١٤٣٦/٢١/١٢ CHAPTER 4: A QUEOUS REACTIONS AND SOLUTION STOICHIOMETRY GENERAL PROPERTIES OF AQUEOUS SOLUTIONS A solution is a homogeneous mixture of two or more substances. A solution in which water is the dissolving medium is called an aqueous solution. ١ ١٤٣٦/٢١/١٢ SOLUTE VS . S OLVENT The substance present in the greatest quantity is called the solvent. Water is considered the universal solvent because of its ability to dissolve many substances. The other dissolved substances are called the solutes . A solvent dissolves a solute. ELECTROLYTES A substance whose aqueous solutions contain ions is called an electrolyte because it will allow electric current to flow through it. Example: NaCl A substance that does not form ions in solution is called a nonelectrolyte. Example: C12 H22 O11 ٢ ١٤٣٦/٢١/١٢ IONIC COMPOUNDS IN WATER When ionic compounds dissolve in water the ions separate and are surrounded by water molecules. Once the ions have separated, they can move and conduct electricity. In the crystalline form they cannot move or conduct electricity. The solvation process helps stabilize the ions in solution and prevents the cations and anions from recombining. IONIC COMPOUND IN WATER Water molecules are polar and have a δ+ and δ- side. The more electronegative oxygen atoms are attracted to the positive ions. The less electronegative hydrogen atoms are attracted to the negative ions. ٣ ١٤٣٦/٢١/١٢ Ionic Compounds : undergo dissociation: process by which many ionic substances dissolve in water, the solvent pulls the individual ions from the crystal and solvates them. _ {NaCl + H2O} Polar water molecule + + H2O Dissociation H O NaCl 2 + - (s) Na (aq) + Cl (aq) Hydration of sodium chloride ٤ ١٤٣٦/٢١/١٢ MOLECULAR COMPOUNDS IN WATER When a molecular compound dissolves in water, the solution usually consists of intact molecules (neutral) dispersed throughout the system. Consequently, most molecular compounds are nonelectrolytes. A few exceptions include strong acids like HCl. Ionic (electrolyte) Molecular (non electrolyte) Molecular (covalent) Compounds: mostly insoluble gases , except polar organic liquids containing O & N (polar : acids, bases, alcohols, etc.) Insoluble gases : NO 2, CH 4, CO 2, O 2, P 2O5, N 2, CO, etc. Polar Covalent {carbon (C) chains containing H,O or N}: CH 3OH, C 6H12 O6, C6H5OH, etc. H2O C6H12 O6(s) C6H12 O6 (aq) H2O C3H5OH (l) C3H5OH (aq) Dissolve without dissociating into ions {Ethanol+Water } ٥ ١٤٣٦/٢١/١٢ STRONG AND WEAK ELECTROLYTES Strong electrolytes are those solutes that exist in solution completely or nearly completely as ions. Weak electrolytes are those solutes that exist in solution mostly in the form of molecules with only a small fraction in the form of ions. ELECTROLYTES : S TRONG AND WEAK A strong electrolyte dissociates completely when dissolved in water. H2O + - HCl (g) H (aq) + Cl (aq) A weak electrolyte only dissociates partially when dissolved in water. H2O + - NH 4OH (aq) NH 4 (aq) + OH (aq) Acetic Acid, H2O + - HC 2H3O2 (aq) H (aq) + C 2H3O2 (aq) molecules ions ٦ ١٤٣٦/٢١/١٢ ELECTROLYTES : A VISUAL CHEMICAL EQUILIBRIUM Chemical equilibrium is the state of balance in which the relative numbers of each type of ion or molecule in the reaction are constant over time. Equilibrium is represented by a double arrow for the yield sign. This shows that the reaction is occurring in both directions. Dynamic Equilibrium ٧ ١٤٣٦/٢١/١٢ STRONG ELECTROLYTES Remember that soluble ionic compounds are strong electrolytes. We consider ionic compounds as those composed of metals and nonmetals – such as NaCl, FeSO 4 and Al(NO 3)3 – or compounds containing the ammonium + ions, NH 4 - such as NH 4Br and (NH 4)2CO 3. SAMPLE EXERCISE 4.1 The diagram below represents an aqueous solution of one of the following compounds: MgCl 2, KCl, or K2SO 4. Which solution does the drawing best represent? The diagram shows twice as many cations as anions, consistent with the formulation K2SO 4. ٨ ١٤٣٦/٢١/١٢ PRACTICE EXERCISE If you were to draw diagrams like the previous one representing aqueous solutions of each of the following ionic compounds, how many anions would you show if the diagram contained six cations? 