Introducing Driving Force #3 - Formation of a Solid

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Paul Craig
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2 Introducing Driving Force #3 - Formation of a Solid A solid that forms in an aqueous reaction is called a precipitate Precipitation reactions are also known as double replacement reactions Cations trade places! 0 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 - break apart 0 Occurs as a result of solvation 0 A process in which water molecules surround the solid compound, which is a crystal lattice structure 0 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 As stated before, when an ionic compound dissociates, the cation and anion break apart 0 Possible cations (positively-charged ions) include: 0 Metals 0 Ammonium (NH 4+ ) 0 Possible anions (negatively-charged ions) include: 0 Nonmetals 0 All other polyatomic ions 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 polyatomic ion stays together! 0 Polyatomics are covalently bonded!

5 What Happens if There is More than One Ionic Compound in Water? When two or more ionic compounds are added to water, each compound will dissociate Ions will move independently and recombine to form new products Remember, cations flip flop places! 2 K + aq + CrO 4 2 aq + Ba 2+ aq + 2 NO 3 aq BaCrO 4 s + 2K + aq + 2 NO 3 aq

6 So, What are the Possible Products of a Double Replacement Reaction? Products must contain a cation and an anion 0 Opposites attract! Products can be: An insoluble product Sometimes, pairs of oppositely charged ions attract each other so strongly that they form an insoluble, neutral ionic solid Called a precipitate The precipitate must have a net zero charges In other words, rebalance the charges! 0 A soluble product aqueous Can predict products based on the solubility rules! A precipitation reaction only occurs IF an insoluble precipitate forms Precipitation Animation

7 The Solubility Rules

9 Practice! 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

10 How to Describe Reactions in Aqueous Solutions There are three different ways to describe a precipitation reaction: 0 Molecular equation 0 Complete ionic equation 0 Net ionic equation

11 What is a Molecular Equation? Shows each of the substances in the reaction as compounds with physical states written next to the chemical formulas 0 Does NOT show the actual form of the reactants and products in solution 2 KCl aq + Pb NO 3 2 aq 2 KNO 3 aq + PbCl 2 (s)

12 What is a Complete Ionic Equation? 0 Shows each of the reactants and products as separate ions if they are water soluble 0 Insoluble substances are not separated and have a symbol (s) written next to them 2K + aq + 2Cl aq + Pb 2+ aq + 2NO 3 aq 2K + aq + 2NO 3 aq + PbCl 2 (s)

13 What is a Net Ionic Equation? 2K + aq + 2Cl aq + Pb 2+ aq + 2NO 3 aq 2K + aq + 2NO 3 aq + PbCl 2 (s) 0 Notice that there are some of the same ions present on both sides of the reaction arrow 0 These ions do not react and are known as spectator ions 0 Commonly eliminated from complete ionic equation by crossing them out 0 Remaining equation is known as the net ionic equation 0 Includes only those components that undergo a change 2Cl aq + Pb 2+ (aq) PbCl 2 (s)

14 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

A reaction in which a solid forms is called a precipitation reaction. Solid = precipitate

A reaction in which a solid forms is called a precipitation reaction. Solid = precipitate Chapter 7 Reactions in Aqueous Solutions 1 Section 7.1 Predicting Whether a Reaction Will Occur Four Driving Forces Favor Chemical Change 1. Formation of a solid 2. Formation of water 3. Transfer of electrons