Prentice Hall © 2003Chapter 4 Chapter 4 Aqueous Reactions and Solution Stoichiometry

Prentice Hall © 2003Chapter 4 Electrolytic Properties Aqueous solutions have the potential to conduct electricity ions Three types of solutions: Strong electrolytes Weak electrolytes Nonelectrolytes 4.1 General Properties of Aqueous Solutions

Prentice Hall © 2003Chapter 4 Ionic Compounds in Water Ions dissociate (break apart) in water In solution, each ion is surrounded by water molecules Transport of ions through solution causes flow of current

Prentice Hall © 2003Chapter 4 Molecular Compounds in Water Molecular compounds in water: no ions are formed nothing present to transport electric charge

Prentice Hall © 2003Chapter 4 Electrolytic Properties

Prentice Hall © 2003Chapter 4 Strong and Weak Electrolytes Strong electrolytes: completely dissociate in solution Acids ionize Soluble Salts Strong Acids (HCl, HBr, HI, HNO 3, HClO 4, HClO 3, H 2 SO 4 ) HCl (aq)  H + (aq) + Cl - (aq) KNOW THE 7 STRONG ACIDS!

Prentice Hall © 2003Chapter 4 Weak electrolytes: produce a small concentration of ions when they dissolve (weak acids) –These ions exist in equilibrium with the un-ionized substance –For example:

Prentice Hall © 2003Chapter 4 Sample Problems # 5 & 9

Prentice Hall © 2003Chapter 4 When two aqueous solutions are mixed and a solid is formed, the solid is called a precipitate 4.2 Precipitation Reactions

Text, P. 118

Prentice Hall © 2003Chapter 4 Sample Problem # 11

Metathesis (Exchange) Reactions Metathesis comes from a Greek word that means “to transpose” AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq)

Metathesis (Exchange) Reactions Metathesis comes from a Greek word that means “to transpose” It appears the ions in the reactant compounds exchange, or transpose, ions AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq)

Metathesis (Exchange) Reactions Metathesis comes from a Greek word that means “to transpose” It appears the ions in the reactant compounds exchange, or transpose, ions AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq)

Prentice Hall © 2003Chapter 4 Exchange (Metathesis) Reactions Metathesis reactions involve swapping ions in solution: (anions/cations switch partners) AX + BY  AY + BX Metathesis reactions will lead to a change in solution if one of three things occurs: –an insoluble solid is formed (precipitate) –weak or nonelectrolytes are formed –an insoluble gas is formed Signs of a chemical reaction!

Prentice Hall © 2003Chapter 4 Sample Problems # 13, 17, 19

Solution Chemistry It is helpful to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution). If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction.

Molecular Equation The molecular equation lists the reactants and products in their molecular form. AgNO 3 (aq) + KCl (aq)  AgCl (s) + KNO 3 (aq)

Ionic Equation In the ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. This more accurately reflects the species that are found in the reaction mixture. Ag + (aq) + NO 3 - (aq) + K + (aq) + Cl - (aq)  AgCl (s) + K + (aq) + NO 3 - (aq)

Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. Ag + (aq) + NO 3 - (aq) + K + (aq) + Cl - (aq)  AgCl (s) + K + (aq) + NO 3 - (aq)

Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The only things left in the equation are those things that change (i.e., react) during the course of the reaction. Ag + (aq) + Cl - (aq)  AgCl (s)

Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The only things left in the equation are those things that change (i.e., react) during the course of the reaction. Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions. Ag + (aq) + NO 3 - (aq) + K + (aq) + Cl - (aq)  AgCl (s) + K + (aq) + NO 3 - (aq)

Prentice Hall © 2003Chapter 4 Ionic Equations Ionic equation: used to highlight a reaction between ions (formation of an insoluble product, molecular compound or a gas) Molecular equation: all species listed as molecules (“the reaction”): HCl(aq) + NaOH(aq)  H 2 O(l) + NaCl(aq) Complete ionic equation: lists all ions (soluble compounds are broken into ions): H + (aq) + Cl - (aq) + Na + (aq) + (OH) - (aq)  H 2 O(l) + Na + (aq) + Cl - (aq) Net ionic equation: lists only unique ions (the insoluble product, molecular compound or a gas) H + (aq) + (OH) - (aq)  H 2 O(l) Cancel Spectator Ions (ions that appear on both sides of the equation)