6 a. NiSO 4 12 b. Ca(NO 3)2 c. Na 3PO 4 2 d. Al 2(SO 4)3 9 PRECIPITATION REACTIONS Reactions that result in the formation of an insoluble product are called precipitate reactions. A precipitate is an insoluble solid formed by a reaction in solution. ٩ ١٤٣٦/٢١/١٢ PRECIPITATION REACTIONS Aqueous solutions, reacting to produce a precipitate (an insoluble compound). Example: KI (aq) + Pb(NO 3)2 (aq) Predict the solubility of compounds in reaction: Pb(NO 3)2(aq) + 2KI (aq) 2 KNO 3(aq) PbI 2(s) Precipitate (ppt) Pb(NO 3)2 KI PbI 2 SOLUBILITY The solubility of a substance at a given temperature is the amount of substance that can be dissolved in a given quantity of solvent at a given temperature. Most important solubility rule: Alkali metal ions and ammonium ions are soluble in water. ١٠ ١٤٣٦/٢١/١٢ SOLUBILITY – ABILITY TO BE DISSOLVED Soluble (aq) a substance which easily dissolves in water Slightly Soluble (s) a substance that only dissolves a tiny bit in water Insoluble (s) substance that does NOT dissolve in water substance remains separate from the H2O molecules (no interaction) substances are insoluble when the ions attract so strongly that they CANNOT be pulled apart by H2O molecules SOLUBILITY : A VISUAL ١١ ١٤٣٦/٢١/١٢ SOLUBILITY RULES To know if an ionic substance is soluble or not, you must consult the solubility rules lists which ions are typically soluble and insoluble also lists exceptions to the rules Note : Acids are not explicitly listed in the rules because all acids are aqueous solutions (thus soluble) SOLUBILITY RULES: for Ionic Compounds (Salts) 1. All salts of alkali metals (IA) are soluble . + 2. All NH 4 salts are soluble . - - - 3. All salts containing the anions: NO 3 , ClO 3 , ClO 4 , - (C 2H3O2 ) are soluble . - - - 2+ + 4. All Cl , Br , and I are soluble except for Hg 2 , Ag , and Pb 2+ salts. 2- 2+ 2+ 2+ 5. All SO 4 are soluble except for Pb , Ba , and Sr . ١٢ ١٤٣٦/٢١/١٢ Solubility Rules for Common Ionic Compounds in Water Soluble Ionic Compounds Exceptions: these are insoluble w/ respective anions - Nitrates, NO 3 none - Acetates, C 2H3O2 none - + 2+ 2+ Chlorides, Cl Ag , Hg 2 (2 Hg atoms), Pb - + 2+ 2+ Bromides, Br Ag , Hg 2 , Pb - + 2+ 2+ Iodides, I Ag , Hg 2 , Pb 2- + 2+ 2+ 2+ 2+ Sulfates, SO 4 Ag , Hg 2 , Pb , Sr , Ba Alkali metal cations none + Ammonium, NH 4 none Insoluble Ionic Compounds Exceptions: these are soluble w/ respective anions 2- + 2+ 2+ 2+ Sulfides, S alkali metal cations, NH 4 , Ca , Sr , Ba 2- + Carbonates, CO 3 alkali metal cations, NH 4 3- + Phosphates, PO 4 alkali metal cations, NH 4 - + 2+ 2+ 2+ Hydroxides, OH alkali metal cations, NH 4 , Ca , Sr , Ba SAMPLE EXERCISE 4.2 Classify the following ionic compounds as soluble or insoluble in water: a. Sodium carbonate: Na 2CO 3, Soluble b. Lead (II) sulfate : PbSO 4 Insoluble ١٣ ١٤٣٦/٢١/١٢ PRACTICE EXERCISE Classify the following compounds as soluble or insoluble in water: a. Cobalt (II) hydroxide: Insoluble b. Barium nitrate: Soluble c. Ammonium phosphate: Soluble EXCHANGE REACTIONS Reactions in which positive ions and negative ions appear to exchange partners conform to the following generic equation: AX + BY AY + BX Such reactions are called exchange reactions, double displacement reactions, or metathesis reactions. ١٤ ١٤٣٦/٢١/١٢ SOLUBILITY RULES -PRACTICE Are the following soluble or insoluble? Be(C 2H3O2)2 soluble MgS insoluble BaSO 4 insoluble K3PO 4 soluble STEPS FOR WRITING DOUBLE DISPLACEMENT REACTIONS 1. Write the formulas for the reactants (balance charges). 2. Write the formulas for the products by switching the cations of the two reactants (balance charges). 3. Balance the equation. 4. Include states of matter when needed. ١٥ ١٤٣٦/٢١/١٢ SAMPLE EXERCISE 4.3 a) Predict the identity of the precipitate that forms when solutions of BaCl 2 and K2SO 4 are mixed. BaSO 4 is insoluble and will precipitate from solution. KCl is soluble. b) Write the balanced chemical equation for the reaction. PRACTICE EXERCISE a) What compound precipitates when solutions of Fe 2(SO 4)3 and LiOH are mixed? Fe(OH) 3 b) Write a balanced equation for the reaction. Fe 2(SO 4)3(aq) + 6 LiOH(aq) 2Fe(OH) 3(s) +3Li 2SO 4(aq) c) Will a precipitate form when solutions of Ba(NO 3)2 and KOH are mixed? no (both possible products, Ba(OH) 2 and KNO 3, are water soluble) ١٦ ١٤٣٦/٢١/١٢ COMPLETE IONIC EQUATIONS o A molecular equation is an equation showing the complete chemical formulas of the reactants and products without showing dissociated ions. o Ex: Pb(NO 3)2(aq) + 2KI (aq) PbI 2(s) + 2KNO 3(aq) o A complete ionic equations is an equation that shows all soluble strong electrolytes as ions. o Ex: 2+ - + - Pb (aq) + 2NO 3 (aq) + 2K (aq) + 2I (aq) PbI 2(s) + + - 2K (aq) + 2NO 3 (aq) NET IONIC EQUATIONS Ions that appear in identical forms on both sides of the yield sign are called spectator ions because they do not actually react. A net ionic equation is an equation in which the spectator ions have been removed. Complete Ionic Equation 2+ - + - + Pb (aq) + 2NO 3 (aq) + 2K (aq) + 2I (aq) PbI 2(s) + 2K (aq) + - 2NO 3 (aq) Net Ionic Equation 2+ - Pb (aq) + 2I (aq) PbI 2(s) ١٧ ١٤٣٦/٢١/١٢ NET IONIC EQUATIONS In a net ionic equation, the numbers of atoms and the charges are conserved in the reaction.