichiometry and the way that "S" is related to each ion's concentration? NOTE: The relationships shown above are true when the only source of the ions in the solution is the salt itself (i.e. when there is no other species present in the solution that produces either of the ions produced by the salt in the equilibrium). Determining if a Precipitate will Form In order to determine whether a precipitate will form or not, we need merely ask the following question: "Will the solubility reaction proceed in the forward or reverse direction based on the ion concentrations present?" Determining the direction in which an equilibrium will proceed is something we have done before. In a previous chapter, we learned about the reaction quotient, Q. We learned the following about Q. 114

3 Q takes the same form as the equilibrium expression. When Q = K, the process being studied is at equilibrium and there is not net reaction. When Q < K, the process shifts toward product until Q becomes equal to K. When Q > K, the process shifts toward reactant until Q becomes equal to K. For precipitation/dissolution equilbria, we change the name from reaction quotient to "ion product" and symbol for Q to either Q IP or "P". example If a solution contains [Ag + ] = [Cl - ] = 0.10 M, will a precipitate of AgCl form? If so, what will [Ag + ] and [Cl - ] be after AgCl has completed precipitation? Ans. The equilibrium is: AgCl (s) <==> Ag + (aq) + Cl - (aq) K (sp) = 1.8 x Precipitation will occur if Q IP > K (sp). Q IP = [Ag + ][Cl - ] = (0.10)(0.10) = Q IP > K (sp), so AgCl will precipitate. Now, how do we determine what [Ag + ] and [Cl - ] will be at equilibrium? Let's set up a reaction chart! AgCl (s) <==> Ag + (aq) + Cl - (aq) initial M 0.10 M change --- -S -S equil S M S M K (sp) = 1.8 x = [Ag + ][Cl - ] 1.8 x = [ S][ S] We cannot assume S is small because most of the ions will precipitate out of solution. However, solving the above equation can be done explicitly by taking the square root of each side and solving for S. Doing so yields S = M... which is many sig figs. more than we should have. Note that [Ag + ] = [Cl - ] = 1.3 x 10-5 M IMPORTANT NOTE: The answer to the second part of the example problem was solved in a very general way. i.e. the method would work for any equilibrium system. However, in the sample above there is a much easier solution. Since AgCl has one to one compound stoichiometry AND since [Ag + ] = [Cl - ] in the starting solution, the final concentrations of both must be equal. That means that K (sp) = 1.8 x = [Ag + ][Cl - ] and [Ag + ] = [Cl - ] square root of K (sp) = 1.3 x 10-5 M. 115

4 Common Ion Effect The solubility of a salt in the presence of another source of ions common to the salt will be lower than in pure water. For example: Find the solubility of lead (II) chloride in pure water and in aqueous 0.10 M NaCl. In pure water the equilibrium table would look like: PbCl 2 (s) <==> Pb 2+ (aq)+2 Cl - (aq) initial M 0 M change S + 2 S equil. --- S M 2S M K (sp) = 1.7 x 10-5 = [Pb 2+ ][Cl - ] x 10-5 = [S][2S] 2 = 4S 3 S = [Pb 2+ ] = 1.6 x 10-2 M, and 2S = [Cl - ] = 3.2 x 10-2 M In aqueous 0.10 M NaCl the equilibrium will be altered to look like this: PbCl 2 (s) <==> Pb 2+ (aq)+ 2 Cl - (aq) initial M 0.10 M change S + 2 S equil. --- S M S M The initial concentration of chloride is due to the presence of dissolved NaCl. K (sp) = 1.7 x 10-5 = [Pb 2+ ][Cl - ] x 10-5 = [S][ S] 2 assume 2S is small compared to 0.10 M, then 1.7 x 10-5 = [S][0.10] 2 S = [Pb 2+ ] = 1.7 x 10-3 M, and [Cl - ] = S = 0.10 M The assumption was valid. Also note that the solubility of the lead (II) chloride is lower in the aqueous NaCl solution than in pure water. The Uncommon Ion Effect Salts will be more soluble in a solution that contains other dissolved salts with no ions in common with the salt in the equilibrium. The reasons are discussed below. 116

