che106_lecture9

che106_lecture9 - Chemistry 106 Lecture 9 Topics: ...

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Unformatted text preview: Chemistry 106 Lecture 9 Topics: Solubility and Precipita=on Reac=ons Chapter 4.1 – 4.2 Solu=ons •  Solu%ons are defined as homogeneous mixtures of two or more pure substances. •  The solvent is present in greatest abundance. •  All other substances are solutes. prepara=on of 0.250 L of 1.00 M solu%on of CuSO4 Ions in Aqueous Solu=on Ionic Theory of Solu6ons •  Many ionic compounds dissociate into independent ions when dissolved in water H2O NaCl (s) → Na (aq ) + Cl (aq ) + − •  Those compounds that “freely” dissociate into independent ions in aqueous solution are called electrolytes. •  Their aqueous solutions are capable of conducting an electric current. Ions in Aqueous Solu=on Ionic Theory of Solu6ons •  When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them. •  This process is called dissocia%on. Solu=on of NaCl and water. Water as a Solvent Water is a very effec=ve solvent for ionic compounds. Solu=on of NaCl and water. Although water is an electrically neutral molecule, one end of the molecule (the O atom) is rich in electrons and has a par=al nega=ve charge, denoted by δ- . Mo=on of ions in solu=on is responsible for the conduc=vity of aqueous solu=ons. Tes=ng Electrical Conduc=vity of a Solu=on (water + NaCl) Ions in Aqueous Solu=on Ionic Theory of Solu6ons •  Not all electrolytes are ionic compounds. Some molecular compounds dissociate into ions. + − HCl(aq) → H (aq) + Cl (aq) •  The resul=ng solu=on is electrically conduc1ng, and so we say that the molecular substance is an electrolyte. Ions in Aqueous Solu=on Ionic Theory of Solu6ons •  Some molecular compounds dissolve, but do not dissociate into ions. H 2O C6 H12O6 (s) (glucose) ⎯ྎ → C6 H12O6 (aq) ⎯ྎ •  These compounds are referred to as nonelectrolytes. They dissolve in water to give a nonconducting solution. Ions in Aqueous Solu=on Ionic Theory of Solu6ons •  A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. •  This yields a nonconduc%ng solu=on. Solu=on of methanol (CH3OH) and water. Ions in Aqueous Solu=on Ionic Theory of Solu6ons •  Electrolytes are substances that dissolve in water to give an electrically conduc=ng solu=on. – Thus, in general, ionic solids that dissolve in water are electrolytes. – Some molecular compounds, such as acids, also dissociate in aqueous solu=on and are considered electrolytes. Ions in Aqueous Solu=on Ionic Theory of Solu6ons If the solution is nonconducting, the circuit is incomplete and the bulb does not light. Apparatus for observing the electrical conductance of a solution. If the solution is conducting, the circuit is complete and the bulb lights. Pure water Tes=ng Electrical Conduc=vity of a Solu=on If you have equal concentra=ons (e.g. 1 mole per liter of solu=on) of the following aqueous solu=ons, which will have the highest conduc=vity? A.  CaCl2 B.  KBr C.  Na2SO4 D. Al(NO3)3 E.  LiCH3OO If you have equal concentra=ons (e.g. 1 mole per liter of solu=on) of the following aqueous solu=ons, which will have the highest conduc=vity? A.  CaCl2 B.  KBr More ions = C.  Na2SO4 higher 4 ions → D. Al(NO3)3 conduc=vity E.  