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Chapter 16 Other Equilibria Week 2 2009

# Chapter 16 Other Equilibria Week 2 2009 - Section 16.6...

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Section 16.6. Slightly Soluble Ionic Salts Ionic salts are collections of cations (M + ) and anions (X - ). When an ionic salt dissolves in water, it does so by the ions separating as they become surrounded by H 2 O molecules. A very soluble ionic salt (e.g., NaCl) dissolves in water completely, giving Na + (aq) and Cl - (aq), and there is no solid left. However, some salts are only slightly soluble , and an equilibrium exists between dissolved and undissolved (i.e. solid) compound. Consider the addition of PbSO 4 (s) (lead sulfate) to water. PbSO 4 (s) Pb 2+ (aq) + SO 4 2- (aq) At equilibrium, the rate at which more PbSO 4 (s) dissolves(i.e. the forward reaction) is equal to the rate at which Pb 2+ (aq) and SO 4 2- (aq) ions come together to give PbSO 4 (s) (the reverse reaction). We say the solution is saturated i.e. the concs are as big as they can be. If the reaction has not yet reached equilibrium, Q c is Q c = ] PbSO [ ] SO ][ Pb [ 4 2 4 2 - + The conc of a solid (= its density) is a constant combine it with Q c . Q c [PbSO 4 ] = Q sp = [Pb 2+ ][SO 4 2- ] Q sp = “ion-product expression” or “solubility product expression” At equilibrium, Q sp = K sp K sp = [Pb 2+ ][SO 4 2- ] ** K sp = “solubility product constant” or just "solubility product" ** K sp , like all other equilibrium constants, only changes with temperature. In general: M p X q (s) p M n+ (aq) + q X z- (aq) K sp = [M n+ ] P [X z- ] q

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We usually only consider systems at equilibrium use K sp (not Q sp ). Occasionally, we will consider Q sp (see later) Examples : Cu(OH) 2 (s) Cu 2+ (aq) + 2 OH - (aq) K sp = [Cu 2+ ][OH - ] 2 CaCO 3 (s) Ca 2+ (aq) + CO 3 2- (aq) K sp = [Ca 2+ ][CO 3 2- ] Ca 3 (PO 4 ) 2 (s) 3 Ca 2+ (aq) + 2 PO 4 3- (aq) K sp = [Ca 2+ ] 3 [PO 4 3- ] 2 ** the greater is K sp , the more soluble is the substance ** e.g., PbSO 4 K sp = 1.6 x 10 -8 insoluble CoCO 3 K sp = 1.0 x 10 -10 more insoluble (or less soluble) Fe(OH) 2 K sp = 4.1 x 10 -15 most insoluble (or least soluble) Calculations Involving Solubility Products Two types: Use K sp to find conc of dissolved ions Use concsto find K sp . ** Make sure the equations are balanced!! ** Example 1 : The solubility of Ag 2 CO 3 is 0.032 M at 20 °C. What is K sp of Ag 2 CO 3 ?
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