5 example: lead (II) chloride would have a higher solubility in an aqueous solution of sodium nitrate than in pure water. However, there is no "easy" mathematical method to account for the change in solubility caused by the uncommon ions. The treatment of these types of systems is covered in higher level chemistry courses. Solubility is often higher than expected for two reasons. First ions can combine in solution to form "ion pairs". For example, when studying the solubility of magnesium fluoride, the reaction of interest is: MgF 2 (s) <==> Mg 2+ (aq) + 2 F - (aq) However, a Mg 2+ (aq) and a F - (aq) can combine to form an ion pair MgF + (aq). This allows more magnesium fluoride to dissolve to replace these ions. The second reason for finding increased solubilities is a little more nebulous. The "ionic atmosphere" of the solution is affected by the presence of high (> M) concentrations of ions. You may think about this in the following way. In very dilute solutions, ions are separated by large distances (on the atomic scale) and numerous solvent molecules "insulate" the ions from the charges on other ions. However, in more concentrated solutions, ions "feel" the presence of other ions and the solvent is made more ionic by the presence of a large amount of ions. This increases the solubility of the sparingly soluble salts. Dissolving Precipitates In the section above you learned that the common ion effect acts to shift the solubility equilibria in a way that decreases the solubility of salts. In this section we will investigate how to enhance the solubility of salts by coupling reactions with the solubility equilibria. Formation of water: Hydroxides generally have low solubilities in water. However, we can make the salts more soluble by dissolving them in an acidic solution. Consider the following example: Aluminum hydroxide has K (sp) = 2 x = [Al 3+ ][OH - ] 3 Clearly this is not a highly soluble compound. However, if we dissolve the hydroxide in a buffer with a ph = 4.00 the salt will dissolve much more readily. (neutral is ph = 7.00, a ph of 4.00 is 1000 times more acidic). It is easy to understand how the presence of additional acid helps dissolve the hydroxide if you examine the following reactions. solubility Al(OH) 3(s) <==> Al 3+ (aq) +3 OH - (aq) acid/base neut.3 OH - (aq) + 3 H + (aq) <==> 3 H 2 O (l) overall 3 H + (aq) +Al(OH) 3 (s) <==> Al 3+ (aq) + 3 H 2 O (l) The first reaction is simply the solubility equilibria. The second reaction is an acid/base neutralization. Look closely at the second reaction and you will recognize that it it 3 times the reverse of the water self-ionization reaction. When these two reactions are added together, the 117

6 overall reaction is found. To find the equilibrium constant for the overall reaction, we simply find the product of the equilibrium constants of the first two reactions: overall K = K (sp) ( 1 / Kw 3) overall K = (2 x )(1.0 x ) = 2 x The overall K is a large number - this indicates that the overall reaction proceeds toward product to a relatively large extent. Now that we have determined the equilibrium constant for the reaction, we can calculate the equilibrium concentrations. Proceed as follows: overall 3 H + (aq) +Al(OH) 3 (s) <==> Al 3+ (aq)+ 3 H 2 O (l) initial 1.0 x 10-4 M M --- change ~ S --- equil. 1.0 x 10-4 M --- S M --- The initial concentration of H + (aq) is determined from the ph of the buffer. The change in the concentration of H + (aq) is ~ 0 because we choose to use a BUFFER in which to dissolve our compound. (buffers resist change in ph). This greatly simplifies the math we must do. Now we can solve for S using the following: overall K = 2 x = [Al 3+ ]/[H + ] 3 S = [Al 3+ ] = M which is a much greater solubility than we would find for aluminum hydroxide in water. Formation of a weak electrolyte: Consider the following example: dissolve calcium fluoride in ph = 2.00 buffer. Fluoride ions are weak bases (they are the conjugate of a weak acid, HF). Calcium fluoride has K (sp) = 1.5 x = [Ca s+ ][F - ] 2 Clearly this is not a highly soluble compound. However, if we dissolve the fluoride in a buffer with a ph = 2.00 the salt will dissolve much more readily. (neutral is ph = 7.00, a ph of 4.00 is 1000 times more acidic). It is easy to understand how the presence of additional acid helps dissolve the fluoride if you examine the following reactions. solubility CaF 2(s) <==> Ca 2+ (aq) + 2 F - (aq) rev. acid dissoc. 2 F - (aq) +2 H + (aq) <==> 2 HF (aq) overall 2 H + (aq) +CaF 2 (s) <==> Ca 2+ (aq) +2 HF (aq) The first reaction is simply the solubility equilibria. The second reaction is 2 times the reverse of an acid dissociation. When these two reactions are added together, the overall reaction is found. To find the equilibrium constant for the overall reaction, we simply find the product of the equilibrium constants of the first two reactions: 118