Prentice Hall © 2003Chapter 4 Sample Problem # 15

Prentice Hall © 2003Chapter 4 Acids Dissociation = ions in a solid move apart in solution. Ionization = a neutral substance forms ions in solution. Acid = substances that ionize to form H + in solution monoprotic(HCl) diprotic(H 2 SO 4 ) many acidic protons = polyprotic 4.3 Acid-Base Reactions

Prentice Hall © 2003Chapter 4 Bases Bases = substances that react with the H + ions formed by acids (e.g. NH 3, Drano ™, Milk of Magnesia ™ )

Prentice Hall © 2003Chapter 4 Strong and Weak Acids and Bases Strong acids and bases are strong electrolytes –They are completely ionized in solution Weak acids and bases are weak electrolytes –They are partially ionized in solution

Prentice Hall © 2003Chapter 4 Text, P. 122

Prentice Hall © 2003Chapter 4 Review: Identifying Strong and Weak Electrolytes STRONG ELECTROLYTES Water soluble and ionic Strong acid or strong base WEAK ELECTROLYTES Water soluble and not ionic Weak acid or weak base Otherwise, the compound is a NONELECTROLYTE

Prentice Hall © 2003Chapter 4 Sample Problems: # 23, 25, 27

Prentice Hall © 2003Chapter 4 Neutralization Reactions and Salts Acid + Base  Salt + Water (Neutralization Reaction) HCl (aq) + NaOH (aq)  H 2 O (l) + NaCl (aq) SA/SB net ionic: H + (aq) + (OH) - (aq)  H 2 O (l) HC 2 H 3 O 2(aq) + NaOH (aq)  NaC 2 H 3 O 2 (aq) + H 2 O (l) WA/SB net ionic: HA (aq) + (OH) - (aq)  A - (aq) + H 2 O (l) Salt Ionic compound Composed of the cation from the base and the anion from the acid

Prentice Hall © 2003Chapter 4 Acid-Base Reactions with Gas Formation Sulfide and carbonate ions can react with H + in a similar way to OH  2HCl (aq) + Na 2 S (aq)  H 2 S (g) + 2NaCl (aq) net ionic: 2H + (aq) + S 2- (aq)  H 2 S (g) HCl (aq) + NaHCO 3(aq)  NaCl (aq) + H 2 O (l) + CO 2(g) net ionic: H + (aq) + (HCO 3 ) - (aq)  H 2 O (l) + CO 2(g)

Prentice Hall © 2003Chapter 4 Sample Problems # 29, 31, 33

Prentice Hall © 2003Chapter 4 Oxidation and Reduction Oxidized: an atom, molecule, or ion becomes more positively charged Oxidation is the loss of electrons Reduced: an atom, molecule, or ion becomes less positively charged Reduction is the gain of electrons Oxidation and reduction always occur simultaneously 4.4 Oxidation-Reduction Reactions

Prentice Hall © 2003Chapter 4 Oxidizing Agent: makes it possible for something else to be oxidized –The agent is reduced Reducing Agent: makes it possible for something else to be reduced –The agent is oxidized

Oxidation and Reduction

Prentice Hall © 2003Chapter 4 Remember: OIL RIG Oxidation Is a Loss of electrons Reduction Is a Gain of electrons And LEO the lion says GER Loss of Electrons is Oxidation Gain of Electrons is Reduction And LEO the GERbil

Prentice Hall © 2003Chapter 4 Oxidation Numbers Oxidation number for an ion: the charge on the ion Oxidation number for an atom: the hypothetical charge that atom would have if it was an ion It keeps track of electron transfers

Prentice Hall © 2003Chapter 4 Oxidation numbers are assigned by a series of rules: 1.If the atom is in its elemental form, the oxidation number is zero. ( Cl 2, H 2, P 4 ) 2.For a monoatomic ion, the charge on the ion is the oxidation state. (F -, O -2, Ca +2 ) Text, P

Prentice Hall © 2003Chapter 4 Oxidation Numbers: Rules 3.Nonmetals usually have negative oxidation numbers: a)Oxidation number of O is usually –2 In the peroxide ion, O 2 2-, the oxygen atom has an oxidation number of –1 b) Oxidation number of H is +1 when bonded to nonmetals and –1 when bonded to metals c) The oxidation number of F is –1. The other halogens have an oxidation number of -1 in most binary compounds. When combined with oxygen, as in oxyanions, however, they have positive oxidation states.