LiCH3OO Ions in Aqueous Solu=on Ionic Theory of Solu6ons •  Strong and weak electrolytes. –  A strong electrolyte is an electrolyte that exists in solu=on almost en=rely as ions. H 2O + − NaCl(s ) ⎯ྎ → Na (aq) + Cl (aq) ⎯ྎ Most ionic solids that dissolve in water do so almost completely as ions, so they are strong electrolytes. Ions in Aqueous Solu=on Ionic Theory of Solu6ons •  Strong and weak electrolytes. –  A weak electrolyte is an electrolyte that dissolves in water to give a rela=vely small percentage of ions. → NH + (aq ) + OH − (aq ) NH 4OH(aq ) ← 4 •  Most soluble molecular compounds are either nonelectrolytes or weak electrolytes. •  Solu=ons of weak electrolytes contain only a small percentage of ions. We denote this by wri=ng the equa=on with a double arrow. Strong Electrolytes Are… •  Strong acids and strong bases •  Soluble ionic salts Comparing Strong and Weak Electrolytes HCl Strong electrolyte = good conduction N H3 Weak electrolyte = poor conduction Summary: Ions in Aqueous Solu=on •  In summary, substances that dissolve in water are either electrolytes or nonelectrolytes. – Nonelectrolytes form nonconduc=ng solu=ons because they dissolve as molecules. – Electrolytes form conduc=ng solu=ons because they dissolve as ions. Summary: Ions in Aqueous Solu=on •  Electrolytes can be strong or weak. – Almost all ionic substances that dissolve are strong electrolytes. – Molecular substances that dissolve are either nonelectrolytes or weak electrolytes. Solubility •  Solubility rules –  Substances vary widely in their solubility (ability to dissolve) in water. –  For example, NaCl is very soluble in water whereas calcium carbonate, CaCO3, is insoluble in water. Limestone (CaCO3) Forma=ons Photo ©Corbis. Solubility Rules for Ionic Compounds Molecular and Ionic Equa=ons •  A molecular equa1on is one in which the reactants and products are wrieen as if they were molecules, even though they may actually exist in solu1on as ions. Ca(OH )2 (aq) + Na 2CO 3 (aq) → CaCO3 (s) + 2NaOH(aq) –  Note that Ca(OH)2, Na2CO3, and NaOH are all soluble compounds but CaCO3 is not. Molecular and Ionic Equa=ons •  An ionic equa1on, however, represents strong electrolytes as separate independent ions. This is a more accurate representa=on of the way electrolytes behave in solu=on. 2− 2+ − + Ca (aq) + 2OH (aq) + 2Na (aq) + CO 3 (aq) → + − CaCO3 (s) ↓ +2Na (aq) + 2OH (aq) •  Molecular and Ionic Equa=ons Complete and net ionic qua=ons e –  A complete ionic equa1on is a chemical equa=on in which strong electrolytes (such as soluble ionic compounds) are wrieen as separate ions in solu=on. Ca( NO 3 )2 (aq) + K 2CO 3 (aq) → CaCO3 (s ) + 2KNO 3 (aq) (strong) (strong) (insoluble) (strong) 2− Ca 2+ (aq) + 2NO 3 (aq) + 2K + (aq) + CO 3 (aq) → − − CaCO3 (s) + 2K + (aq) + 2NO 3 (aq) Molecular and Ionic Equa=ons •  Complete and net ionic equa=ons. –  A net ionic equa1on is a chemical equa=on from which the spectator ions have been removed. –  A spectator ion is an ion in an ionic equa=on that does not take part in the reac=on. 2− Ca 2+ (aq) + 2NO 3 (aq) + 2K + (aq) + CO 3 (aq) → − − CaCO3 (s) + 2K + (aq) + 2NO 3 (aq) Molecular and Ionic Equa=ons •  Complete and net ionic equa=ons – Let’s try an example. First, we start with a molecular equa=on. 2HNO3 (aq) + Mg(OH )2 (s) → 2H 2O(l ) + Mg( NO 3 )2 (aq) Nitric acid, HNO3, and magnesium nitrate, Mg(NO3)2, are both strong electrolytes. •  + Molecular and Ionic Equa=ons equa=ons Complete and net ionic –  Separa=ng the strong electrolytes into separate ions, we obtain the complete ionic equa=on. − 2H (aq) + 2NO 3 (aq) + Mg(OH )2 (s) → 2+ − 2H 2O(l ) + Mg (aq) + 2NO 3 (aq) –  Note that the nitrate ions did not par=cipate in the reac=on. These are spectator ions. Molecular and Ionic Equa=ons equa=ons Complete and net ionic •  –  Elimina=ng the spectator ions results in the net ionic equa=on. + − 2H (aq) + 2NO 3 (aq) + Mg(OH )2 (s) → 2+ − 2H 2O(l ) + Mg (aq) + 2NO 3 (aq) 2H + (aq) + Mg(OH )2 (s ) → 2H 2O(l ) + Mg 2+ (aq) This equa=on represents the “essen=al” reac=on. Types of Chemical Reac=ons •  Most of the reactions we will study fall into one of the following categories: – Precipitation Reactions – Acid-Base Reactions – Oxidation-Reduction Reactions Precipita=on