7 overall K = K (sp) ( 1 / Ka 2) overall K = (1.5 x )/(6.9 x 10-4 ) 2 = 3.2 x 10-4 The overall K is a larger number than K sp - this indicates that the overall reaction proceeds toward product to a larger extent. Now that we have determined the equilibrium constant for the reaction, we can calculate the equilibrium concentrations. Proceed as follows: overall 2 H + (aq) +CaF 2 (s) <==> Ca 2+ (aq) +2 HF (aq) initial 1.0 x 10-2 M M 0 M change ~ S + 2S equil. 1.0 x 10-2 M --- S M 2S M The initial concentration of H + (aq) is determined from the ph of the buffer. The change in the concentration of H + (aq) is ~ 0 because we choose to use a BUFFER in which to dissolve our compound. (buffers resist change in ph). This greatly simplifies the math we must do. Now we can solve for S using the following: overall K = 3.2 x 10-4 = [Ca 2+ ][HF] 2 /[H + ] 2 S = [Ca 2+ ] = M which is a much greater solubility than we would find for calcium fluoride in water. Formation of a complex ion: Salts which contain an ion that can form a complex ion can be dissolved to a greater extent through the formation of that complex ion. Consider the following... AgCl is relatively insoluble (K sp = 1.8 x ). However, silver can form a complex with ammonia. You can see how this would change the position of the solubility equilibria by examining the following reactions and calculating the solubility of Ag Cl in M ammonia. solubility AgCl (s) <==> Ag + (aq) + Cl - (aq) complex form. Ag + (aq) +2 NH 3 (aq) <==> [Ag(NH 3 ) 2 ] + (aq) overall 2 NH 3 (aq) + AgCl (s) <==> Cl - (aq) +[Ag(NH 3 ) 2 ] + (aq) The first reaction is simply the solubility equilibria. The second reaction is a complex formation equilibria. When these two reactions are added together, the overall reaction is found. To find the equilibrium constant for the overall reaction, we simply find the product of the equilibrium constants of the first two reactions: overall K = (K (sp) )(K f ) overall K = (1.8 x )/(1.7 x ) = 3.1 x 10-3 The overall K is a larger number than K sp - this indicates that the overall reaction proceeds toward product to a relatively large extent. 119

8 Now that we have determined the equilibrium constant for the reaction, we can calculate the equilibrium concentrations. Proceed as follows: overall 2 NH 3 (aq) +AgCl (s) <==> Cl - (aq)+[ag(nh 3 ) 2 ] + (aq) initial 0.10 M M 0 M change - 2S S + S equil M - 2S M --- S M S M Now we can solve for S using the following: overall K = 3.1 x 10-3 = [Ag(NH 3 ) 2 + ] [Cl - ]/[NH 3 ] 2 S = [Cl - ] = 5.0 x 10-3 M which is a much greater solubility than we would find for silver chloride in water. Keywords common ion complex ion equilibrium rules -coefficent -multiple -reciprocal formation constant, K f ion product, P net ionic equations -spectator ions -adding reactions ph precipitate reaction quotient, Q solubility, S solubility product constant, K sp 120