Prentice Hall © 2003Chapter 4 Oxidation Numbers: Rules 4.The sum of the oxidation numbers for a polyatomic ion is equal to the charge on the molecule –The sum of the oxidation numbers is zero for a neutral compound

Prentice Hall © 2003Chapter 4 Sample Problems # 39, 41

Prentice Hall © 2003Chapter 4 Oxidation of Metals by Acids and Salts Metals are oxidized by acids to form salts: Zn(s) +2HCl(aq)  ZnCl 2 (aq) + H 2 (g) During the reaction, 2H + (aq) is reduced to H 2 (g) Metals can also be oxidized by other salts: Net ionic: Fe(s) +Ni 2+ (aq)  Fe 2+ (aq) + Ni(s)

Prentice Hall © 2003Chapter 4 Activity Series Some metals are easily oxidized whereas others are not Activity series: a list of metals arranged in decreasing ease of oxidation Notice the TYPES of elements & their location on the series

Text, P. 131

Prentice Hall © 2003Chapter 4 From Your Handout

Prentice Hall © 2003Chapter 4 Activity Series, Continued The higher the metal on the activity series, the more active that metal (“No Reaction” is possible) –An element can only “replace” an element that is below it on the activity series. Form: AX + B  BX + A –a “spontaneous reaction” “Activity Series” for halogens: F 2 > Cl 2 > Br 2 > I 2 Form: AX + Z 2  AZ + X 2 “Like replaces like”

Identifying Redox Equations In general, all chemical reactions can be assigned to one of two classes: 1)oxidation-reduction, in which electrons are transferred: Single-replacement, combination, decomposition, and combustion 2)this second class has no electron transfer, and includes all others: Double-replacement and acid-base reactions

Prentice Hall © 2003Chapter 4 Sample Problems #43 & 45 on P. 147

Prentice Hall © 2003Chapter 4 Molarity Solution = solute dissolved in solvent Solute: present in smallest amount Water as solvent (aqueous solutions) Change concentration by using different amounts of solute and solvent Molarity: Moles of solute per liter of solution If we know molarity and liters of solution, we can calculate moles (and mass) of solute 4.5 Concentrations of Solutions

Prentice Hall © 2003Chapter 4 Molarity Remember: V * M = moles

Prentice Hall © 2003Chapter 4 Sample Problems P. 147 # 51, 53, 55, 57 (c and d) –Can you EXPLAIN how to prepare these solutions?

Prentice Hall © 2003Chapter 4 Dilution We recognize that the number of moles are the same in dilute and concentrated solutions. So: M dilute V dilute = moles = M concentrated V concentrated A lways A dd A cid to Water

Prentice Hall © 2003Chapter 4 Sample Problems P. 148 # 59 (also do an HCl dilution and explain how to prepare the solution), 61

Prentice Hall © 2003Chapter 4 There are two different types of units: –laboratory units (macroscopic units) –chemical units (microscopic units) Convert! Grams  moles (using molar mass) Volume or molarity  moles (using M = mol/L) Use the stoichiometric coefficients to move between reactants and products 4.6: Solution Stoichiometry and Chemical Analysis

Prentice Hall © 2003Chapter 4 Figure 4.18, P. 139 of text 4.6: Solution Stoichiometry and Chemical Analysis

Prentice Hall © 2003Chapter 4 Titration: Controlled combination of a solution of unknown concentration with a solution of known concentration Primary Standard: A relatively pure compound with a –high molar mass –known pattern of reactivity –used as a basis of comparison for the determination of the concentration of an unknown Secondary Standard: A solution with a molarity that is very accurately known Standard Solution: A solution of known concentration. Titration Terms

Prentice Hall © 2003Chapter 4 Equivalence Point: Stoichiometrically equivalent amounts of substances are combined –Indicator: Dyes used to indicate the equivalence point (color change) Phenolphthalein: colorless in an acid, pink in a base End Point: the point of color change that coincides with the equivalence point. Titration Terms, continued

Titrations

Prentice Hall © 2003Chapter 4 Sample problems P. 148 # 65, 67, 69 Thought Provoking questions: # 71, 73, 75

Prentice Hall © 2003Chapter 4 End of Chapter 4: Aqueous Reactions and Solution Stoichiometry