Reac=ons •  A precipita=on reac=on occurs in aqueous solu=on because one product is insoluble. –  A precipitate is an insoluble solid compound formed during a chemical reaction in solution. –  For example, the reaction of sodium chloride with silver nitrate forms AgCl(s), an insoluble precipitate. NaCl(aq ) + AgNO3 (aq ) → AgCl(s) ↓ + NaNO3 (aq ) NaCl(aq) + AgNO3(aq) → AgCl(s) +NaNO3(aq) Reac=on of sodium chloride and silver nitrate. Photo courtesy of American Color. AgCl solid crystals form. 2KI(aq) + Pb(NO3)2(aq) → PBI2(s) +2KNO3(aq) Precipita=on Reac=ons: Solubility •  Predic=ng Precipita=on Reac=ons. –  To predict whether a precipitate (insoluble solid compound) will form, we need to look for poten=al insoluble products. –  Table 4.1 lists solubility rules for ionic compounds. These rules apply to the most common ionic compounds. Solubility Rules for Ionic Compounds Precipita=on Reac=ons •  Predic=ng Precipita=on Reac=ons. –  Suppose you mix together solu=ons of nickel(II) chloride, NiCl2, and sodium phosphate, Na3PO4. NiCl 2 + Na 3 PO 4 → –  How can you tell if a reac=on will occur, and if it does, what products to expect? Precipita=on Reac=ons •  Predic=ng Precipita=on Reac=ons. –  Precipita=on reac=ons have the form of an “exchange reac%on.” NiCl 2 + Na 3 PO 4 → Ni 3 ( PO 4 )2 + NaCl –  An exchange (or metathesis) reac%on is a reac=on between compounds that, when wrieen as a molecular equa=on, appears to involve an exchange of ca=ons and anions. Metathesis (Exchange) Reac=ons •  Metathesis comes from a Greek word that means “to transpose.” •  It appears the ions in the reactant compounds exchange, or transpose, ions. AgNO3 (aq) + KCl (aq) !! AgCl (s) + KNO3 (aq) Precipita=on Reac=ons •  Predic=ng Precipita=on Reac=ons. –  Now that we have predicted poten=al products, we must balance the equa=on. N 2 3 iCl + 2Na 3 PO 4 → Ni 3 ( PO 4 ) 2 + 6 NaCl –  We must then verify that NiCl2 and Na3PO4 are soluble and then check the solubili%es of the products. Precipita=on Reac=ons •  Predic=ng Precipita=on Reac=ons. –  Table 4.1 indicates that our reactants, nickel(II) chloride and sodium phosphate are both soluble. 3NiCl 2(aq) + 2 Na 3 P O 4 (aq) → Ni 3 ( PO 4 )2 (s) + 6 NaCl (aq) –  Looking at the poten=al products, we find that nickel(II) phosphate is not soluble although sodium chloride is. Precipita=on Reac=ons •  Predic=ng Precipita=on Reac=ons. –  We predict that a reac=on occurs because nickel(II) phosphate is insoluble and precipitates from the reac=on mixture. –  To see the reac=on that occurs on the ionic level, we must rewrite the molecular equa=on as an ionic equa=on. Precipita=on Reac=ons •  Predic=ng Precipita=on Reac=ons. –  First write strong electrolytes (the soluble ionic compounds) in the form of ions to obtain the complete ionic equa=on. 2+ − + 3− 3Ni (aq ) + 6Cl (aq ) + 6Na (aq ) + 2PO 4 (aq ) → Ni 3 ( PO4 )2 (s) + 6Na + (aq) + 6Cl − (aq) Precipita=on Reac=ons •  Predic=ng Precipita=on Reac=ons. –  Ajer canceling the spectator ions, you obtain the net ionic equa%on. 3− 2+ − + 3Ni (aq ) + 6Cl (aq ) + 6Na (aq ) + 2PO 4 (aq ) → Ni 3 ( PO4 )2 (s) + 6Na + (aq) + 6Cl − (aq) 2+ 3− 3Ni (aq) + 2PO 4 (aq) → Ni 3 ( PO 4 )2 (s) This equa%on represents the “essen%al” reac%on. Wri=ng Net Ionic Equa=ons 1.  Write a balanced molecular equa=on. 2.  Dissociate all strong electrolytes. 3.  Cross out anything that remains unchanged from the lej side to the right side of the equa=on. 4.  Write the net ionic equa=on with the species that remain. ...
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This note was uploaded on 10/11/2011 for the course CHE 106 taught by Professor Freedman during the Fall '08 term at Syracuse.

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