Introducing Driving Force #3 - Formation of a Solid

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2 Introducing Driving Force #3 - Formation of a Solid In each of the next two types of chemical reactions, the reactants are aqueous solutions Reactants are ionic substances (solutes) dissolved in water (the solvent) If a solid forms when two aqueous solutions are mixed, the reaction is called a precipitation reaction The solid is called a precipitate Precipitation reactions are a special type of double replacement reactions The products of DR reactions can be predicted based on the dissociated ions

3 What Does Dissociated Mean??? When soluble ionic compounds are dissolved in water, they dissociate or break apart Dissociation occurs as a result of solvation 0 A process in which water molecules surround the solid compound, which is a crystal lattice structure 0 The polar water molecules are attracted to outermost layer of ions and literally pull individual ions away from the lattice Dissociation of an Ionic Compound 0 Soluble covalent compounds DO NOT dissociate in water 0 Remains as one whole compound

4 Dissociation of Ionic Compounds 0 Once dissociated, each ion moves independently of each other Ba(NO 3 ) 2 (aq) Ba 2+ aq + 2 NO 3 (aq) 0 Notice how the whole nitrate polyatomic ion stays together! 0 Polyatomic ions are covalently bonded! 0 When two or more ionic compounds are added to water, each compound will dissociate 0 Ions will move independently and recombine to form new products 2 K + aq + CrO 4 2 aq + Ba 2+ aq + 2 NO 3 aq BaCrO 4 s + 2K + aq + 2 NO 3 aq

5 So, What are the Possible Products of a Double Replacement Reaction? New products are a flip-flop of the cations and anions Products can be: An insoluble, neutral ionic solid Called a precipitate and it occurs when pairs of oppositely charged ions attract each other so strongly that they overcome the attraction of the water molecules and fall out of solution The precipitate must have a net zero charges In other words, rebalance the charges! 0 A soluble, neutral aqueous ionic compound A precipitation reaction only occurs IF an insoluble precipitate forms Precipitation Animation

6 How Do We Predict Whether a Precipitation Reaction Will Occur? 0 The solubility of a substance at a given temperature is the amount of the substance that can be dissolved in a given quantity of solvent at the given temperature 0 Any substance with a solubility less than 0.01 mol/l will be considered insoluble 0 To predict whether certain combinations of ions form insoluble compounds, we must consider some guidelines concerning the solubilities of common ionic compounds SOLUBILITY RULES!

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8 More on the Solubility Rules 0 The solubility rules are phrased as if you had a sample of a given solute and wanted to see if you could dissolve it in water 0 These rules can also be applied, however, to predict the identity of the solid produced in a precipitation reaction 0 In other words, a given combination of ions will not be soluble in water whether you take a pure compound out of a reagent bottle or if you generate the insoluble combination of ions during a chemical reaction

9 The Solubility Rules BaSO 4 is not soluble in water This means that not only that a pure sample of BaSO 4 taken from a reagent bottle will not dissolve in water, but also that if a Ba 2+ ion and SO 4 2- ion end up together in the same solution, they will precipitate as BaSO 4

10 Practice with Our Pink Sheet! Determine whether the following ionic compounds are soluble (aq) or insoluble (s) using your solubility rules pink sheet: 0 MgF 2 0 CuSO 4 0 AgCl 0 Na 2 SO 4 0 NH 4 OH 0 BaCO 3 0 Hg 2 I 2

11 Solubility and Electrical Conductivity 0 Depending on the type of solute dissolved, the resulting solution may conduct electricity 0 A substance whose aqueous solutions contain ions is called an electrolyte 0 Conducts electricity due to the fact that the ions carry electrical charge from one electrode to the other, completing the circuit 0 Strong electrolytes are those solutes that exist in solution completely or nearly completely as ions 0 Many are water-soluble ionic compounds 0 Examples: NaCl, HCl (molecular compound), KOH, etc. 0 Weak electrolytes are those solutes that exist in solution mostly in the form of neutral molecules with only a small fraction in the form of ions 0 Examples: HC 2 H 3 O 2 0 A substance that does not form ions in solution is called a nonelectrolyte

12 How to Describe Reactions in Aqueous Solutions Just as in single replacement reactions, you can describe a precipitation reaction through a molecular equation, complete ionic equation, or net ionic equation 0 Since we take the actual chemical reaction in a precipitation process to be the formation of a solid, the spectator ions are those that do not participate in the formation of the solid 0 Precipitation reactions have TWO spectator ions 0 Remember, single replacement reactions only have one! 0 We can then eliminate them from the molecular and complete ionic equation to produce a net ionic equation

13 Representing a Precipitation Reaction - Example 0 A solution of potassium chloride reacts with lead (II) nitrate. 0 Balanced molecular equation: 2 KCl aq + Pb NO 3 2 aq 2 KNO 3 aq + PbCl 2 (s) 0 Balanced complete ionic equation: 2K + aq + 2Cl aq + Pb 2+ aq + 2NO 3 aq 2K + aq + 2NO 3 aq + PbCl 2 (s) 0 Balanced net ionic equation: 2Cl aq + Pb 2+ (aq) PbCl 2 (s)

14 A Note on Net Ionic Equations 0 Just because we leave out the spectator ions out when writing a net ionic equation for a reaction does NOT mean that the spectator ions do not have to be present 0 Remember, the spectator ions are needed to provide a balance of charge in the reactant compounds for the ions which combine to form the precipitate

15 Practice! 0 Balance the following equations. Then, write the complete ionic equation and cross out the spectator ions to give the net ionic equation for each of the reactions below. Include physical states for each species. 0 LiCl ( ) + AgNO 3 ( ) AgCl ( ) + LiNO 3 ( ) 0 Complete Ionic 0 Spectator Ions 0 Net Ionic 0 Na 2 S ( ) + CaCl 2 ( ) NaCl ( ) + CaS ( ) 0 Complete Ionic 0 Spectator Ions 0 Net Ionic