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Equilibrium in Solutions

Equilibrium in Solutions. When discussing whether or not an ionic solid dissolves well into water, we talk about its Solubility Product Constant, or K sp Which tells us the ratio of dissolved solid versus undissolved solid at equilibrium. Consider the following reaction:.

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Equilibrium in Solutions

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  1. Equilibrium in Solutions • When discussing whether or not an ionic solid dissolves well into water, we talk about its Solubility Product Constant, or Ksp Which tells us the ratio of dissolved solid versus undissolved solid at equilibrium. • Consider the following reaction:

  2. BaSO4(s) <---------> Ba2+(aq) + (SO4)2-(aq) • This is a reaction where an ionic solid is being redissolved into water….BUT NOT WELL! • Most of this solid would be just that at the bottom of a glass of water….a solid! • The Ksp for this reaction would be: [Ba2+ ]1 [SO42-]1 ---------------------- [BaSO4(s)]1 • Ksp is similar to KC, because it tells us the ratio of products to reactants AT EQUILIBRIUM!!!

  3. BaSO4(s) ---------> Ba2+(aq) + (SO4)2-(aq) • Let’s say that a sample of BaSO4(s) is added to pure water and allowed to come to equilibrium at 250C. We measure the Ba2+ concentration and it is 3.87 x 10-5M. We measure the (SO4)2- concentration and it is……? • The same! For every Ba2+ that dissolves, we get the same number of (SO4)2- ions! • This tells me that most of the solid still remains at the bottom of the container as an undissolved solid!

  4. BaSO4(s) ---------> Ba2+(aq) + (SO4)2-(aq) • The constant for this reaction is: Ksp = [Ba+2(aq)][SO4-2(aq)] ------------------ [BaSO4(s)] [3.87 x 10-5M] 1[3.87 x 10-5M]1 Ksp = ---------------------------------------- = 1.5 x 10-9 [1]1 1.5 x 10-9 = .0000000015 = 15 10,000,000,000 • This means that only 15 out of 10 billion molecules are going to dissolve in water! • Why is the concentration of the solid equal to 1?

  5. BaSO4(s) ---------> Ba2+(aq) + (SO4)2-(aq) • Remember, solids and liquids change only in amount - not in concentration! • If you assume that you start with 1M of the solid, since so little of it dissolves, you still have 1M of the solid at the end! • When an ionic solid dissolves in water, there are so few ions in solution compared to undissolved solid that we assume the concentration of the leftover solid to be 1! • We are really measuring how much of the solid dissolved by just looking at the ion concentration! • But don’t ionic solids dissolve well in water?????

  6. Ionic solids DO dissolve well in water - at least most of them! • You won’t see Ksp values for substances like NaCl, MgSO4, CaCl2, etc. • Remember the solubility rules? These substances are REALLY soluble in water! • They would have a Ksp close to 1, meaning they dissolve completely!

  7. What does Ksp really tell us??? Let’s take two compounds that do not dissolve well in water…. • A solid such as CaCrO4, which has a Ksp = 7.1 x 10-4, would dissolve comparatively well compared to…. • a solid such as HgS, which has a Ksp = 2 x 10-53, which dissolves poorly in water!

  8. Why is Ksp important to us? • Ionic solids with small Ksps do not dissolve well in water, and fall out as…. PRECIPITATES!!! • These solids are used as paint colorings, chalks, and for medical procedures, like X-rays, upper and lower GI’s

  9. 5. The Ksp for barium iodate at 250C is 6.00 x 10-10. What is the molar solubility of each ion at 250C in solution? Barium iodate is used to purify edible oils. • What does the term molar solubility mean? • It is the amount of each ion dissolved in water at equilibrium! • Let’s write the reaction for barium iodate dissolving in water, and set up an ICE equation – • 1 Ba(IO3)2(s) Ba+2(aq) + 2 IO3-1(aq) I 1M 0 0 C -x +x +2x E 1M x 2x • Ksp = 6.00 x 10-10 = [Ba+2(aq)]1[IO3-1(aq)]2 [1] • 6.00 x 10-10 = [x]1[2x]2 = 4x3 • 1.5 x 10-10 = x3 • 5.31 x 10-4M = x, so Ba+2 = 5.31 x 10-4M, and IO3-1 = .00106M • Why does the concentration of the solid stay 1M?

  10. The Common Ion Effect…. • What is the common ion effect? • It is when there is already one of the ions in solution, making it harder for the solid to dissolve! • 14. What is the molar solubility of calcium oxalate solid in .15M calcium chloride solution? Compare this molar solubility to that for calcium oxalate in pure water, which is 4.8 x 10-5. The Ksp for calcium oxalate is 2.3 x 10-9. • A .15M CaCl2 solution breaks up 100% into Ca+2(aq) and C2O4-2(aq), because it follows the solubility rules! • This, in effect, adds Ca+2(aq) ions into the solution! • 1 CaC2O4(s) Ca+2(aq) + C2O4(aq) I 1M .15M 0 C -x +x +x E 1M .15+x x • Ksp = 2.3 x 10-9 = [Ca+2(aq)]1[C2O4-2(aq)]1 [1] • 2.3 x 10-9 = [.15 + x]1[x]1 = x2 + .15x • 0= x2 + .15x - 2.3 x 10-9 • 1.53 x 10-8M = x, so Ca+2 = .15M, and C2O4-2 = 1.53 x 10-8M • Why does the concentration of the Ca+2 stay .15M? Ca+2 ions already added Solid CaC2O4

  11. Does water dissolve itself??? • We know that water has a partial charge - does it pull itself apart? • Water actually has a Ksp, which is also called: Kw • Water naturally dissociates into H+ and (OH)- ions. Only one in ten million water molecules actually dissociates, though

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